Intercalation of fatty acids into layered double hydroxides Nontete Suzan Nhlapo Magister Scientiae

Intercalation of fatty acids into layered double hydroxides Nontete Suzan Nhlapo Magister Scientiae
Intercalation of fatty acids into layered double
hydroxides
by
Nontete Suzan Nhlapo
Submitted in partial fulfilment of the requirements for the degree
Magister Scientiae
In the Department of Chemistry
Faculty of Natural and Agricultural Sciences
University of Pretoria
Pretoria
Supervised by: Prof. Walter W. Focke
October 2008
ABSTRACT
Surfactant-mediated intercalation of aliphatic fatty acids into a commercial,
layered
double
hydroxide
(LDH)
with
the
approximate
composition
of
[Mg0.689Al0.311(OH2)] (CO3)0.156·nH2O was explored. The reactions were conducted
at elevated temperatures with the LDH powder suspended in a fatty acid oil-water
emulsion. The acidic fatty acid, e.g. stearic acid, reacts with the basic carbonate
anions from LDH-CO3. In the process, CO2 is released as a gas and the fatty acids
are intercalated as a bilayer. A high concentration of anionic or non-ionic
surfactants, i.e. sodium dodecylsulphate or Tween 60, facilitates the intercalation
process by emulsifying the molten fatty acids and dispersing the LDH particles.
The presence of carboxylate anions in the interlayer region was confirmed by the
carboxylate absorption peaks observed in the region 1700–1000 cm-1 on Fouriertransform infrared spectroscopy (FT-IR). Several bands were observed, i.e. ionised
and non-ionised. An increase in the d-spacing of the d003 plane of the brucite-like
LDH layers was observed on X-ray diffraction (XRD) analysis of all the LDH
intercalates. The d-spacing increased linearly with the length of the carboxylic acid
chain. Sharp reflection peaks were obtained on XRD, showing the high crystallinity
of the LDH intercalates. The thermal decomposition of these materials was
explored
on
thermogravimetric
or
differential
thermogravimetric
analysis
(TGA/DTA) and temperature-scanned XRD. The mole ratio of Mg to Al was
obtained by XRF and the morphology by scanning electron microscopy (SEM).
The present method works well with long-chain aliphatic fatty acids at
temperatures above or at the melting point of the desired acid. Temperature
proved to be the most important parameter to control during the preparation
process, i.e. at low temperatures incomplete reactions were obtained. The method
is convenient, economical and environmentally friendly. It employs the readily
available carbonate form of LDH as a starting reagent, water is used as medium
rather than organic solvents, there are no high-temperature calcinations, and an
inert atmosphere is not required.
i
Keywords: Layered double hydroxides, fatty acids, surfactants
ii
ACKNOWLEDGEMENTS
First and foremost, I would like to thank God Almighty for his grace, loving
kindness, help and protection all times. “Trust in the Lord with all your heart, do
not lean on your own understanding, for he is good, his mercy is everlasting, and
his truth endures to all generations” (Psalm 100:5).
This study would not have been possible without the contributions of the
following:
Prof. W.W. Focke, my advisor, supervisor and promoter – thank you for your
constant support, valuable suggestions, encouragement and guidance throughout
the study.
Financial assistance from the National Research Foundation (NRF) and the
University of Pretoria, gratefully acknowledged.
My family: my mom and dad (Masephoso and Dan), my brothers (Simon, Phike
and Mfokazana), my sisters (Nozika, Nomthemba and Nonhlupheko) and my Uncle
Chris who motivated, encouraged and supported me in difficult times. Thank you
for being there for me.
My friends and colleagues at the Institute of Applied Materials (IAM): Lumbi Moyo,
Bhekani Magagula, Pedro Massinga, Kolela Ilunga, Herminio Muiambo, Darren
Swanepoel, Mpolokeng Makhanya and Muzi Makhaye. Thank you very much for
your inputs.
I also wish to thank Mila Maksa, Dr Sabine Verryn and Dave Lillies for the analyses
– your patience and guidance are highly appreciated.
iii
DECLARATION
I, the undersigned, declare that the dissertation which I hereby submit for the
degree MSc at the University of Pretoria is my own work, and has not previously
been submitted by me for degree purposes or examination at this or any other
tertiary institution.
……………………………..
Nontete Suzan Nhlapo
iv
TABLE OF CONTENTS
Abstract ...................................................................................................... i
Acknowledgements .................................................................................. iii
Declaration…………………………………………………………………………......iv
List of Figures..........................................................................................viii
List of Tables ........................................................................................... xii
List of Schemes ...................................................................................... xiii
List of Abbreviations ...............................................................................xiv
1
Introduction and Aim of Study ........................................................... 1
1.1
Dissertation Outline............................................................................. 1
1.2
Clay Minerals ...................................................................................... 2
1.3
Historical Background.......................................................................... 3
1.4
Problem Statement ............................................................................. 6
1.5
Aims and Objectives............................................................................ 7
2
Literature Review ............................................................................... 9
2.1
LDH Structure and Composition ........................................................... 9
2.2
Intercalation ..................................................................................... 15
2.3
Review of the Intercalation Methods .................................................. 16
2.3.1 Direct ion exchange ................................................................ 16
2.3.2 Rehydration............................................................................ 20
2.3.3 Direct synthesis by coprecipitation ........................................... 22
2.3.4 Thermal melt/reaction method ................................................ 25
2.3.5 Sol-gel ................................................................................... 25
2.4
Characterisation of LDHs ................................................................... 26
2.4.1 FT-IR ..................................................................................... 26
2.4.2 TG/DTG ................................................................................. 27
2.4.3 PXRD ..................................................................................... 28
2.4.4 Other characterisation techniques............................................ 30
v
2.5
Carboxylic/Fatty Acids ....................................................................... 31
2.6
Surfactants ....................................................................................... 32
2.7
Potential Applications of LDH ............................................................. 36
2.7.1 Catalysis ................................................................................ 36
2.7.2 Pharmaceutical, medical and cosmetic applications ................... 37
2.7.3 Polymers ................................................................................ 38
2.7.4 Other applications…………………………………………………………………39
3
Experimental..................................................................................... 41
3.1
Reagents and Suppliers ..................................................................... 41
3.2
Experimental Set-up.......................................................................... 41
3.3
Standard Intercalation Method........................................................... 42
3.4
Effect of Surfactant on Intercalation................................................... 44
3.5
Leaching out the Excess Monocarboxylic Acid on LDH Intercalates ....... 44
3.6
Preparation of Mixture of Magnesium Stearate and LDH Stearate......... 45
3.7
Material Analysis ............................................................................... 46
3.7.1 Instrumentation...................................................................... 46
4
Results and Discussion ..................................................................... 48
4.1
Elemental Analysis ............................................................................ 48
4.2
Thermal Decomposition..................................................................... 48
4.3
FT-IR ............................................................................................... 54
4.4
State of Intercalated Carboxylic Acid .................................................. 57
4.5
X-ray Diffraction................................................................................ 59
4.6
Effect of Surfactant on Intercalation................................................... 65
4.7
Effect of Reaction Temperature on Carboxylate Anion Intercalation...... 69
4.8
Differential Scanning Calorimetry (DSC).............................................. 71
4.9
Temperature-scanned XRD ................................................................ 76
4.10
Particle Morphology........................................................................... 80
5
Conclusion......................................................................................... 83
6
References ........................................................................................ 85
Appendix A:
A.1
Experimental Methods ................................................ 105
Synthesis procedure for monocarboxylate intercalated LDH ................105
vi
Appendix B:
Thermal Analysis ......................................................... 108
B.1:
Expected TG mass loss after the first and last thermal events.............108
B.2:
Thermogravimetric curves and the derivative fatty acid intercalated LDH..
.......................................................................................................109
Appendix C:
FT-IR Results............................................................... 116
Appendix D:
XRD Results………………………………………………..…...120
Appendix E:
SEM Results ................................................................. 124
vii
LIST OF FIGURES
Figure 1:
Structure of LDH (adapted from Carlino, 1997)................................ 11
Figure 2:
Schematic presentation of the reconstruction method followed to
produce LDH intercalated material (adapted from Morioka et al., 1995) ...... 21
Figure 3:
Diffraction of X-rays on crystal lattice according to Bragg ................. 29
Figure 4:
Representations of (a) a surfactant molecule; (b) a surfactant micelle;
and (c) the surfactant head in water and tail in oil ..................................... 34
Figure 5:
Chemical structure of sodium dodecylsulphate (SDS) – a typical
example of an anionic surfactant .............................................................. 35
Figure 6:
Schematic diagram of the experimental set-up ................................ 42
Figure 7:
Carbon numbers and the melting points (∆) of the monocarboxylic
acids… .................................................................................................... 44
Figure 8:
Schematic presentation of the experimental extraction set-up .......... 45
Figure 9:
TG and DTG curves of LDH-CO3 and stearic acid in air ..................... 50
Figure 10: TG curves of LDH-octanoate, laurate, myristate, stearate and behenate
prepared at 80 °C (octanoate and stearate), 70 °C (laurate), 60 °C
(myristate) and 90 °C (behenate) ............................................................. 51
Figure 11: DTG curve of LDH-octanoate, laurate, myristate, stearate and behenate
prepared at 80 °C (octanoate and stearate), 70 °C (laurate), 60 °C
(myristate) and 90 °C (behenate) ............................................................. 52
Figure 12: Effect of reagent stoichiometry on the degree of stearic acid
intercalation at 80 °C, where (••) represents the present data and (∆
∆)
represents the values obtained by Itoh et al. (2003) for sodium stearate .... 54
Figure 13: FT-IR spectra of the precursor LDH-CO3, stearic acid and the surfactant
SDS used in the intercalation reactions ..................................................... 55
Figure 14: Comparison of FT-IR spectra of LDH-octanoate, laurate, myristate,
stearate and behenate prepared at 80, 70, 60, 80 and 90 °C respectively ... 57
Figure 15: FT-IR spectra of fatty acid (octanoic, lauric, myristic, stearate and
behenic) intercalated LDH showing only the carboxyl region....................... 58
Figure 16: XRD pattern of fatty acid (octanoic, lauric, myristic, stearic and
behenic) intercalated LDH samples prepared at 80 °C (LDH-octanoate, laurate
viii
and stearate), 60 °C (LDH-myristate) and 90 °C (LDH-behenate) respectively
.............................................................................................................. 60
Figure 17: Schematic representation of the bilayer structure of fatty acid
intercalated LDH with corrected slant angle (adapted from Carlino, 1997) ... 62
Figure 18: Effect of carboxylic acid chain length on the d-spacing of the LDH
intercalates prepared by the SDS-mediated intercalation method, represented
) and LDH-SDS (∆
∆) (i.e. the
by (
). The d-spacing values for LDH-CO3 (
sample in which an attempt was made to intercalate acetate) are shown, as
well as the d-spacing values reported by Borja and Dutta (1992) (
) and Itoh
et al. 2003 (□). ........................................................................................ 63
Figure 19: (a) Idealised regular arrangement of aluminium ( ) and magnesium
atoms ( ) in the brucite-like metal hydroxide sheet of [Mg2+xAl1-x(OH)6]
(CO3)(1-x)/2.nH2O) with α ≅ 0, the lattice parameter a = 0.305 (Belloto et al.,
1996); and (b) the hexagonal close packing structure of the stearate chains.
.............................................................................................................. 65
Figure 20: FT-IR spectra of LDH-SDS and LDH-Tween 60 in comparison with LDHCO3......................................................................................................... 66
Figure 21: XRD pattern of LDH-SDS and LDH-Tween 60 synthesised at 80°C in
comparison with LDH-CO3 ........................................................................ 67
Figure 22: TG/DTG curves of the product obtained by dispersing the LDH in water
in the presence of the surfactants SDS (LDH-SDS) and Tween 60 (LDH-Tween
60), in comparison with LDH-CO3 heated from 25–700 °C in air ................. 68
Figure 23: FT-IR spectra of LDH-acetate obtained at room temperature in
comparison with the surfactant SDS.......................................................... 69
Figure 24: Effect of reaction temperature on the degree of lauric and stearic acid
intercalation ............................................................................................ 71
Figure 25: DSC melting endotherms for technical-grade stearic acid, pure stearic
acid (99%), LDH-stearate prepared at 80 °C using SDS and magnesium
stearate. The measured enthalpies were -185, -221, -241 and -173 kJ/kg
respectively ............................................................................................. 72
Figure 26: DSC melting endotherm and hot-stage microscopic image of LDHstearate prepared at 80 °C using Tween 60 .............................................. 73
ix
Figure 27: Microscopic images of LDH-stearate (SDS) taken at 100, 136, 152 and
170 °C .................................................................................................... 74
Figure 28: XRD pattern of LDH-stearate in comparison with the mixture of LDHstearate and magnesium stearate ............................................................. 75
Figure 29: DSC curve for LDH-laurate prepared at 80°C using SDS.................... 76
Figure 30: Effect of temperature on the X-ray diffraction spectra of LDH-stearate
synthesised at 80 °C using Tween 60 (scans taken at 5 °C/min intervals) ... 77
Figure 31: Effect of temperature on the intensity of the selected X-ray diffraction
peaks of LDH-stearate (SDS) (scans taken at 5 °C/min intervals)................ 78
Figure 32: Changes in the X-ray diffraction spectra on heating LDH-stearate (SDS)
to 100 °C and cooling it to 30 °C (scans taken at 5 °C/min intervals).......... 79
Figure 33: Changes in X-ray diffraction spectra of LDH-laurate on heating to 135
°C and cooling to ambient (scans taken at 5 °C/min intervals) ................... 80
Figure 34: SEM images showing: (a) the ‘sandrose’ morphology of LDH-CO3
crystals; (b) the flake-like habit of LDH-stearate (SDS); (c) the delamination
of the LDH-stearate crystals after extraction with ethanol; (d) the LDHstearate crystals obtained with Tween 60; and (e) the LDH-laurate crystals..
.............................................................................................................. 81
Figure B1: TG/DTG curves of acetate intercalated LDH synthesised at room
temperature ...........................................................................................110
Figure B2: TG/DTG curves of butyrate intercalated LDH synthesised at room
temperature and at 80 °C .......................................................................111
Figure B3: TG/DTG curves for hexanoate intercalated LDH at room temperature
.............................................................................................................111
Figure B4: TG/DTG curves of octanoate intercalated LDH synthesised at room
temperature and at 80 °C .......................................................................112
Figure B5: TG/DTG curves of decanoate intercalated LDH synthesised at 50 °C and
from 37 to 32 °C ....................................................................................112
Figure B6: TG curves of LDH-laurate prepared at 54–48, 60, 65, 70 and 80 °C .113
Figure B7: Derivative of weight loss curves of LDH-laurate obtained at 54-48, 60,
65, 70 and 80 °C ....................................................................................113
Figure B8: TG/DTG curves of LDH-myristate prepared at 59–54 and at 60 °C ...114
x
Figure B9: TG curves of LDH-stearate prepared at 50, 60, 70, 75, 75–70, 80 and
85oC using SDS and at 80 °C using Tween 60 ..........................................114
Figure B10: Derivative of weight loss curves of LDH-stearate prepared at 50, 60,
70, 75, 80 and 85 °C ..............................................................................115
Figure B11: TG/DTG curves of LDH-behenate prepared at 90 and 85–80 °C .....115
Figure B12: TG/DTG curves of the product obtained by dispersing the LDH in
distilled water in the presence of the surfactants SDS (LDH-SDS) and Tween
60 (LDH-Tween 60) at 80 °C ...................................................................116
Figure C1: FT-IR spectra of short-chain carboxylates: acetate, butyrate, hexanoate
and decanoate acid intercalated LDH, obtained at room temperature and at
50 °C for decanoate................................................................................117
Figure C2: FT-IR spectra of LDH-octanoate prepared at room temperature and at
80 °C .....................................................................................................118
Figure C3: FT-IR spectra of LDH-laurate prepared at 80, 70, 65 and 60 oC and
from 53–58 °C........................................................................................119
Figure C4: FT-IR spectra of LDH-stearate prepared at 60, 70, 75, 80 and 85 °C
using SDS and at 80 °C using Tween 60 ..................................................120
Figure D1: XRD pattern of LDH-acetate prepared at room temperature ............121
Figure D2: XRD pattern of LDH-butyrate prepared at 80 °C .............................121
Figure D3: XRD pattern of LDH-octanoate prepared at 80 °C ...........................122
Figure D4: XRD pattern of LDH-laurate prepared at 80 °C ...............................122
Figure D5: XRD pattern of LDH-myristate prepared at 60 °C ............................123
Figure D6: XRD pattern of LDH stearate prepared at 80 °C using SDS ..............123
Figure D7: XRD pattern of LDH-behenate prepared at 90 °C ............................124
Figure D8: XRD pattern of a mixture of LDH-stearate and magnesium stearate
prepared at 80 °C...................................................................................124
Figure E1: SEM image of LDH-acetate ............................................................125
Figure E2: SEM image of LDH-butyrate ...........................................................125
Figure E3: SEM image of LDH-octanoate.........................................................126
Figure E4: SEM image of LDH-behenate .........................................................126
xi
LIST OF TABLES
Table 1:
Naturally occurring minerals similar to hydrotalcite (adapted from Frost
et al., 2003b and Auerbach 2004 .................................................................... 5
Table 2:
Presentation of the atomic radius of some of the reported di-and
trivalent metal cation in LDH brucite like layers (adopted from Auerbach 2004
and Cavani et al., 1991) ............................................................................... 12
Table 3:
Reagents used and suppliers .............................................................. 41
Table 4:
XRF composition analysis (mass %) LDH-CO3 and LDH-stearate
synthesised with SDS and Tween 60 ashed at 700°C ..................................... 48
Table 5:
TG data for LDH-CO3, octanoate (80 °C), laurate (70 °C), myristate (60
°C), stearate (80 °C) and behenate (90 °C) intercalated LDH prepared using
SDS….. ....................................................................................................... 53
Table 6:
Observed XRD data of LDH intercalated samples ................................. 61
Table 7:
Differential scanning calorimetry (DSC) results for selected compounds 72
Table A1: Synthetic procedure followed for intercalation of fatty acids in LDH ......106
xii
LIST OF SCHEMES
Scheme 1:
Schematic
presentation
of
the
coprecipitaion
method
(from
Costantino et al., 2007)............................................................................ 23
Scheme 2:
Schematic presentation of the hydrolysis of fat or oil in aqueous
NaOH, yielding glycerol and three fatty acids, where R, R’, and R’’ = C11 –
C19 (adapted from MacMurry, 2000) ........................................................ 32
Scheme 3:
Schematic
representation
of
the
hydrolysis
process
for
the
production of surfactant, where R = C11- C19 (adapted from MacMurry,
2000)
................................................................................................. 33
Scheme 4:
Schematic presentation of the LDH decomposition process ........... 50
xiii
LIST OF ABBREVIATIONS
AEC
anionic exchange capacity
DSC
differential scanning calorimetry
DTA
differential thermal analysis
DTG
differential thermogravimetry
FT-IR
Fourier transform infrared spectroscopy
LDH
layered double hydroxide
LDO
layered double oxide
NTA
nitrilotriacetate
PXRD
powder X-ray diffraction
SDBS
sodium dodecylbenzenesulphate
SDS
sodium dodecylsulphate
SEM
scanning electron microscopy/micrograph
SOBS
sodium octylbenzenesulphonate
SOS
sodium octylsulphate
TG
thermogravimetry
TGA
thermogravimetric analysis
XRD
X-ray diffraction
XRF
X-ray fluorescence
xiv
1
1.1
INTRODUCTION AND AIM OF STUDY
Dissertation Outline
Layered double hydroxides (LDH) have recently received considerable attention in
a wide variety of industries, as well as in research. Much research has been done
on the preparation of inorganic and organic anion intercalated LDH. However,
most of the reported preparations had some limitations. The present study reports
on a simple and environmentally friendly method for the preparation of fatty acid
intercalated LDH.
Chapter 1 starts with a review of the history of clay minerals. A brief survey of the
main problems encountered when preparing LDH intercalated materials using the
previously reported methods is given. The aims and objectives of this study are
also presented.
Chapter 2 gives a detailed review of the literature. The structure and composition
of LDH and the preparation of LDH intercalates with various organic and inorganic
anions are presented. Methods for preparing LDH intercalates that are
investigated include direct anion exchange, rehydration, thermal melt reaction
methods and coprecipitation. The techniques used to characterise LDH and LDH
intercalates, carboxylic acids and the surfactants are briefly reviewed. Potential
applications of LDH intercalates are also considered in this chapter.
Chapter 3 describes the experimental work carried out in this study. The main
purpose of this study was to prepare various fatty acids intercalated LDH with the
aid of a surfactant. The reagents and suppliers, the intercalation procedure
followed, the methods used to further purify the LDH intercalates obtained, as well
as a description of all the instrumentation used to carry out the analysis are
presented in this chapter.
Chapter 4 presents the results and discussion. This chapter starts with the results
obtained for elemental analysis, followed by the thermal decomposition of the
1
selected precursors and of the LDH intercalates obtained. In the case of the
Fourier transform-infrared spectroscopy (FT-IR) results, the observed bands and
their assignments are discussed in detail. This provides information on the
presence and the state of the fatty acids in the interlayer region of the brucite-like
LDH layers. The X-ray diffraction (XRD) results are used to determine the type of
intercalation via the dependence of the interlayer spacing on the length of the
carboxylic chain. The effect of reaction temperature, reagent stoichiometry and
surfactant on intercalation is explored. The thermal behaviour of the LDH
intercalates obtained is studied using differential scanning calorimetry (DSC) and
hot stage microscopy. The phase changes of the intercalates are considered by
means of the change in the X-ray diffraction spectra as a function of temperature
during the heating and cooling of the LDH-stearate and laurate intercalates. The
particle morphology is presented by means of SEM images and compared with
that of the LDH-CO3 precursor.
Finally, the conclusions are presented in Chapter 4 which also summarises the
work carried out in the present study. The literature references are given in
Chapter 5. Raw data are given in the Appendices.
1.2
Clay Minerals
Clay minerals are two-dimensional structural minerals found in nature, with
particle sizes of less than 2 µm (Zammarano et al., 2006). They are made up of
different chemical compositions and characterised by a common platey
morphology (Bergaya, 2008). Clays are divided into two main groups, i.e. cationic
(smectites) and anionic (hydrotalcite-like minerals) clays (Vaccari, 1998). The
difference between anionic and cationic clays is in the layer charge, i.e. layers are
either negatively (cationic) or positively (anionic) charged (Zammarano et al.,
2006). The interlayer region of clays consists of cations or anions to compensate
for the layer charge. The free space also contains water molecules (Cavani et al.,
1991). Clays vary in nature and in the number of exchangeable ions. The physical
and chemical properties of these minerals are dependent on the nature and
2
particle size (Frost et al., 2002). Clays are characterised by their ability to
exchange ions from a solution (Vaccari, 1998).
Smectite clays are obtained mainly from natural rocks containing quartz and
calcite (Vaccari, 1998). They are characterised by their swelling behaviour in
water, a typical example being montmorillonite. However, other cationic clays
exhibit non-swelling behaviour, e.g. mica (Bergaya, 2008). Smectite clay consists
of an octahedral sheet of MO4(OH)2 bonded to a tetrahedral sheet of MO4
(Rajamathi et al., 2001). Mg, Al and Fe cations occupy the octahedral sites, while
Si occupies the tetrahedral sites. The net negative charge is a result of the
isomorphic substitution of Si cations by Al and/or Mg cations. The neutrality is
maintained by the presence of the interlayer cation (Vaccari, 1998). Bujdák (2006)
states that the orientation of cations and the arrangement of clay minerals is
affected by the density of the surface charge.
The individual clay layers are composed of two, three or four sheets arranged to
form hexagonal networks (Bergaya, 2008). Clays have excellent properties such as
low cost, non-toxicity, flexibility and ion-exchange ability (Vaccari, 1999). Organic
modification of clay is performed to change the properties from hydrophobic to
hydrophillic (Vaccari, 1998). Smectite clays are characterised by a lower charge
density on the octahedral sheet which makes exchange reactions easier (Auerbach
et al., 2004, p 10). The low charge density also results in weaker interactions
between the interlayer region and the clay layers, making the exfoliation process
easier (Auerbach et al., 2004. p 92).
1.3
Historical Background
Early reports on clay minerals date back to 1940–1950 in relation to catalysis
(Vaccari, 1998). The research started with the hydrated aluminosilicate minerals
called zeolites in 1938 (Bergaya, 2008). Zeolites were modified by exchanging the
interlayer cations for use as catalysts or catalyst supports. Since zeolites have
small particle sizes, it was difficult to use these minerals for all the desired
applications (Bergaya, 2008). Modification of clay minerals started with bentonite
3
in 1942 at the University of Chicago (Vaccari, 1998; Bergaya, 2008). Modified
bentonite found applications as catalysts, catalyst supports, adsorbents and as an
ion exchanger (Bergaya, 2008). Cationic clays then became important minerals for
a wide variety of applications in polymers, pharmaceuticals and biologicals, and in
nanotechnology (Fischer, 2003; Evans and Duan, 2006).
Natural anionic clay mineral is known as hydrotalcite. Hydrotalcite is a hydrated
mineral containing magnesium, aluminium and carbonate with the general formula
Mg6Al2(OH)16CO3.4H2O (Reichle, 1986b). Natural hydrotalcite was first discovered
in 1848 in Sweden as a white-coloured mineral that can be easily crushed into
powder (Cavani et al., 1991). Hydrotalcite is found naturally as deposits from
ground water or as a weathering product of primary oxides (Frost et al., 2003a
and 2003b). Unlike cationic clays, hydrotalcite is rare in nature, and it is found in
small quantities in a limited number of geographic areas such as Norway and the
Urals area of Russia (Cavani et al., 1991).
Hydrotalcite was discovered at the same time as its two polytypes: pyröaurite and
sjögrenite (Belloto et al., 1996). At the time, they were referred to as ‘mixed
hydroxyl-carbonates of magnesium and iron’, while others referred to them as
‘mixed hydroxides’ (Brindley and Kikkawa, 1979). However, there are some known
naturally occurring minerals with structural compositions similar to that of
hydrotalcite (Frost et al., 2003b) (see Table 1). The only notable difference
between these minerals is in the cationic composition and interlayer anions
(Cavani et al., 1991). The difference between hydrotalcite and manasseite is in the
unit cell: hydrotalcite has a rhombohedral unit cell, whereas manasseite has a
hexagonal unit cell (Belloto et al., 1996).
4
Table 1:
Naturally occurring minerals similar to hydrotalcite (adapted from
Frost et al., 2003b and Auerbach, 2004)
Mineral
Composition
Unit cell
Hydrotalcite
Mg6Al2(OH)16CO3.4H2O
Rhomohedral, 3R
Manasseite
Mg6Al2(OH)16CO3.4H2O
Hexagonal, 2H
Desautelsite
Mg6Mn2(OH)16CO3.4H2O
Rhombohedral, 3R
Pyröaurite
Mg6Fe2(OH)16CO3.4.5H2O
Rhombohedral, 3R
Sjögrenite
Mg6Fe2(OH)16CO3.4.5H2O
Hexagonal, 2H
Stitchtite
Mg6Cr2(OH)16CO3.4H2O
Rhombohedral , 3R
Babertonite
Mg6Cr2(OH)16CO3.4H2O
Hexagonal, 2H
Reveesite
Ni6Fe2(OH)16CO3.4H2O
Rhombohedral, 3R
Takovite
Ni6Al2(OH)16CO3.4H2O
Rhombohedral, 3R
The synthetic hydrotalcite is referred to as layered double hydroxide, abbreviated
to LDH (Reichle, 1986a). The name LDH was derived from an early work by
Feitknecht. The first LDH was prepared in 1942 by Feitknecht using the
coprecipitation method. This was achieved by mixing the dilute metal salts with
the basic solution.
The product was named Doppelschichtstrukturen, meaning
double sheet structure. In his hypothesis, Feitknecht assumed that LDH has a
structure with intercalated hydroxide layers (Cavani et al., 1991). Allmann, Taylor
and co-workers later corrected Feitknecht’s hypothesis based on the study of
single-crystal XRD (Allmann, 1968; Cavani et al., 1991). The study proved that the
cations are found in the layers, while the anions and water molecules are located
in the interlayers (Cavani et al., 1991).
The first patent on LDH was published by Miyata (1970). Following this
publication, Miyata and co-workers (1973, 1975, 1977, 1980 and 1983) prepared a
reasonable amount of LDH containing organic and inorganic anions. The chemical
and physical properties of LDH intercalates were examined for use in catalysis.
Feitknecht’s method was adopted by Reichle (1985 and 1986a) to study the
thermal decomposition of LDH. Later, other preparation methods were discovered
and LDH was prepared with a wide variety of compositions for a wide variety of
5
applications (Meyn et al., 1990; Dimotakis and Pinnavaia, 1990; Newman and
Jones, 1998).
1.4
Problem Statement
Naturally hydrotalcite is found as a mixture of spinnels and/or other minerals such
as muscovite and heavy metals such as lead (Frost et al., 2002). This might be
due to the existence of non-equilibrium conditions during the formation of the
hydrotalcite deposit. Currently, there are no known methods for purifying or
separating natural hydrotalcite. This makes it difficult for these minerals to be
used in the desired applications. An alternative route is to prepare LDH on a
laboratory scale. The main advantage of preparing LDH in this way is that it can
be prepared with the desired combination of divalent and trivalent cations for
specific purposes (Reichle, 1985 and 1986b). There are several methods of
preparing LDH intercalates (Carlino, 1997). The reported methods include
coprecipitation, ion-exchange, rehydration and the thermal melt or melt reaction
method (Williams and O’Hare, 2006).
The reported methods require prolonged periods of synthesis, an inert atmosphere
and high temperatures for calcination. In ion-exchange the common problem is
contamination with the anion from the precursor or the solvent and sometimes
incomplete reactions (Jackrupca and Dutta, 1995). In coprecipitation the
intercalates show M(OH)2 and M(OH)3 mixed phases or precursor contamination
from mixed salts solutions. In some cases amorphous intercalates are reported.
A common problem with all these methods is avoiding CO2 contamination from the
atmosphere. This problem is encountered mainly in the preparation of LDH with
anions other than carbonates (Chibwe and Jones, 1989). Iyi et al. (2004) reported
the decarbonation of Al-rich LDH using NaCl-HCl solutions. The degassed water is
used only for washing the samples. This process is reported to be dependent on
the Mg:Al ratio and the NaCl-HCl concentration.
6
A solution to some of these problems was also devised (Zhang et al., 2004). In
the method the LDH-CO3 was dissolved in an excess of aqueous carboxylic acid
and the required pure product was obtained by precipitation involving addition of
the mixture to a basic solution. Presumably, this approach is limited to
water-soluble acids, e.g. hydroxy-carboxylic acids such as citric, tartaric and malic
acids.
1.5
Aims and Objectives
The modification of layered double hydroxides (LDH) offers many advantages in a
wide variety of applications. In polymers, organo-modified LDH is used to improve
the physical and chemical properties of polymeric materials (Fischer, 2003; Khan
and O’Hare, 2002). The anion-exchange capacity plays a major role in the
preparation of polymer clay nanocomposites (Hibino and Jones, 2001). This is
achieved by modifying the surface polarity of the clay. Clay modification is usually
achieved by intercalating the desired compound in between the layers of the host,
depending on the final properties of the required nanocomposite (Leroux and
Besse, 2001). The interlayer anions can be exchanged with the desired ones. In
the pharmaceutical and medical industries, pharmaceutical drugs and biologically
active compounds are intercalated for purposes such as controlled drug release
(Choy, 2004 and 2007; Ambrogi et al., 2002; Evans and Duan, 2006). LDH has
attracted considerable attention as flame retardants, ion exchangers, catalysts and
catalysts supports, and as PVC stabilisers (Evans and Duan, 2006).
The aim of the present work was to find a simple and economical way to prepare
fatty-acid intercalated LDH using readily available chemicals, without employing an
inert atmosphere and high-temperature calcinations. Fatty acids were used
because of their ability to weaken the interactions between the adjacent clay
layers. This results in the neutralisation of the brucite-like LDH layers. Fatty acids
have the ability to replace the interlayer carbonate with carboxylate anions,
resulting in the formation of a mono- or bilayer intercalated LDH structure through
hydrophobic interactions. In this study water was used as a medium rather than
an organic solvent. Surfactants were used due to their ability to interact with the
7
surface to form micelles, leaving the LDH layers free to capture the desired
carboxylate anions. The surfactants kept the LDH particles in suspension and also
ensured better dispersion.
8
2
2.1
LITERATURE REVIEW
LDH Structure and Composition
The structure of LDH is derived from that of the naturally occurring mineral brucite
(Mg(OH)2). It consists of two-dimensional structural sheets (Cavani et al., 1991).
The structure of brucite consists of Mg2+ ions co-ordinated six-fold to hydroxyl
groups. Similarly, each cation in LDH is surrounded by six hydroxyl groups. These
hydroxides share the edges to form an infinite sheet (Carrado and Kostapapas,
1988). The LDH structure results from a partial replacement of divalent cations
(Mg2+) by trivalent cations (Al3+) in octahedral sites (Hickey et al., 2000). This
substitution results in a net positive charge on the LDH layer. Lopez et al. (1997)
claim that the formation of basic centres in hydrotalcite appears in the bridge
oxygens between two magnesium atoms. These basic centres are due to the
defects in oxygen linked to fully co-ordinated magnesium (Lopez et al., 1997).
The interlayer region of LDH contains anions that neutralise the excess positive
charge on the brucite-like layers. The galleries also contain water molecules
(Cavani et al., 1991; Rajamathi et al., 2001) that are free to move by breaking
and forming new bonds (Cavani et al., 1991). The LDH structure contains different
types of water molecule, i.e. hydrogen bonded to the interlayer anions and
hydrogen bonded to the –OH groups on the surface of the brucite-like layers
(Cavani et al., 1991; Yun and Pinnavaia, 1995; Van der Pol et al., 1994). The ionic
mobility of the anion is dependent on the interlayer water content. The anion
mobility determines the acid base properties and the ion-exchange behaviour of
the LDH (Yun and Pinnavaia, 1995).
The interlayer thickness is dependent on the number, size and strength of the
bonds between the anions and hydroxyl groups (Yun and Pinnavaia, 1995; Cavani
et al., 1991). Dehydrated LDH can absorb water from the surroundings (Hou et
al., 2003). The amount of interlayer water is dependent on the size and nature of
the anion, water vapour pressure and temperature. This results in the formation of
mono-, di- or trilayers (Miyata, 1975; Khan and O’Hare, 2002; Miyata and Kumura,
9
1973). Yun and Pinnavaia (1995) identified two types of water molecule for all
air-dried LDH-CO3 samples. The water due to capillary condensation between LDH
crystallites is called ‘interparticle pore water’. The last layer of water, called
‘adsorbed surface water’, is bonded to the gallery and external surfaces. Kagunya
and co-workers (1996 and 1997) identified two types of water molecule water in
LDH. The first is the intrinsic water, which is the structural water intercalated in
LDH as a monolayer, and the second is the extrinsic water, consisting of water
molecules bound to the external surfaces.
Unlike brucite, LDH is characterised by its three-dimensional structure as a result
of the electrostatic and hydrogen bonds between the layers and the interlayer
region (Cavani et al., 1991; Ogawa and Kaiho, 2002). The general formula is
[M(II)1-xM(II)x(OH)2]x+ An-x/n .mH2O
where
M(II) represents a divalent cation Mg2+, Fe2+, Cd2+, Co2+, Zn2+ or Cu2+
M(III) represents Al3+, Cr3+, Fe3+, or Ga3+
An- is an interlayer anion CO32-, SO42-, NO3- or Clx ranges from 0.2 to 0.33 (Costantino et al., 1998).
The crystal layer structure is shown in Figure 1. The brucite-like sheets are
stacked on top of each other (Cavani et al., 1991). The surface area of LDH is
usually lower than 100 m2/g and the layer thickness is approximately 0.48 nm
(Kanoh et al., 1999; Chibwe and Jones, 1989; Choy et al., 2007). The brucite-like
sheet spacing varies from 0.78 nm to 0.76 nm, depending on the value of x
(Chibwe and Jones, 1989; Carlino and Hudson, 1994).
10
Brucite-like layers
CO32-
CO32-
CO32-
CO32-
CO32-
Divalent cation, Mg2+
CO32-
Trivalent cation, Al3+
Interlayer water
CO32-
Figure 1:
CO32-
CO32-
CO32-
CO32-
Interlayer region
Structure of LDH (adapted from Carlino, 1997)
The di- and trivalent cations are metallic cations with oxidation states of +2 and
+3 respectively. These cations can be exchanged with desired ones for specific
purposes (Carrado and Kostapapas, 1988). The cations with atomic radii similar to
that of Mg2+ and Al3+ can be accommodated in the octahedral sites of brucite-like
layers (Cavani et al., 1991). In the literature, wide varieties of cations in LDH have
been reported (Auerbach et al., 2004). Table 2 shows some of the reported diand trivalent cations and their atomic radii. LDH containing the rare earth metals
Ce3+, Eu3+ has been reported (Chang et al., 2006; Fernandez et al., 1997).
11
Table 2: Atomic radii of some of the reported di- and trivalent metal cations in
LDH brucite-like layers (adapted from Auerbach 2004 and Cavani et al., 1991)
Divalent, M2+
Radius, nm
Trivalent, M3+
Radius, nm
Mg
0.072
Al
0.054
Ni
0.072
Co
0.063
Co
0.065
Ga
0.062
Zn
0.074
In
0.081
Fe
0.061
Y
0.090
Mn
0.083
V
0.074
Cu
0.073
La
0.103
Cd
0.097
Ti
0.076
Be
0.030
Cr
0.069
In the literature, LDH with tetravalent cations in place of trivalent cations has been
reported. These include Zn-Si-LDH and Zn-Sn-LDH (Saber and Tagaya, 2003a,
2003b and 2007). The other reported tetravalent cations are V4+, Ti4+, Zr4+
(Intissar et al., 2003). However, the replacement of trivalent cations by ones with
radii that differ from that of Mg2+ and Al3+ may result in a different final product,
i.e. not LDH. Das et al. (2004) tried to replace Al3+ with Zr4+ in Zn3-Al, Zn2-Al and
Mg3-Al-LDH. This resulted in the formation of Zn hydroxycarbonates and Zn-Al-Zr
mixed oxides. The replacement of trivalent cations with tetravalent cations results
in LDH with lower basal spacing and a decrease in the layer thickness compared
with Mg-Al-LDH (Muramatsu et al., 2007; Saber and Takagi, 2007). In some cases
the trivalent cation has also been replaced by the hexavalent cation, Mo6+, in
Zn/Mo-LDH with Mo:Zn atomic ratios of 3:7, 2:8 and 1:9 (Muramatsu et al.,
2007).
LDH containing three different cations on the layers has been reported and this
includes di-, tri- and tetravalent cations, i.e. Zn-Al-Sn-LDH, Cd-Al-Fe-LDH and MgZn-Al, Cd-Al-Fe-LDH, Ni-Al-Cr and Ni-Al-Fe (Saber and Tagaya, 2003b; PerezRamirez et al., 2007; Kooli et al., 1995).
12
The charge density and stability of LDH layers is dependent on the M(II)/M(III)
ratio (Boclair and Braterman, 1999a and 1999b). LDH with a variety of
M(II)/M(III) ratios has been reported (Itoh et al., 2003). The exchange capacity is
controlled by controlling this ratio (Newman and Jones, 1998; Leroux et al., 2003;
Reichle, 1986b).
Although in nature hydrotalcite is found containing carbonates, in practice, a
variety of charge-balancing anions may be incorporated in between the LDH
layers. The anions may vary in geometry, size and charge, resulting in a large
class of isostructural materials with different physicochemical properties (Evans
and Duan, 2006). Some of the reported anions are listed below.
•
Organic anions: Carboxylates, acetate, sebacate, carprate, laurate, palmitate,
myristate, stearate and oleate oxalate, citrate, tartarate, succinate, adipate and
malate, pamoate and aliphatic α, ω-dicarboxylic acids (Kandare and
Hossenlopp, 2006; Prevot et al., 1998; Kanoh et al., 1999; Miyata and Kumura,
1973).
•
Inorganic anions: Halides, (X-), CO32-, NO3-, OH-, SO42-, Cl-, I-, Br-; MnO4-,
CrO42-, Cr2O72-, ClO4- (Bontchev et al., 2003; Choudhary et al., 2004; Malherbe
and Besse, 2000).
•
Surfactants: Sodium dodecylsulphate (SDS), sodium dodecylbenzenesulphate
(SDBS), sodium octylsulphate (SOS), sodium octylbenzenesulphonate (SOBS),
dodecyl glycol ether sulphate, sodium tetradecyl sulphate, octanesulphonic acid
sodium salt, octylbenzene sulphonate acid sodium salt, octyldodecylsulphonic
acid sodium salt and sodium dodecylbenzene sulphonic acid salt (Costa et al.,
2008; Meyn et al., 1993; Trujillano, 2006).
•
Antibiotic and pharmaceutical drugs: Gramidicin, amphoterin B, ampicillin,
nalidixic (Trikeriotis and Chanotakis, 2007), phenylphosphonic acid (Carlino et
al., 1996), dichlorophenac (Dupin et al., 2004) and L-ascorbic acid (Aisawa et
al., 2007).
•
Biochemical anions: Various amino acids (Fudala et al., 1999), DNA and ATP
(Choy et al., 1999).
13
•
Complex and polymeric anions: CoCl42-, NiCl42-, Fe(CN)42-, Fe(CN)63-,
Ru(CN)6-, [P2O7]4- , [V2O7]4-, (Malherbe and Besse, 2000), nitrilotriacetate
(NTA) (Kaneyoshi and Jones, 1998) poly (ethylene) (Leroux and Besse, 2001)
and inorganic-organic pigment – azo dye methyl orange (Costantino et al.,
1999).
The basal spacing is dependent on the size and nature of the interlayer anions
(Bar-on and Nadiv, 1988). In carboxylate anions, the basal spacing is mostly
dependent on the length of the chain (Carlino, 1997). The anion orientation is
dependent on the anion concentration and the reaction temperature (Auerbach et
al., 2004, p 179). The ionic radii of the anions determine the thickness of the
brucite-like layers (Miyata, 1980). The amount of the adsorbed anions is
determined by the ratio of divalent to trivalent layer cations (Hansen and Taylor,
1991). Mg-Al-LDH is stable in the pH range of 3 to 10 and its capacity is about 220
meq/100 g (Miyata and Kumura, 1973). The order of LDH anion preference is
known and is as follows (Miyata, 1983; Auerbach et al., 2004):
NO3-<Br-<Cl-<F-<OH-<MoO42-<SO42-<CrO42-<HAsO42-<HPO42-<naphthol yellow <CO32-
Weak anions are preferred for exchange reactions because they can be easily
replaced (Miyata, 1983). LDH has a strong electrostatic interaction with the
divalent anions due to their high anion charge density (Leroux and Besse, 2001).
However, Kooli et al. (1996) claim that anions with smaller radii are more strongly
bonded than those with larger radii. The only limitation is that the desired anion
must not form complexes with the layer cations.
The crystallinity and the textual and structural properties of LDH are highly
dependent on the nature of the interlayer anions, on the preparation route and on
the conditions (Bar-on and Nadiv, 1988; Costantino et al., 1998). In ion-exchange
reactions, replacement of anions with large inorganic or organic anions may lead
to the formation of pillared materials (Dimotakis and Pinnavaia, 1990). The
solubility is greatly affected by the nature of the exchangeable species, i.e. the
14
layer cations and anions. The smaller the crystal size, the higher the solubility of
the resulting LDH material (Choy et al., 2007; Costantino et al., 1998).
2.2
Intercalation
The intercalation reaction can be defined as a reaction in which an ion or molecule
is inserted in between the layers of the crystal lattice, leaving the basic structure
unchanged (Williams and O’Hare, 2006). The insertion process is reversible (Khan
and O’Hare, 2002; Chibwe and Jones, 1999). After intercalation, this clay can later
be incorporated into polymers to improve the chemical and physical properties of
the materials, i.e. tensile strength, thermal stability, and optical and magnetic
properties, and also to change the surface properties of the host from hydrophobic
to hydrophilic (Khan and O’Hare, 2002; Adachi-Pagano et al., 2000).
It is necessary for a host material to have a charged layered structure (Khan and
O’Hare, 2002). The intercalation compounds are formed when the mobile guest
species comes into contact with the host lattice (Evans and Duan, 2006; Morioka
et al., 1995). Layered hosts adapt to the geometry of the inserted guest species
by adjusting the interlayer separation, resulting in increased basal spacing. The
intercalation process may involve ion exchange. In LDH, if the anionic guest
contains long aliphatic chains, the anions may self-assemble to form a mono- or
bilayer structure due to hydrophobic interactions between the layers and the
anions (Khan and O’Hare, 2002).
Intercalation reactions involve an electrostatic interaction between the guest
anionic charge and the cationic sites in LDH layers (Morioka et al., 1995). Only
anion guest species can be intercalated between the brucite-like layers of LDH.
The amount of anionic species and water molecules intercalated depends on the
charge density of the LDH layers (Adachi-Pagano et al., 2000). The higher the
layer charge density, the higher the content of intercalated anions and water
molecules. This results in a strong interaction between the layers, leading to a
tight stacking of the sheet (Adachi-Pagano et al., 2003). The high layer charge
15
density of LDH makes exfoliation difficult. However, exfoliation of LDH has been
reported (Leroux and Besse, 2003; Evans and Duan, 2006).
2.3
Review of the Intercalation Methods
LDH is characterised by a smooth, flexible structure which exhibits excellent
chemical and physical properties (Newman and Jones, 1998). The final properties
of LDH are dependent on the method of preparation employed. Highly crystalline
materials can be produced by optimising experimental parameters such as pH,
time and reaction temperature (Reichle, 1986b). The particle size, surface area
and morphology are highly affected by these parameters. Ageing plays a major
role in determining the textural properties of the final material (Costantino et al.,
1999). Carlino (1997) and Newman and Jones (1998) reviewed the methods for
preparing organic anion intercalated LDH. Carlino (1997) identified five methods
that can be used to intercalate carboxylate anions in LDH. The methods are:
1. ion exchange
2. coprecipitation
3. rehydration
4. thermal melt
5. glycerol effect.
However, only coprecipitation and rehydration were found to be effective since
they gave single-phase products on powder X-ray diffraction (PXRD).
The indirect method employs a suitable LDH precursor prepared by direct
synthesis. Crepaldi et al. (1999) identified three indirect techniques for preparing
intercalated LDH: 1) direct ion exchange; 2) LDH reconstruction from the layered
double oxide (LDO) form obtained by calcinations of a suitable precursor; and
3) anion replacement by elimination of the precursor interlamellar species.
2.3.1
Direct ion exchange
In the direct ion-exchange method the guest anions are intercalated by dispersing
the LDH in an aqueous solution containing an excess of the desired anion. The
16
pre-existing interlayer anion is partially replaced when the guest anion diffuses
through the LDH matrix. The exchange reaction is carried out under an inert
atmosphere to avoid the incorporation of carbonate from the atmosphere (Cavani
et al., 1991). The guest anion and LDH layers must be stable at the pH of
exchange (Miyata and Kumura, 1973). A higher layer charge density increases the
exchange capacity. Monovalent anions like Cl- are usually preferred in exchange
reactions because they can be easily exchanged. Carboxylic acid anions can be
intercalated by shaking the LDH in a solution of the desired carboxylic acid or its
salts (Carlino, 1997).
Borja and Dutta (1992) and Dutta and Robins (1994) successfully exchanged
LDH-Cl- with laurate, myristate and palmitate anions into Mg3Al-LDH and
Li3Al-LDH. This was achieved by shaking the LDH-Cl in ethanolic solutions in the
presence of the carboxylic acids, C12 to C16. However, the ethanol was
incorporated along with the carboxylic acids. This was confirmed with
thermogravimetry (TG) by 30% mass loss upon heating to 90 °C. Jackrupca and
Dutta (1995) also confirmed it with myristate-exchanged LiAl-LDH. Apart from
ethanol contamination, the LDH intercalates were also contaminated with the
chloride anions from the precursor, confirmed by X-ray fluorescence spectroscopy
(XRF).
Miyata and Kumura (1973) reported an exchange reaction of anions in Zn3-Al-LDH
with a series of α, ω dicarboxylic acids. Itoh et al. (2003) exchanged the Cl- anions
in LDH-Cl with sodium aliphatic carboxylate salts from C16 to C26 by treatment
with an aqueous solution of carboxylates under a nitrogen atmosphere. A similar
procedure was used by Kanoh et al. (1999) who exchanged the chloride anions
with stearate anions from sodium stearate and also C8 to C10. The intercalation
was unsuccessful for C8 to C10 and this was attributed to the hydrophobicity of
these carboxylates at lower temperatures. Carboxylate anions were intercalated as
bilayers. However, the LDH intercalates were contaminated with the chloride anion
from the precursor.
17
The exchange of interlayer nitrate anions with sodium fatty acid salts in water was
reported by Meyn et al. (1990). Tartarate and citrate anions were found not to
react easily with the LDH. Anbarasan et al. (2008) described synthetic methods
involving the dispersion of LDH-CO3 in water in the presence of dodedecane-1-12diol, dodecanedioic acid, stearic acid and heptadecanoic acid. The reaction was
carried out at 70 °C with vigorous stirring under a nitrogen atmosphere for
48 hours. Intercalation was successful and the article claims that the surfactants
interacted with the LDH through ionic bonding.
Saber and Tagaya (2003 a, 2003 b, 2007) exchanged the carbonate anion in Zn2+
and Si4+ in LDH with mono-(n-caprate, myristate and stearate), dicarboxylate
(sebacate, suberate and dodecanoate) and aromatic 4-chlorophthalic acid. The
carbonate
Zn/Si-LDH
and
Zn/Sn-LDH
samples
were
first
prepared
by
coprecipitation, followed by exchange reactions in carboxylic acid sodium salts.
A stearate-intercalated LDH containing the di-, tri-, and tetravalent cations Zn-AlSn-LDH was also prepared (Saber and Tagaya, 2003a, 2003b). The reactions were
carried out in an argon atmosphere. Although the intercalation reactions were
successful, the materials were found to be contaminated with carbonate anions.
A similar procedure was followed to prepare n-caprate and suberate-intercalated
Zn2+ and Mo6+ containing LDH (Muramatsu et al., 2007). The carbonate impurity
was also retained in the final material. Broad diffraction peaks were obtained,
indicating poor crystallinity.
Prevot et al. (1998) reported the exchange of Cl- anions by dicarboxylates,
tartarate
and
succinate.
The
LDH
intercalates
were
contaminated
with
atmospheric carbonate anions, as evident from a low intense diffraction peak at
11.4° on XRD. In the exchange of NO3- anions in LDH-NO3 with alkylsulphate
derivatives, the samples retained the nitrate anions from the precursor (Meyn et
al., 1993). In the exchange of Cl- anions with the anionic surfactants SDS, SDBS,
SOS and SOBS, poorly crystalline materials were obtained (You et al., 2002). The
exchange of surfactants was also reported (Meyn et al., 1993).
18
The exchange of nitrate and chloride anions by anionic nitrilotriacetate (NTA)
complexes [M(NTA]- (where M is Cu2+, Ni2+ for Zn/Cr LDH) has been reported
(Gutmann et al., 2000). The NO3- and Cl- anions were found to compete for the
interlayer sites in LDH for the [Ni2+(NTA)]- complex. The exchange of chloride
anions for the platinum complexes MgAlPt, ZnAlPt and CuAlPt in LDH has also
been reported (Beaudot et al., 2001). In both cases the LDH intercalates were
contaminated with anions from the precursor. The exchange of Cl- with a series of
complexes has been reported (Malherbe and Besse, 2000). Newman and Jones
(1998) reported anion-exchange reactions of layered Zn, Cu, Ni, or La hydroxide
nitrates with different aqueous media and the organic anions acetate,
terephthalate and benzoate.
Anion exchange of NO3- in M-Al4-NO3-LDH (M = Zn, Cu, Ni, and Co) with a series
of dicarboxylates, mono- and disulphonates was reported by Williams and O’Hare
(2006). LDH was reacted with excess salt at 60 °C for 24 hours. The LDH
intercalates were then dried for 3 hours under vacuum. The authors claim that
with dicarboxylate the reactions were completed in less than 2 minutes. Bontchev
et al. (2003) studied the monovalent anion-exchange preferences of LDH in
comparison with the hydrothermal and rehydration methods. However, incomplete
ion-exchange reactions were obtained. The pH of the synthesis solution for
anion-exchange reactions was reported by Kukkadapu et al. (1997).
Prevot et al. (1999) described an exchange reaction method involving the
extraction, dissolution and reconstruction. The Al3+ cations were first extracted by
oxalate anions to produce aluminium oxalate complex. The Cl- anions in
ZnAl-LDH-Cl are exchanged with oxalate anions simultaneously when the LDH is
being reconstructed at 250 °C. Crepaldi et al. (1999) described an exchange
reaction method involving salt formation between anionic and cationic surfactants.
This salt surfactant is later removed to an organic phase. Dimotakis and Pinnavaia
(1990) described an exchange reaction using a swelling agent. The exchange of
hydroxides is claimed to be easier with this method.
19
2.3.2
Rehydration
The rehydration method also referred to as ‘reconstruction’ is based on the
so-called ‘memory effect’. Miyata (1975) and Leroux and Taviot-Guého (2005)
define the memory effect as the ability of mixed oxides (LDO) obtained by thermal
decomposition of LDH to reconstruct their original layered structure in aqueous
media. The mixed oxides (LDO) formed feature interesting properties such as high
surface area, small crystal sizes and stability to thermal treatments (Rocha et al.,
1999). High temperature causes solid state diffusion between the cations while
the crystalline structure is destroyed. The hydroxyl groups and the interlayer
carbonate anions are lost and the LDH lamellar structure collapses. During this
process the divalent cations migrate to tetrahedral sites, resulting in the formation
of the mixed oxides MgO and MgAl2O4 (Rocha et al., 1999; Labajos et al., 1992).
The method involves hydrothermal reconstruction of LDH from LDO in the
presence of the desired anion (Kaneyoshi and Jones, 1998; Miyata and Okada,
1997; Crepaldi et al., 2002; Chibwe and Jones, 1989). The reaction is carried out
under an inert atmosphere to avoid carbonate contamination and calcination is
performed in the temperature range 400–500 °C (Cavani et al., 1991). The
calcination process is reported to be reversible, provided the heating does not
exceed 600 °C (Reichle, 1986a). However, when applied to organic anions it is
difficult to avoid carbonate contamination and in some instances mixed phases
may be produced (Dimotakis and Pinnavaia, 1990). During the LDH calcination
process, water evaporates while interlayer carbonate anions are released as gas
(Rocha et al., 1999; Costantino et al., 2007).
Dimotakis and Pinnavaia (1990) described the rehydration method in the presence
of the swelling agent glycerol. The reconstruction was carried out in a
water-glycerol solution, followed by reaction with the carboxylic acid. The absence
of glycerol resulted in the mixed phases observed on XRD. Therefore, the swelling
agent facilitated carboxylic acid intercalation. Greenwell et al. (2007) described the
synthesis of dicarboxylate anion (adipate, succinate, malonate and glutarate)
20
intercalated LDH using the co-hydration route. The method consisted of stirring
the MgO and Al2O3 in distilled water at 60 °C, followed by addition of excess
dicarboxylic acid. Broad diffraction peaks and the carbonate impurity were
observed on XRD and FT-IR.
Morioka et al. (1995) reconstructed LDH by treatment with water under a nitrogen
atmosphere (see Figure 2). The LDH-CO3 was calcined and reconstructed to
LDH-OH by treatment with water. Water-treated samples were dried in vacuo,
followed by the reaction with acid chlorides in acetonitrile. The authors claim that
the LDH intercalates obtained were esterified. The reconstruction method was
used to intercalate dicarboxylic acid, tartarate and succinate (Prevot et al., 1998).
No reconstruction was observed for tartarate anions. The LDH intercalates were
also contaminated with carbonate, as evidenced by FT-IR.
+ + + + +
+ + + + +
+ + + + +
CO32-;
H2O
+ + + + +
H2O
Calcination
OH OH OH
+ + + + +
400 - 450 οC
OH OH
Mixed oxides
OH
+ + + + +
Ion exchange
+ + + + +
Anionic guest
H2O
+ + + + +
+ + + + +
Figure 2: Schematic presentation of the reconstruction method followed to
produce LDH intercalated material (adapted from Morioka et al., 1995)
21
Chibwe and Jones (1989) used this method to prepare MgAl-LDH intercalated with
sebacic acid, dodecyl sulphate and potassium salts. The LDH was calcined in air at
450 °C for 18 hours. Poorly crystalline LDH intercalates contaminated with
carbonate were obtained. Sako and Okuwaki (1991) reported using the
reconstruction method for the synthesis of benzene carboxylate intercalated LDH.
Stearate and oleate intercalated LDH were reported using reconstruction under
hydrothermal conditions (Inomata and Ogawa, 2006). Incomplete intercalation of
the oleate anions was observed at lower temperatures. Fogg et al. (1998) used
the reconstruction method followed by anion exchange of Cl- in Li-Al2-LDH-Cl with
a series of sodium dicarboxylate anions. LDH reconstruction in the presence of the
anionic surfactants SDS and SDBS was reported by Costa et al. (2008). The
atmospheric carbonate anion was incorporated along with the surfactants. This
was indicated by FT-IR spectroscopy.
Del-Arco et al. (2003) attempted the reconstruction of Mg2-Al-LDH in the presence
of chromium oxalate complexes, but the method did not lead to the desired
product. Aisawa et al. (2006) reported intercalation of L-ascorbic acid in Mg3-Al,
Mg3-Fe and Zn3-Al-LDHs. In comparison with coprecipitaion and ion exchange,
intercalation barely occurred with coprecipitation and rehydration.
2.3.3
Direct synthesis by coprecipitation
Coprecipitation is one of the methods commonly used for the preparation of LDH
intercalated materials. The metal (MII) and M(III) salts are mixed together and
added to a solution of a base containing the desired anion (Carlino, 1997).
Coprecipitation is achieved at either constant or increasing pH, depending on the
conditions applied (Reichle, 1986b). The reaction is carried out under conditions of
super-saturation.
It is necessary to precipitate at a pH higher than or equal to the one at which the
LDH structure is more stable to avoid the formation of M(OH)2 or M(OH)3
impurities (Cavani et al., 1991). The general coprecipitation reaction adapted from
22
Costantino et al. (2007) is shown in Scheme 1. The morphology and particle size
depends on the conditions of super-saturation (Aramendia et al., 2002). To
prepare the intercalated LDH, the guest anion must have a high affinity for the
brucite-like layers, otherwise the LDH intercalate may be contaminated with
counter-anions from the metal salts. Hydrothermal treatment is usually performed
to
improve
the
crystallinity.
The
method
is
divided
into
two
parts,
i.e. coprecipitation at low and high super-saturations.
NaOH
MII(salt) + MIII(salt) + Guest (anion solution)
MIIMIII-(Guest)-LDH
pH = 10
Scheme 1: Schematic presentation of the coprecipitaion method (from Costantino
et al., 2007)
Coprecipitation at low super-saturation
At low super-saturation the reaction is carried out by slow addition of M(II) and
M(III) salts in an excess solution of the desired anion (Auerbach, 2004). The ratio
of M(II) to M(III) must be known to prevent the formation of impurity phases. The
pH is maintained between 7 and 10 by addition of basic solution, in the
temperature range of 333 to 353 K, and low concentrations of the reagents.
Washing is carried out with warm water at temperatures not exceeding 393 K
(Cavani et al., 1991). Aramendia et al. (1999) prepared LDH with the cation
combinations Mg3-Al and Mg3-Ga using this method. The metal nitrate salts were
employed as the starting materials, followed by anion exchange.
This method was employed for the intercalation of a series of dicarboxylic acids,
succinic, adipic, subaric, sebacic and dodecanedionic acid, as well as Cl-, CO32-,
NO3 and SO42- intercalated in MnAl, and ZnAL-LDH tartarate and succinate in Zn3Al
and Zn2Cr-LDH from the carboxylate sodium salts (Prevot et al., 1998). Poorly
crystalline LDH intercalates contaminated with anions from the precursor were
obtained. The researchers claim that the orientation change of dicarboxylic acid is
23
influenced by the length of the carboxylic acid chain. Acetate was also intercalated
in Zn/Ni-LDH, and terephthalate and benzoate in Mg-Al-LDH (Kandare and
Hossenlopp, 2006; Newman and Jones, 1998). Kooli et al. (1996) claimed that the
terephthalate anion intercalation follows an indirect reaction.
Zhang et al. (2004) described a coprecipitation method for the preparation of
citrate, oxalate, tartarate and malate pillared LDH intercalates. The method
involved the dissolution of a suspension of LDH-CO3 by addition of the desired
amount of carboxylic acid. The acid-LDH mixture was re-precipitated by addition
of an aqueous solution of NaOH. The method works well for the intercalation of
dicarboxylate anions in LDH. Intercalation of acetate in Co-Al, Ni-Al and Mg-Al-LDH
has been reported (Kelkar and Schutz, 1997). This method involved the
peptisation of aluminium in the presence of acetic acid. Poorly crystalline materials
were obtained.
Coprecipitation at high super-saturation
In this method mixed di- and trivalent salt solutions are added to a basic solution
of the desired carboxylic acid (Reichle, 1986b). At high super-saturation, poorly
crystalline materials are produced. The continuous change of pH causes the
formation of the amorphous phases M(OH)2 and M(OH)3 (Adachi-Pagano et al.,
2003). The method is commonly used for the synthesis of pillared LDH
intercalates, mostly for applications in catalysis (Reichle, 1986b; Costantino and
Pinnavaia, 1995). Dredzon (1988) synthesised pillared terephthalate LDH for
application in catalysis. Carlino and Hudson (1994) employed this method in the
synthesis of caprate pillared LDH intercalates. Bilayer caprate intercalated LDH
was obtained. Indole-2-carboxylate intercalated with Zn/Al-LDH has also been
reported (Hussein and Long, 2004).
Costantino et al. (1998) described a method involving the addition of urea for the
preparation of hydrotalcite. The method involves the decomposition of urea in a
basic solution. Urea is added for better pH control and to allow homogenous
precipitation as the pH increases. This leads to the formation of the fewer, well
24
crystallized particles. Rao and coworkers (2005) also used this method under
hydrothermal conditions. No repetitive washings were required. Costa et al. (2008)
produced highly crystalline Mg/Al-LDH with this method. The method is mostly
used to prepare mono-dispersed LDH particles as a result of urea decomposition.
It is good for the preparation of catalysts (Adachi-Pagano et al., 2003).
Zhao et al. (2002) described a coprecipitation method involving separate
nucleation and aging. The nucleation process is carried out by rapid mixing of the
precursors in a colloid mill, followed by ageing. The crystallite is formed during
nucleation and during ageing it undergoes growth, breakage, agglomeration
and/or Ostwald ripening. Materials with higher crystallinity and high aspect ratios,
and smaller crystallites with narrow size distribution are obtained (Zhao et al.,
2002). The method was recently employed for the preparation of LDH with
different interlayer anions, divalent and trivalent cations (Feng et al., 2006).
2.3.4
Thermal melt/reaction method
This method is used to prepare organo-intercalated LDH. The thermal reaction
method, also referred to as the ‘melt reaction’ method, involves reacting molten
acid with LDH. Carboxylic acid intercalation is achieved by heating the LDH-acid
mixture slowly at a heating rate of less than 1 °C/min, followed by cooling at a
rate of 10 °C/min. The method was first reported for the synthesis of sebacate
intercalated LDH (Carlino and Hudson, 1994). Capric acid intercalated Mg2-Al-LDH
has also been reported (Carlino and Hudson, 1995). The LDH intercalate that was
obtained contained an unreacted Mg-A-LDH-CO3 phase, as evident from PXRD.
2.3.5
Sol-gel
A sol-gel can be defined as a process through which a product is formed by means
of the gradual change of liquid involving the conversion of molecules in a sol
(colloidal suspension of a solid in a liquid) to a gel. This method was used for the
synthesis of LDH (Baron et al., 2001). Two metal alkoxide solutions are mixed
together to form a gel, followed by thermal treatment. The method involves
hydrolysis of an organic precursor, achieved by addition of a strong acid such as
25
HCl. For example, if HCl is used for hydrolysis, acid addition results in LDH-Cl.
Precipitation of the LDH phase occurs when working at a suitable pH. The method
was used to study the thermal stability of LDH with a series of aluminium anions
(Ramos et al., 1997). The crystallinity depends on the nature of the precursors
and the hydrolysis acid used (Prinetto et al., 2000).
He et al. (2004) reported the synthesis of LDH-CO3 using a water-in-oil emulsion
solution. Octane, water and surfactant were mixed together at constant and
variable pH. This method leads to a mesoporous LDH material.
2.4
Characterisation of LDHs
Various analytical techniques
have been employed to
characterise LDH
intercalated materials. Some of the most commonly used analytical techniques are
discussed in Sections 2.7.1 to 2.7.4 below. These include Fourier-transform
infrared
spectroscopy
(FT-IR),
powder
X-ray
diffraction
(PXRD)
and
thermo-gravimetric techniques (TG/DTG).
2.4.1
FT-IR
FT-IR is one of the molecular vibrational spectroscopic techniques used for both
quantitative and qualitative analysis (Pungor, 1995, p74). The infrared region is
the region found in the wave number range 1.3 x 104 to 3.3 x 101 in the
electromagnetic spectrum. This region is between the microwave and UV-visible
light absorption spectra (Skoog et al., 1996, p 502). FT-IR is used to investigate
the structural bonding and chemical properties of compounds (Madejová, 2003).
When a molecule absorbs radiation, the bonds stretch, vibrate or bend (Socrates,
1980, pp 1-3). Each molecule absorbs a specific IR radiation at a different
frequencies – this is referred to as the ‘molecular fingerprint’. Therefore each
functional group has its own frequency and this is useful for revealing the
presence or absence of these groups from the spectrum.
In this study samples are prepared using the KBr pellet-pressing method
(Madejová, 2003). The pellet is prepared by crushing a small amount of sample
26
(2–5 mg) with 100 mg of dried KBr powder. The mixture is then pressed in a die
under a specified pressure. KBr is used because it features a simple spectrum with
no water or intense peaks (Sibilia, 1988, pp 13-19). The reference spectrum is
then subtracted from the sample (Madejová, 2003).
In pure clays –OH absorptions can be detected (3 500-4 000 cm-1) and the
interlayer carbonate in the case of LDH-CO3 (Labajos et al., 1992; Williams and
O’Hare, 2006). The inorganic lattice vibrations of the M-O and M-OH modes can
be confirmed below 1000 cm-1 (Williams and O’Hare, 2006). In organo-LDHs the
presence of the intercalated anionic species can be confirmed, e.g. in carboxylate
intercalated LDH, the anion is identified by a strong asymmetric and symmetric
stretching band in the region 1560 – 1400 cm-1 (Carlino, 1997). The undissociated
form will be confirmed by the carbonyl stretch in the region 1725 – 1700 cm-1
(Borja and Dutta, 1992; Newman and Jones, 1998; Carlino and Hudson, 1994).
The purity of the materials can also be confirmed. The carbonate impurity is
observed by the band at 1360 cm-1 and other anions, such as nitrates, at
1360 cm-1 (Williams and O’Hare, 2006).
2.4.2
TG/DTG
TG is the analytical technique used to study the changes in thermal properties, i.e.
the sample weight of the material as a function of temperature (Sibilia, 1988, p
205). The properties of the material are monitored under specified atmospheric
conditions while the sample is subjected to a controlled temperature programme
(Charles, 1988, p 1). Properties such as thermal stability, decomposition and the
composition of the materials can be studied.
TG/DTG is mostly used to study the thermal decomposition and stability of the
LDH intercalated materials (Williams and O’Hare, 2006). LDH contains different
water molecules and an increase in temperature causes water to be released in
the form of vapour. TG indicates the temperature at which this event takes place
(Van de Pol et al., 1994). Differential TG (DTG) enhances these events by showing
the decomposition peaks in the range observed on TG (Sibilia, 1988, pp 206-207).
27
LDH intercalates decompose in three stages, namely loss of interlayer water
(referred to as ‘dehydroxylation’) at 300 °C, dehydroxylation of brucite-like layers
in the range 300-500 °C, and loss of interlayer anion (Reichle, 1986b; Williams
and O’Hare, 2006). The temperature at which the interlayer anion is lost depends
on the nature of the intercalated anion. The decomposition of LDH results in the
formation of thermally stable mixed oxides that are mostly used in catalysis
(Reichle, 1986b). The amount of intercalated anion can be estimated from the TG
data.
2.4.3
PXRD
PXRD is one of the most powerful techniques for investigating the composition,
purity and structural orientation of a material. The technique involves directing
X-rays to the crystals. The radiation will be either reflected or diffracted at
different angles. The interaction of X-rays with crystals results in the formation of
secondary diffracted beams, as shown in Figure 3. The relationship between
diffracted X-rays and the interplanar spacing is given by Bragg’s law:
nλ = 2dhkl sin θ
(1)
where
n = diffraction order
λ = wavelength of the X-ray beam
d = interplanar spacing
θ = diffraction angle
hkl = diffraction or Miller indices of the plane where a, b and c are the axes.
XRD data are recorded as a plot of 2θ against the intensity (Sibilia, 1988, p 115;
Pungor, 1995, p 151).
28
2
d
1
B
C
θ
A
Figure 3:
Atomic plane
Diffraction of X-rays on crystal lattice according to Bragg
PXRD is used to study the purity of the LDH intercalated material. The phase
purity of the material is determined by the sharpness and/or the broadness of the
diffraction peaks. The broader reflections correspond to the amorphous phase,
while the sharper reflections correspond to the crystalline phase. From the
diffraction data the d-spacing of the intercalated LDH material can be determined.
The reflection with the greatest d-spacing corresponds to the d-spacing of the
intercalated LDH (Carlino, 1997).
In LDH the d-spacing depends on the size and orientation of the intercalated anion
while n is the stacking sequence of the brucite-like layers (Williams and O’Hare,
2006). Carlino (1997) described three
major reflections for carboxylate
intercalated LDH, i.e. the greatest basal spacing, d003, the half-height harmonic,
d006, which is equivalent to half d003, and the d009 reflection, which is equivalent to
one third of d003. The rest are referred to as lesser reflections. The long-chain
carboxylic acid guest can be intercalated to give mono- or bilayer structures. The
d-spacing of a mono- and bilayer intercalated LDH structure can then be
expressed by the following correction to the correlation equation presented by
Carlino (1997) and Meyn et al. (1990):
Monolayer: dL = d0 + 1.27 n cos α + d1
(2)
Bilayer:
(3)
dL = d0 + 2.54 n cos α + d2
where
29
n = number of carbon atoms in the carboxylic acid
dL = observed basal spacing from XRD data
d0 = distance between the terminal ionised carboxyl group and the centre of the
Mg-Al-(OH)x layer
d1 = distance between the terminal methyl group and the centre of the Mg-Al(OH)x layer
α = slant angle of the carboxylate chain from the normal to the LDH layer plane.
The interlayer height can be expressed by the following equation (Anbarasan et
al., 2005):
L = d-spacing – layer thickness
(4)
The bond lengths of metal-bonded oxygen can also be calculated from the PXRD
data. Belloto et al. (1996) determined Al-O and Mg-O bond lengths in CO3-LDH.
The Mg-O bond length was found to be higher at 0.211 nm than the Al-O bond
length at 0.190 nm.
2.4.4
Other characterisation techniques
Other techniques include scanning electron microscopy (SEM), differential
scanning calorimetry (DSC) and X-ray fluorescence spectroscopy (XRF). SEM is
used to study the surface topography of LDH intercalates – the particle
morphology, porosity and phase composition within the material can be revealed.
DSC, like TG, is also used to check the chemical and physical properties of a
material corresponding to the temperature changes. It is used to detect the
melting points (enthalpy of melting) and phase transitions of materials. This
technique has not been explored very much in the study of LDH intercalates. XRF
is employed for trace element analysis and is also used for the determination of
the divalent to trivalent cation ratios.
30
2.5
Carboxylic/Fatty Acids
Carboxylic acids are characterised by the presence of the –COOH functional group.
The suffix –oic is added to the end of the radical name (Gunstone, 1996). In linear
fatty acids, the functional group is located at the end of the carbon chain.
Carboxylic acids containing an even number of carbons from 16 to 36 are referred
to as ‘fatty acids’ (MacMurry, 2000). The carboxylic acid anion is referred to as
‘carboxylate’ (Carlino, 1997). There are two types of fatty acid, namely saturated
and unsaturated. The saturated fatty acids have long chains without double bonds
or other functional groups along the chain. In unsaturated fatty acids, the chain
consists of double or triple bonds and/or other functional groups along the chain
(Markley, 1947).
Some fatty acids are found in ester and or sterol forms (Markely, 1947). In nature,
these acids are found in plant or vegetable oils and animal fats. Typical examples
are fish oil and cotton seed oil, which consist of 10-30% and 15-30% palmitic acid
respectively, and fat from cow’s milk, which contains 4% butyric acid (MacMurry,
2000; Gunstone, 1996; Markley, 1947). The short-chain fatty acids are retained in
products containing milk fat (Gunstone, 1996). Fatty acids with carbon numbers
greater than 18 are present in seed oils (Markley, 1947).
The fatty acids are extracted from natural resources by hydrolysis in aqueous
NaOH, resulting in glycerol and fatty acid. (see Scheme 2 adapted from MacMurry
(2000). Unsaturated fatty acids also exist in nature, e.g. 4-5 decanoic acid
(Gunstone, 1996).
31
O
CH2O
C
O
CHO
CH2O
CH2O H
R
1. - OH
C
O
R
C
R
2. H3O
Fat or oil
CHO H
RCOOH
+
R'COOH
+
CH2O H
R''COOH
Glycerol
Fatty acids
Scheme 2: Schematic presentation of the hydrolysis of fat or oil in aqueous
NaOH, yielding glycerol and three fatty acids, where R, R’, and R’’ = C11 – C19
(adapted from MacMurry, 2000)
Fatty acids exist in both the solid and liquid state. Short-chain carboxylic acids are
usually liquids with low melting points and are isolated from plant or animal fat by
carbonylation (Markley, 1947). The melting point and molecular mass of fatty
acids increases with an increase in the chain length. Carboxylic acids have the
ability to form hydrogen bonds with each other (MacMurry, 2000). The O-H bond
is weak, which results in less stable molecules (Gunstone, 1996). Carboxylic/fatty
acids are proton donors and dissociate into RCOO- and H+ in aqueous solutions
(MacMurry, 2000). Fatty acid salts are amphiphillic molecules produced by the
reaction between carboxylic acid and a base. The functional group (carboxyl
group) can be modified to produce surfactants (Lange, 1999).
With FT-IR, carboxylic acids are easily identifiable. They are characterised by
strong absorption bands at 1710 and 1760 cm-1 due to the C=O (Carlino, 1997)
functional group. The O-H bond of the carboxyl group results in a broad
absorption band in the range 2500- 3300 cm-1 (Socrates, 1980). Free acids absorb
at 1760 cm-1 while dimerics absorb at 1710 cm-1.
2.6
Surfactants
Surfactants, also referred to as ‘surface active agents’, are amphiphilic molecules
composed of parts of different polarity. These are usually dubbed the ‘head’ and
32
the ‘tail’ respectively. The head is hydrophilic (polar) and the tail hydrophobic
(non-polar) (see Figure 4 (a)). The tail often consists of a long hydrocarbon chain
(Moilliet et al., 1961, p 6). Surfactants self-assemble to form micelles in solution
(Lange, 1999, p 1). Khan and O’Hare (2002) define self-assembly as a process in
which the small pre-existing subunits organise themselves into an ordered state or
structural arrangement due to the electrostatic attraction, chemisorption,
hydrophobicity and hydrophillicity of the materials.
Some surfactants occur naturally, while others are chemically modified lipids
(Lange, 1999, p 3). Naturally occurring surfactants contain triglyceride ester from
plant or animal oils (Lange, 1999). The separation is achieved by hydrolysis,
resulting in glycerol and fatty or carboxylic acids with chain lengths from C8 to
C22 (see Scheme 3). Synthetically, some surfactants are produced from plant and
vegetable oils, followed by hydrolysis (Moilliet et al., 1996, p 7). Surfactants are
crystalline, amorphous solids and they can also be found in liquid form.
O
CH2O
C
CH2O H
OR
O
CHO
CH2O
O
1. NaOH
C
O
CR
C
OR
2.H2O
3R
C
O- Na
+
+
CHO H
Surfactant
Fat or oil
CH2O H
Glycerol
Scheme 3: Schematic representation of the hydrolysis process for the production
of surfactant, where R = C11- C19 (adapted from MacMurry, 2000)
Surfactants orient themselves into extended structures at surfaces and also in
water to form micelles (see Figure 4 (b)). Formation of a micelle allows the head
to stay in the water phase while the tail is not (see Figure 4 (c)). In aqueous
solution the surfactant molecules can form different microstructures, depending on
factors such as temperature, composition and the surfactant’s nature (Dong et al.,
33
2008). Disk-like and rod-like surfactant micelles are known (Hoffmann and Ebert,
1988). Surfactants have the ability to solubilise hydrocarbons. The solubility is
mainly dependent on the critical micelle concentration, which is defined as the
concentration at which micelles begin to form (Lange, 1999).
When dissolved in water/medium, the surfactant reduces the surface tension of
the water/medium. The surfactant head can be positively (cationic) or negatively
(anionic) charged or it can be non-ionic. It can also have both a positive and a
negative charge (amphoteric or zwitternoic). Surfactants can be divided into four
main groups, depending on the nature and the charge of the hydrophilic head. In
a solution, charged surfactants can be adsorbed at the interfaces due to the
electrostatic force (Lange, 1999, p 2).
Hydrophobic tail
Hydrophillic head
(a). Surfactant molecule
(b). Surfactant micelle
oil
Water
Hydrophobic tail
Hydrophillic head
(c). Surfactant head in water and tail in oil
Figure 4: Representations of (a) a surfactant molecule; (b) a surfactant micelle;
and (c) the surfactant head in water and tail in oil
34
Anionic surfactants consist of a negatively charged head. A typical example is
sodium dodecylsulphate. This type of surfactant consists of a hydrocarbon tail of
12 carbons and a negatively charged sulphate head (see Figure 6). Anionic
surfactants are characterised by their high surface activities. They adsorb easily on
positively charged mineral surfaces. The adsorption process involves electrostatic
and hydrophobic interactions with the mineral (Carrasco et al., 2008; Reis et al.,
2004). It depends mainly on the nature of the structural groups on the surface,
the molecular structure of the surfactant, the pH, the temperature and the
concentration of the media (Reis et al., 2004; Carrasco et al., 2008). In the
modification of minerals such as clay, the presence of surfactant changes the
physico-chemical properties of the mineral surfaces via hydrophobic and
electrostatic interactions (Fischer, 2003). Sodium alkyl sulphonate has attracted
attention due to its high solubility in the presence of magnesium and calcium ions
(Meyn et al., 1990).
O
O
S
O Na+
O
Figure 5: Chemical structure of sodium dodecylsulphate (SDS) – a typical
example of an anionic surfactant
Cationic surfactants are usually composed of quaternary nitrogen. This type of
surfactant interacts strongly with water molecules due to the presence of alkyl
ammonium halides (Lange, 1999, p 3).
Zwitternoic surfactants are electro-neutral salts with a hydrophobic head that has
both positive and negative charges. The negative charge arises from carboxylate
or sulphate groups. These kinds of surfactant are characterised by their low
toxicity, biodegradability and by not being irritant to skin (Wydro, 2007). They are
amphoteric in nature and therefore their behaviour is similar to that of cationic
and anionic surfactants (Lange, 1999, p. 4).
35
Non-ionic surfactants have an uncharged head. Unlike other surfactants, the
interaction is governed by steric and osmotic forces (Lange, 1999, p 5). Their
main application is in detergents, specifically fabric softeners. Their head groups
consist of long ethoxylated chains. The availability of different kinds of surfactant
allows tailoring for particular applications (Wydro, 2007).
Surfactants are mostly used in household products such as detergents, shampoos,
cosmetics, stain removers, fabric softeners, etc. They can be used as suspension
stabilisers and emulsifiers, for better solubility and dispersion (Wydro, 2007). In
clay science, surfactants are used to modify the physical and chemical properties
of clays, i.e. to improve the hydrophilicity in order to prepare polymer composites
and/or nanocomposites (Fischer, 2003).
2.7
Potential Applications of LDH
Recently, much research has focused on potential applications of LDH intercalated
materials. LDHs have found a wide variety of applications in the medical,
pharmaceutical, catalysis and polymer industries. Some of these potential
applications are discussed in the following sections.
2.7.1
Catalysis
LDH has attracted attention in the field of catalysis because of its small particle
size, large specific surface area and the wide variety of chemical compositions
attainable. In applications involving catalysis or ion exchange, it is desirable that
the LDH be carbonate-free and have a high layer charge. LDH and LDO are very
efficient catalysts for different chemical reactions (Cavani et al., 1991). Some of
the reported LDH and/or LDO base catalysed reactions are the Michael additiontype reaction, Aldol condensation of aldehydes and ketones, Claisen Schimide,
Knoevenagel and Henry reactions (Prescott et al., 2005; Evans and Duan, 2006).
36
LDO is characterised by high thermal stability, making it useful as a catalyst or
catalyst support. LDO has been found to be catalytically active for the
polymerisation of propiolactones and polypropylene oxide and for hydrogendeuterium exchange of acetone and toluene (Carrado and Kostapapas, 1988).
In
heterogeneous
catalysis,
LDH
and
LDO
are
useful
precursors
of
multi-component oxide catalysts (Reichle, 1986b). LDH can also be used as a
support for the metals used for the catalytic reduction of nitrates with hydrogen.
LDO can be used for improving the catalytic reduction of nitrates in water
(Palomares et al., 2004).
Semi-conductor pillars incorporated in LDH material have been reported to provide
excellent photocatalytic activity (Fujishiro et al., 1999; Guo et al., 2001). Catalysts
prepared with LDH at low temperatures are characterised by higher activity,
stability and lifetime, and there is no necessity for alkali metal additions; they
therefore have good activity and thermal stability (Cavani et al., 1991).
Organo-modified Zn/Al-LDH has been reported for the polymerisation of ethylene
as a catalyst support and reinforcement material (He and Zhang, 2007).
2.7.2
Pharmaceutical, medical and cosmetic applications
The most important properties that make LDH useful in these industries are their
low toxicity, buffering effect and exchange capacity (Choy et al., 2007). The
problem facing modern pharmacology is to produce active medication that can be
released at the required rate in the human body. LDH has proved to be useful as a
matrix for most pharmaceutical and biologically active agents for this purpose
(Choy et al., 2007).
Solubility plays a major role in drug liberation, adsorption and bioavailability
(Costantino et al., 2007). Drugs must be highly soluble in biological fluid but some
available drugs show poor solubility. Trikeriotis and Chanotakis (2007) reported
the intercalation of the antibiotics gramidin, amphoterin B, ampicillin and nalidixic
in LDH. Dupin et al. (2004) incorporated dichlorophenac, an anti-inflammatory
37
drug used mostly to treat fever, pain and inflammation in the body. This resulted
in drugs being released at the required time and under appropriate conditions.
The release of the drugs involves a long de-intercalation process, giving the
medicine the required prolonged period of action (Choy et al., 1999; 2007). DNA
and nucleoside monophosphate intercalated LDH has also been reported (Choy et
al., 1999). These LDH nano-hybrids can be used for delivering the DNA into the
cells.
LDH is an antacid, and can be used to neutralise the free HCl in the stomach
gastric juices (Miyata and Okada, 1977; Choy et al., 2007). Currently, LDHs are
receiving considerable attention in the development of biosensors for enzymes,
particularly for medical diagnosis. Urea biosensors based on LDH have been
reported for the determination of urea in the human body in order to diagnose
diabetes and dysfunction of the liver or kidneys. Carbonate-containing LDH can be
used as agents for peptic ulcer treatment (Miyata and Okada, 1977; Miyata, 1980;
1983).
Solar radiation affects human skin badly, resulting in premature skin ageing, skin
cancer and burning of the skin. Skin damage is caused mostly by sunscreens,
moisturisers and skin lighteners which loose their specific functions when
penetrating the skin (Anselim, 2001; Perioli et al., 2006). The skin lotions must
remain on the skin for a required period of time (Anselim, 2001). LDH has shown
the ability to solve some of these problems. It has proved to be a good matrix for
intercalating sunscreens. This offers advantages such as photostability, easy
formulation and no skin contact, and also removes shine and covers blemishes
(Choy et al., 2007). Perioli et al. (2006) intercalated 2-phenyl-1H-benzimidazole-5sulfonic acid in LDH. This proved to be good for photoprotection and can also be
used in underarm deodorants.
2.7.3
Polymers
LDH can be used in a polymer matrix to produce nanocomposite materials with
reproducible chemical and physical properties (Adachi-Pagano, 2003; Fischer,
38
2003) Anion-exchange capacity (AEC) plays a major role in the preparation of
nanocomposites. The smaller the AEC, the easier the formation of a
nanocomposite (Leroux and Besse, 2001). Employing LDH in the preparation of
nanocomposites results in materials with increased tensile and thermal properties,
reduced permeability and solvent uptake, and lower flammability (Zammarano et
al., 2006; Frost et al., 2003a). LDH nanocomposites can be used in many
applications, including batteries (Leroux et al., 2003).
PVC is one of the thermoplastic resins that are thermally unstable. Most thermal
stabilisers in use contain toxic substances like lead, metal soaps and tin
compounds that are environmental pollutants. Nanocomposites based on PVC and
modified LDH have been investigated to improve the thermal stability of PVC (Lin
et al., 2006). LDHs are used as halogen scavengers in polyolefin production, for
the production of ceramic aluminium nitride from LDH poly (acrylonitrile)
complexes (Meyn et al., 1990).
2.7.4
Other applications
LDH has been reported for applications in cleaning and water treatment. The
process involves the removal of organic contaminants. The development of
stabilisers and sorbents for hydrophobic organic compounds is a major challenge.
LDHs have attracted attention due to their use as adsorbents and ion exchangers.
LDH formation offers a mechanism for the disposal of radioactive wastes and also
for removing heavy metals from water contaminated by heavy metals (Lin et al.,
2006). Dodecylsulphate intercalated LDHs have been used for trapping chlorinated
pollutants in water (Allada et al., 2002). Li/Al-LDH is reported to be an effective
adsorbent for Cr (VI), while in de-intercalated form it can be used to recover the
contaminants from the adsorbents used (Lin et al., 2006). Mg-Al oxides can be
used for the treatment of waste acids as both a neutraliser and an adhesive of
anions (Kameda et al., 2002).
LDHs can be used in separation processes for the removal of environmentally
hazardous acid mine drainage or as scavenger (Frost et al., 2003a). LDH
39
incorporated into polyelectrolytes can be employed to modify the proton
conductivity and diffusion coefficient of the membrane in direct methanol fuel cell
applications (Lee et al., 2005).
40
3
3.1
EXPERIMENTAL
Reagents and Suppliers
All the reagents were used without further purification, unless otherwise stated.
The hydrotalcite, also referred to as LDH-CO3, contained silica and magnesium
carbonate as minor impurities. In all the experiments distilled water was used.
Table 3 gives all the reagents used, their chemical grades and the suppliers.
Table 3:
Reagents used and suppliers
Reagent
Supplier
Hydrotalcite (HT-325 grade)
Charmotte Holdings
Sodium dodecylsulphate (SDS) 98%
Fluka
Behenic (docosanoic) acid (80% technical grade)
Fluka
Tween 60 (polyoxyethylene-20-sorbatin monostearate)
Sigma
Stearic acid (65-90 °C)
Biozone Chemicals
Caprylic (octanoic) acid
Croda Chemicals
Acetic acid (98%)
Saarchem
Acetone (99%) C.P.
Radchem Laboratory
Suppliers
Hexanoic acid 98%
Croda Chemicals
Butyric acid
Fluka
Ammonia solution
Promark Chemicals
Ethanol 96% rectified
Dana Chemicals
Ethanol A.R. 99.9% absolute
Radchem Laboratory
Suppliers
Lauric (dodecanoic) acid
3.2
Croda Chemicals
Experimental Set-up
A schematic presentation of the experimental set-up is shown in Figure 6. All the
LDH intercalated samples were prepared using this set-up. All the reactions were
carried out in a glass beaker. The hot plate was used to heat the reactants to the
41
required temperature by setting it on the temperature controller. A mechanical
stirrer was used to stir the reactants in order to avoid agglomeration. Constant
low-speed stirring was employed throughout the experiments. A pH meter was
used to determine the reaction pH for all the experiments.
Mechanical stirrer with speed
control
Temperature controller
Thermometer
Stirrer
Glass beaker
Hot plate
5
4
6
5
7
4
3
8
3
2
9
2
1
11
6
7
8
9
1
10
Top desk
Figure 6:
3.3
Schematic diagram of the experimental set-up
Standard Intercalation Method
Typical intercalation experiments were conducted according to the following
procedure:
1. The molecular mass of LDH-CO3 used was estimated at ca. 234.66 g/mol.
20 g of LDH-CO3 ([Mg0.689Al0.311(OH)2](CO3)0.156.1.5H2O]) was used which
amounted to 0.0852 mol. Thus 1 AEC was equivalent to 0.085 mol
monocarboxylic acid. Therefore 4.12 and 4.5 AEC amounted to 0.351 mol
and 0.384 mol monocarboxylic acid respectively.
42
2. 20 g of LDH-CO3; 0.351 mol or 0.384 mol monocarboxylic acid; and 40 g
(0.139 mol unless otherwise stated) of surfactant, SDS or Tween 60, were
suspended in 1 500 ml of distilled water. The mixture was heated to and
maintained at the required reaction temperature, e.g. at 80 °C for 9 hours,
and cooled down overnight at room temperature. The cycle was repeated
four times. Carboxylic acid was added partially in three cycles, i.e. until the
overall total amount had been added. During the last cycle the mixture was
simply allowed to stir without acid addition. The pH of the mixture was
controlled by drop-wise addition of NH4OH solution. The mixture was
allowed to cool down slowly at room temperature. The solids were
separated from the mother liquor by centrifugation, washed once with
distilled water, four times with ethanol and once with acetone. After each
washing the solids were separated from the liquid by centrifugation. The
product (LDH-carboxylate) was allowed to dry at room temperature. In
some instances the solids were further purified by Soxhlet extraction with
absolute ethanol to remove the excess acid.
The above procedure was followed to prepare ethanoic (acetic), butanoic
(butyric), hexanoic (caproic), octanoic (caprylic), decanoic (capric), dodecanoic
(lauric), tetradecanoic (myristic), octadecanoic (stearic) and docosanoic (behenic)
acid intercalated LDH. See Figure 7 for the melting points of these acids. All the
experimental parameters, temperatures, pHs and product yields obtained for all
the samples are shown in Appendix A.
43
100
Monocarboxylic acids
Melting point, °C
80
60
40
20
0
-20
0
4
8
12
16
20
24
Number of carbons
Figure 7: Carbon numbers and the melting points (∆) of the monocarboxylic acids
3.4
Effect of Surfactant on Intercalation
The effect of surfactant on intercalation was checked using the same standard
procedure as in Section 3.3. Two samples were prepared using the surfactants
SDS and Tween 60. In each case 40 g of the surfactant was added to 1 500 ml of
distilled water at 80 °C, followed by addition of 20 g LDH-CO3. The mixture was
allowed to stir in four cycles as above, but without acid addition.
In some instances the samples were prepared using a smaller amount of
surfactant. 2 g of SDS or Tween 60 was added to 1 500 ml of distilled water at
80 °C, followed by the addition of an excess amount of stearic acid (0.384 mol)
and LDH-CO3. The acid was added partially in three cycles, as stated in
Section 3.3.
3.5
Leaching out the Excess Monocarboxylic Acid on LDH Intercalates
The main focus was on intercalating stearate anions. The excess stearic acid was
leached out using a Soxhlet extractor. 3.9 g of stearate-intercalated LDH was
44
suspended in 100 ml of 99.9% absolute ethanol. The suspension was heated to
80 °C in a water bath under reflux for an hour. The mixture was cooled down
slowly at room temperature. The product was recovered by centrifugation and
allowed to dry at room temperature. The experimental set-up is shown in
Figure 8.
Temperature controller
Condenser
Water out
Water bath
LDH stearate in ethanol
Water in
Hot plate
Top bench
Figure 8: Schematic presentation of the experimental extraction set-up
3.6
Preparation of Mixture of Magnesium Stearate and LDH Stearate
To make sure that the LDH stearate obtained is an intercalate rather than simply
magnesium stearate, a mixture of magnesium stearate and LDH stearate was
prepared. The procedure described in Section 3.3 was employed. 40 g of the
surfactant SDS, 20 g of LDH-CO3, 2.25AEC (54.401 g) of stearic acid and 2.25 AEC
(113.07 g) of magnesium stearate were dissolved in 1 500 ml of distilled water at
80 °C. The mixture was heated in four cycles as described in Section 3.3, but in
this case the magnesium stearate was added partially in three cycles. The product
was recovered and dried at room temperature.
45
3.7
Material Analysis
3.7.1
Instrumentation
Elemental analysis was performed by means of XRF. The carboxylate intercalated
LDH sample was first ashed in a furnace at 700 °C for four hours to produce LDO.
The elemental analysis was performed on the LDO sample. The samples were
ground to <75 µm in a tungsten carbide milling vessel, then roasted at 1 000 °C
for determination of the loss on ignition. The loss on ignition value was
determined after addition of 1 g of sample to 9 g of Li2B4O7 fused into a glass
bead. Major element analysis was executed on the fused bead using an
ARL9400XP+ spectrometer. Another aliquot of the sample was pressed into a
powder briquette for trace element analysis.
Thermal degradation of the samples was checked on a simultaneous TGA/SDTA
Mettler Toledo 851e instrument. 15 mg of the sample was placed in an open 70 µl
aluminium pan. The sample was heated from 25 to 700 °C at a heating rate of
10 °C/min in air.
Identification of organic interlayer anions was performed on a Bruker FT-IR
machine operating with Opus software, Version 2.1. The standard KBr
pellet-pressing method was used. The pellets were prepared by crushing
approximately 2 mg of sample together with 100 mg of KBr powder. The mixture
was pressed in a die under vacuum for 5 minutes. The FT-IR absorption spectra
were recorded by allowing the infrared radiation to pass through the pellet in the
frequency range of 400 to 4 000 cm-1. The data were collected from 32 scans at a
resolution of 2 cm-1. The data obtained were averaged and background-corrected
using a pure KBr pellet.
DSC data were collected on a DSC Q100 TA instrument. 5–10 mg samples were
placed in an open aluminium pan and heated from –20 °C to 150 °C and back to
-20 °C at a rate of 10 °C/min and a flow rate of 50 ml/min using nitrogen.
46
Phase identification was carried out by XRD analysis on a PANalytical X-pert Pro
diffractometer with variable divergence and receiving slits and an X'celerator
detector using Fe-filter Co K-alpha radiation (0,17901 nm) operating with X'Pert
High Score Plus software. The temperature-scanned XRD data were obtained
using an Anton Paar HTK 16 heating chamber with Pt heating strip. Scans were
measured between 2θ = 1 and 40 °C in a temperature range of 25–150 °C at
intervals of 25 °C with a waiting time of 1 min and a measurement time of 6 min
per scan. 99% pure Si (Aldrich) was added to the samples so that the data could
be corrected for sample displacement using the X’Pert High Score Plus software.
The results are presented as variable slit data as that allows for better data
visualisation.
Sample morphology was checked on a JEOL 840 SEM (scanning electron
microscope). A small fraction of sample was placed on carbon tape on a metal
sample holder. The excess powder was removed by air blasting. SEM uses
electrons to produce images and the sample must be electrically conductive. To
make the samples conductive, SEM auto-coating unit, E2500 Polaron equipment
LTD sputter coater, was used. The samples were placed in a chamber at vacuum
and argon gas was introduced. They were coated five times with gold. The goldcoated samples were viewed at low magnifications.
47
4
4.1
RESULTS AND DISCUSSION
Elemental Analysis
Table 4 reports the XRF results for the chemical analysis of the LDH-CO3 grade
HT 325 that was used as raw material and the chemical composition of the ashed
LDH stearate synthesised using the surfactants SDS and Tween 60. The reported
composition of the LDH-CO3 is consistent with the Mg:Al ratio of 2.21:1 (mol
basis). The theoretical anionic exchange capacity (AEC) of this LDH-CO3 is
402 meq/100 g. The ashed LDH-stearate (SDS) sample suggests instead a Mg:Al
ratio of 1.93:1 (mol basis) and indicates the presence of 0.45 mol of sodium
atoms for every mol of aluminium atoms for the LDH stearate sample synthesised
using SDS. These results indicate that some sodium stearate was co-intercalated.
The LDH stearate synthesised using Tween 60 suggests a Mg:Al ratio of 2.20:1.00
(mol basis). There is no change in the mol ratio obtained compared with LDH-CO3.
In this case only stearate anion was intercalated. The results show that non-ionic
surfactants can be used to prevent co-intercalation of sodium. This is evident from
the absence of sodium in the LDH stearate sample prepared using Tween 60.
Table 4:
XRF composition analysis (mass %) of LDH-CO3 and LDH-stearate
synthesised with SDS and Tween 60 ashed at 700 °C
Sample
MgO
Al2O3
SiO2
CaO
Fe2O3 Na2O
NiO
LOI%
LDH-CO3
35.05
20.09
1.05
0.26
0.10
0.00
0.08
43.31
Ash (SDS)
49.43
32.38
1.55
0.27
0.27
8.85
0.05
7.23
Ash (Tween
55.23
31.76
1.24
0.36
0.15
0.01
0.04
12.00
60)
4.2
Thermal Decomposition
Figure 9 shows the thermogravimetric mass loss and the mass loss derivative
curves obtained for pure LDH-CO3 heated from 25 to 700 ºC in air. The mass loss
of LDH-CO3 starts at room temperature and is complete by 700 ºC. Three major
mass loss steps are observed on TG enhanced by three peaks on DTG. The first
48
sharp peak at 236 ºC observed in the range 25-250 ºC with a 86.3% mass loss is
due to the loss of interlayer water (Rey et al., 1992; Rocha et al., 1999). The
second peak centred at 325 ºC in the range 250–370 ºC is due to the loss of
hydroxyl groups from the brucite-like layers (Rey et al., 1992). The third peak at
428 ºC in the temperature range 370–700 ºC is reported as being due to a
combination of dehydroxylation and loss of interlayer anion or decarbonation
(Reichle, 1985; Miyata and Okada, 1995). The Mg-Al-LDH structure starts to
decompose at temperatures higher than 400 ºC and the MgO phase starts to form
(Kanezaki, 1998).
Despite the claim by Kanezaki (1998) that decarbonation occurs at temperatures
above 400 and 500 ºC, Hibino et al. (1995) found that in this temperature range
the process is not complete for Al-rich compounds. Rey et al. (1992) claim that the
decarbonation process starts from 227 ºC and overlaps with the dehydration, and
that the Al content has no influence on the thermal behaviour of hydrotalcites. All
the thermal processes were found to be to be reversible at room temperature in
contact with the atmosphere (Rey et al., 1992). Kloprogge and Frost (1999) claim
that decarbonation occurs in the region 80 to 230 ºC.
The present results show 58.3% total residue at 700 ºC on TG. The expected TGA
residues for [Mg2Al(OH)6](CO3)1/2.1.5H2O after the first and final steps are 88.45
and 56.08% respectively. The full calculation of the expected % mass loss after
the first and last thermal events is shown in Appendix B. The experimentally
observed values for the LDH-CO3 are 86.33 (at T = 250 °C) and 58.3% (at T =
700 °C) respectively.
49
100
-0.4
Mass loss, % .
80
-0.7
LDH-CO 3
60
-1.0
-1.3
40
-1.6
-1.9
20
Stearic acid
Derivative curve, %/°C .
-0.1
-2.2
0
0
100
200
300
400
500
600
-2.5
700
Temperature, °C
Figure 9: TG and DTG curves of LDH-CO3 and stearic acid in air
The thermal decomposition product of the [Mg2Al(OH)6](CO3)1/2.nH2O can be
represented as follows:
25-250 °C
[Mg2Al(OH)6](CO3)1/2nH2O
[Mg2Al(OH)6](CO3)1/2
dehydration
dehydroxylation and
decarbonation
2MgO + 0.5 Al2O3
Scheme 4: Schematic presentation of the LDH decomposition process
Kanezaki claims that water molecules thermally oxidise the interlayer carbonate
and carbon dioxide, and that hydroxyl anions are released by the reaction
(Kanezaki, 1998)
[CO32- + H2O]interlayer
CO2(g) + 2OH-(g)
50
Stearic acid shows only one mass loss step. The peak observed is centred at 225
°C on DTG. This peak is attributed to the vaporisation of carboxylic acid and its
degradation products. This mass loss is also complete at 700 °C.
Figures 10 and 11 show the TG and DTG curves for fatty acid intercalated LDH
from 25–700 °C in air. The individual curves are shown in Appendix B. Three
thermal events similar to those observed in LDH-CO3 are observed. These are loss
of interlayer water, dehydroxylation and a combination of dehydroxylation and the
loss of interlayer anion.
100
Residual mass, % .
80
60
LDH-octanoate
LDH-laurate
40
LDH-myristate
LDH-stearate
LDH-behenate
20
0
0
100
200
300
400
500
600
700
Temperature, °C
Figure 10: TG curves of LDH-octanoate, laurate, myristate, stearate and
behenate prepared at 80 °C (octanoate and stearate), 70 °C (laurate), 60 °C
(myristate) and 90 °C (behenate)
The sharp DTG peaks centred at 82, 90, 85, 97 and 100 °C corresponding to ca.
4% mass loss are attributed to the loss of interlayer water for the LDH-octanoate,
laurate, myristate, stearate and behenate samples respectively. The peak at
51
225 °C observed in LDH-CO3 in Figure 9 is suppressed and new peaks are
observed at 250, 237, 260, 380 and 425 °C for the LDH-octanoate, laurate,
myristate, stearate, and behenate samples respectively. An attempt was made to
intercalate acetic, butyric, hexanoic and decanoic acid anions in LDH. However,
the intercalation failed. The TG/DTG results are shown in Appendix B.
Derivative mass, %/°C
0.000
LDH-octanoate
LDH-laurate
LDH-myristate
LDH-stearate
LDH-behenate
-4.200
0
100
200
300
400
500
600
700
Temperature, °C
Figure 11: DTG curve of LDH-octanoate, laurate, myristate, stearate and
behenate prepared at 80 °C (octanoate and stearate), 70 °C (laurate), 60 °C
(myristate) and 90 °C (behenate)
The amount of intercalated carboxylate in the interlayer was estimated from TG
data as follows:
The interlayer water content was estimated from the mass loss recorded at
148 °C. This is based on the fact that at this temperature the physically adsorbed
and interlayer water has been lost. The effective clay content was calculated from
the residue at a high temperature of 700 °C. The carboxylic acid content was
52
calculated from the difference (see Table 5). This was based on the fact that the
residue contains MgO, Na2O and Al2O3 as is evident from the presence of sodium
in the samples prepared using SDS. A high carboxylate content was observed for
LDH-laurate, stearate and behenate. This high organic content is attributed to the
presence of sodium in the interlayer region.
Table 5: TG data for LDH-CO3, octanoate (80 °C), laurate (70 °C), myristate (60
°C), stearate (80 °C) and behenate (90 °C) intercalated LDH prepared using SDS
Sample
Residual
Residual
% Effective
%
mass at
mass at
clay content
Carboxylate
148 °C
700 °C
LDH-CO3
97.78
58.33
100
LDH-
91.98
23.63
43.06
56.94
LDH-laurate
90.70
14.71
27.17
72.82
LDH-myristate
93.77
25.94
46.36
53.64
LDH-stearate
95.63
12.48
21.87
71.28
LDH-behenate
95.73
9.00
15.75
84.24
octanoate
Figure 12 illustrates the effect of reagent stoichiometry on stearate intercalation
obtained at 80 °C in comparison with the data obtained by Itoh et al. (2003). The
degree of stearate intercalation in the present data lies slightly above the
theoretical limit for the product obtained at a feed composition of stearate/LDH =
4.12, purified by Soxhlet extraction with absolute ethanol. In contrast, Itoh et al.
(2003) obtained values that are slightly below the theoretical limit at a feed
composition of 5 mm of the sodium stearate, with appropriate amounts of LDH, at
60 °C.
53
Stearate/LDH intercalated .
3
Theoretical limit
2
1
This study
Itoh et al. 2003
0
0
2
4
6
Stearate/LDH in feed
8
10
Figure 12: Effect of reagent stoichiometry on the degree of stearic acid
intercalation at 80 °C, where (•) represents the present data and (∆) represents
the values obtained by Itoh et al. (2003) for sodium stearate
4.3
FT-IR
Figure 13 shows the FT-IR results of the precursors (LDH-CO3, stearic acid and the
surfactant SDS) used in the intercalation reactions. LDH-CO3 shows a broad band
at 3455 cm-1 due to the (-OH) hydroxyl stretching vibration of free hydrogen and
hydrogen bonded to the octahedral layer and water molecules (Labajos et al.,
1992). The shoulder at 3063 cm-1 is due to the hydrogen bonding of H2O to CO32ions in the interlayer space (Perez-Ramirez, 2001). The carbonate peak is
observed at 1360 cm-1. The shoulder at 917 cm-1 is due to the M-OH deformation
mode. The bands at 763, 672 and 549 cm-1 are due to the Mg-OH translation
mode, the ν4 (in-plane bending) vibrations of CO32- and the Al-OH translation
mode respectively (Kloprogge and Frost., 1999).
Stearic acid shows a broad O-H stretching mode in the range 3300-2500 cm-1. The
C-H symmetric and asymmetric stretching vibrations are observed at 2954, 2914
and 2870 cm-1. Carboxylic acid salts are characterised by a strong absorption at
54
1700 cm-1 as is evident from C=O in dimeric carboxylic acids. The C-H bending
and scissoring modes are observed at 1472 and 1464 cm-1. The C-O-H bending
mode is observed at 1410 cm-1 and the C-O stretching modes at 1313 and
1297 cm-1. The CH2 wagging modes are observed in the range 1300-1250 cm-1.
The number of these peaks is dependent on the length of the carboxylic acid
chain, i.e. for even-number carbons the number of peaks will be equal to half
carbon numbers, while for odd-number carbons they will be half plus one
(Socrates, 1980). The O-H out-of-plane bending mode is observed at 936 cm-1.
% Transmittance
Stearic acid
4000
SDS
LDH-CO 3
3400
2800
2200
1600
1000
400
Wavenumber, cm-1
Figure 13: FT-IR spectra of the precursor LDH-CO3, stearic acid and the
surfactant SDS used in the intercalation reactions
The O-H stretching mode of the surfactant, SDS, is observed at 3447 cm-1. The
strong absorptions at 2915 and 2848 cm-1 are due to the -CH symmetric and
asymmetric stretching of the alkyl chain. A strong and sharp absorption due the
bending and scissoring mode of the –CH2- surfactant tail is observed at 1467 cm-1
(Crepaldi et al., 2002). The band at 1382 cm-1 is attributed to the CH3 deformation
55
mode. SDS is mainly characterised by the presence of –SO4. The bands due to the
asymmetric and symmetric stretching mode of -SO4 are observed at 1206 and
1062 cm-1. These bands are sometimes observed at 1214 and 1132 cm-1 or 1229
and 1065 cm-1 (Crepaldi et al., 2002; Costa et al., 2008). The shift to lower
frequencies is attributed to the disturbance due to –SO3 (Crepaldi et al., 2002).
Irrespective of C-H absorptions, carboxylic acids and SDS are easily distinguished
by the presence of the carbonyl region for carboxylic acids and sulphate in the
surfactant SDS.
Figure 14 compares the FT-IR spectra of fatty acid (octanoic, lauric, myristic,
stearic and behenic) intercalated LDH. The separate spectra are shown in
Appendix C. A broad band due to the -O-H vibration mode bonded to metal in the
brucite-like layered sheet is observed in the range 3448–3387 cm-1 for all the
samples (Frost et al., 2002). The shoulder at 3247–3225 cm-1 is attributed to the
water molecules that are hydrogen bonded to the interlayer anion (Perez-Ramirez
et al, 2001). The strong and sharp-intensity peaks at 2915 and 2848 cm-1 are
attributed to the asymmetric and symmetric stretching modes of the –CH2- group
of the alkyl chain respectively (Labajos et al., 1992).
LDH-octanoate, laurate and myristate show a very weak-intensity peak at
2936 cm-1 due to the –CH2 vibration mode. However, these peaks are absent in
the stearate and LDH-behenate samples. Several absorption bands are observed
in the carboxyl region of 1750–600 cm-1 and this is discussed separately. The MOH deformation and translation modes are observed at 983, 769 and 775 cm-1
(Perez-Ramirez et al., 2001; Labajos et al., 1992). The O-M-O and M-O bending
and stretching modes are observed at 723 and 667 cm-1 and the –CH2- rocking
mode at 716 cm-1 (Perez-Ramirez et al., 2001).
56
LDH-Octanoate
% Transmittance
LDH-laurate
LDH-myristate
LDH-stearate
LDH-behenate
4000
3600
3200
2800
2400
2000
1600
1200
800
400
-1
Wavenumber, cm
Figure 14: Comparison of FT-IR spectra of LDH-octanoate, laurate, myristate,
stearate and behenate prepared at 80, 70, 60, 80 and 90 °C respectively
4.4
State of Intercalated Carboxylic Acid
Figure 15 shows the FT-IR spectra of the carboxyl group in the vibration region
1750-600 cm-1 of octanoate, laurate, myristate, stearate and behenate
intercalated LDH obtained at 80, 70, 60, 80 and 90 °C respectively. The absence
of a peak due to undissociated carboxylic acid at 1720 cm-1 and at 1210 cm-1 for
sulphate indicates the purity of the intercalates obtained. LDH-Stearate and
behenate show absorption bands similar to those obtained by Borja and Dutta
(1992). A weak absorption band is observed at 1634 cm-1 in the LDH-stearate and
behenate samples. This peak is attributed to the –OH stretching mode of the
interlayer water (Labajos et al., 1992). However, Borja and Dutta (1992) obtained
the bands at 1637 cm-1 and 1588 cm-1, and these bands were attributed to
carboxylic acid being intercalated in the form –RCOOH, where H+ is ionised in the
interlayer as C(O)O-δH+δ. The band at 1588 cm-1 is present in all the samples. The
57
LDH-octanoate shows a weak band at 1559 cm-1 due to the symmetric stretching
mode of the ionised –C-O group in the interlayer (Carlino and Hudson, 1994). This
band is observed at 1558, 1555, 1538 and 1536 cm-1 for the LDH-laurate,
myristate, stearate and behenate samples respectively. The asymmetric mode is
observed at 1425, 1426, 1413 and 1412 cm-1 in the LDH-laurate, myristate,
stearate and behenate samples respectively (Perez-Ramirez et al., 2001).
However, these bands are absent in the LDH-octanoate sample. The mediumintensity bands at 1467, 1468, 1466, 1472 and 1470 cm-1 for the LDH-octanoate,
laurate, myristate, stearate and behenate samples respectively are attributed to
the –CH2 bending mode of the carboxylic acid chain (Borja and Dutta, 1992).
LDH-Octanoate
% Transmittance
LDH-laurate
LDH-myristate
LDH-stearate
LDH-behenate
1800
1600
1400
1200
1000
800
600
-1
Wavenumber, cm
Figure 15: FT-IR spectra of fatty acid (octanoic, lauric, myristic, stearate and
behenic) intercalated LDH showing only the carboxyl region
The high affinity of LDH for carbonate anion is confirmed by the presence of the
band at 1360 cm-1 due to carbonate impurity from the LDH-CO3 precursor. These
bands are observed for the LDH-laurate, myristate, stearate and behenate
58
samples and are absent in LDH-octanoate. These results indicate that the
carboxylic acid was intercalated in different forms, i.e. ionised and non-ionised, in
between the LDH layers. Intercalation of short-chain fatty acids (acetic, butyric,
hexanoic and decanoic) failed completely, as already mentioned. However, the
FT-IR spectra are also shown in Appendix C. Kanoh et al. (1999) tried to
intercalate short-chain fatty acids, but failed. This was attributed to the
hydrophilicity of the acids at lower temperatures.
4.5
X-ray Diffraction
The PXRD pattern (obtained using cobalt Kα) of fatty acids, octanoate, laurate,
myristate, stearate and behenate intercalated LDH prepared at 80 °C (octanoate
and stearate), 70 °C (laurate), 60 °C (myristate) and behenate (90 °C) in
comparison with LDH-CO3 is shown in Figure 16. The individual XRD patterns are
shown in Appendix D.
Two diffraction peaks are observed at 0.76 nm (2θ = 13.49°) and 0.38 nm
(2θ = 27.21°). The first and greatest reflection d003 corresponds to the d-spacing
of LDH. In this case a d-spacing of 0.76 nm is observed.
Several reflections are observed and the main reflections d003; d006 and d009 are
shown in Table 6. LDH intercalates show unreacted LDH-CO3. This is evident from
the reflections at 2θ = 13.49° and 27.21°. These are observed for the samples
LDH-laurate, myristate, stearate and behenate and are absent in LDH-octanoate.
This is in agreement with the FT-IR results obtained for all the samples. The sharp
reflection peaks observed are an indication that the LDH intercalates obtained are
highly crystalline. However, Carlino and Hudson (1995) reported the intercalation
of octanoate in LDH using coprecipitation and the thermal reaction method, and
obtained a polyphasic XRD pattern.
59
LDH-CO 3
Intensity,a.u
LDH-octanoate
LDH-laurate
LDH-myristate
LDH-stearate
LDH-behenate
0
5
10
15
20
25
30
2θ
θ /°(Co Kα
α)
Figure 16: XRD pattern of fatty acid (octanoic, lauric, myristic, stearic and
behenic) intercalated LDH samples prepared at 80 °C (LDH-octanoate, laurate and
stearate), 60 °C (LDH-myristate) and 90 °C (LDH-behenate) respectively
An increase in the basal spacing from 0.76 nm is observed for all the samples.
This is evident from the XRD data obtained. The basal spacings increased to 2.72,
3.66, 4.22, 5.04 and 5.81 nm for the LDH-octanoate, laurate, myristate, stearate
and behenate samples respectively. These basal spacings suggest a bilayer
intercalated LDH structure (Carlino, 1997; Meyn et al., 1993). The bilayer
structure is illustrated in Figure 17.
Borja and Dutta (1992) reported a monolayer structure in intercalated laurate,
myristate and palmitate with LiAl2-LDH. However, with Mg3Al-LDH a bilayer
structure was obtained. These structures were achieved using an ion-exchange
method.
60
In the present study the observed basal spacings result in interlayer gallery
heights of 2.24, 3.18, 3.74, 4.56 and 5.33 nm for the LDH-octanoate, laurate,
myristate, stearate and behenate samples respectively. These interlayer gallery
heights were calculated from the difference of 0.84 nm between the basal spacing
and the thickness of the brucite-like LDH layers (Chibwe and Jones, 1989; Kanoh
et al., 1999).
Table 6: Observed XRD data of LDH intercalated samples
Sample
Observed reflections
d003
d006
d009
LDH-octanoate: 2θ/°
3.77
5.02
7.50
d/nm
2.72
2.04
1.37
LDH-laurate: 2θ /°
2.80
5.56
8.29
d/nm
3.66
1.85
1.24
LDH-myristate: 2θ/°
2.43
4.83
7.22
d/nm
4.22
2.13
1.42
LDH-stearate: 2θ/°
2.04
4.01
5.97
d/nm
5.04
2.56
1.72
LDH-behenate: 2θ/°
1.76
2.61
3.39
d/nm
5.81
3.93
3.03
61
d0
+ + + + + + + + + +
0.127 n cos α
d
d2
α
0.127 n cos α
d0
+ + + + + + + + + +
Figure 17: Schematic representation of the bilayer structure of fatty acid
intercalated LDH with corrected slant angle (adapted from Carlino, 1997)
The effect of the chain length of carboxylic acid on the d-spacing was studied by
plotting the observed d-spacing against the number of carbon atoms in carboxylic
acid (see Figure 18). The data suggest that the d-spacing of the LDH intercalates
increases linearly with an increase in the length of the carboxylic acid chain.
Figure 18 also plots the d-spacing reported by Borja and Dutta (1992) and the
values reported by Itoh et al. (2003). The literature values are slightly higher than
the ones obtained in this study. The difference might be due to the presence of
impurities that were incorporated into the clay galleries, i.e. ethanol in the case of
the results obtained by Borja and Dutta (1992), and sodium ions in the case of the
results obtained by Itoh et al. (2003). Miyata and Kumura (1973) and Meyn et al.
(1993) also reported a linear increase in d-spacing with an increase in chain length
for dicarboxylate and surfactant intercalated LDH.
62
7
LDH layer
a
d-spacing, nm
6
carboxylate
layers
5
4
d = 0.842 + 0.240 n
3
LDH-SDS
2
1
LDH-CO 3
0
0
4
8
12
16
20
24
Carbons in carboxylic acid
Figure 18: Effect of carboxylic acid chain length on the d-spacing of the LDH
intercalates prepared by the SDS-mediated intercalation method, represented by
(). The d-spacing values for LDH-CO3 () and LDH-SDS (∆) (i.e. the sample in
which an attempt was made to intercalate acetate) are shown, as well as the dspacing values reported by Borja and Dutta (1992) () and Itoh et al. 2003 (□).
It is assumed that the slant angle of the alkyl chain length is dependent on the
length and that the methylene bond lengths are equal to 0.127 nm (Carlino,
1997). The d-spacing of the bilayer intercalated LDH-carboxylate is calculated
using equation 3. The present results, suggesting a slope of slope of 0.24 nm per
unit charge, are similar to those reported by Kanoh et al. (1999) and Itoh et al.
(2003) (see equation 5).
d = 0.842 + 0.240 n
(5)
The slope of 0.24 nm per –CH2- unit is similar to previously reported values of
0.235 and 0.2454 nm (Itoh et al., 2003; Kanoh et al., 1999). This slope
corresponds to the slant angles of 19.3°, 22.3 and 15° using equation 3. These
63
values are slightly higher than the theoretical values reported in the literature.
This is because of the presence of the sodium ion and ethanol impurities observed
(Itoh et al., 2003; Kanoh et al., 1999). The aluminium atoms are probably
randomly distributed within the Mg-Al-(OH)x sheets. However, an idealised regular
arrangement is shown in Figure 19. This figure supports the estimation of the
projected surface area per formula weight of the LDH. This was based on the
predicted brucite-like Mg-Al-(OH)x sheet area per LDH formula weight ([Mg2+xAl1x(OH)6]
(CO3)(1-x)/2.nH2O), which is given by equation6 below.
A(LDH) =
3 3 2
a
2
(6)
The lattice parameter a = 0.305 nm for the present LDH-CO3 (Belloto et al., 1996).
This results in the projected LDH area ALDH = 0.2404 nm2. Based on the
assumption that the sample has a hexagonal close-packing structure, the
cross-sectional area per stearic acid chain is given by equation 7.
A(chain) =
2
3
t2
(7)
The reflection 2θ = 25.422° for CoKα results in a layer spacing of 0.407 nm and
an area per stearic acid chain of 0.191 nm2. Therefore, for bilayer intercalated
LDH, incorporating a correction for the slant angle leads to the following maximum
intercalation level (equation 8):
2
A
9 a
 Carboxylate 
= 2 LDH cos α =   cos α


LDH
Achain
2 t 

max
(8)
In the present case the slant angle is estimated at α = 19.1°. This yields a limit of
2.39 mol carboxylate/LDH for close-packed carboxylate chains.
64
a
(a)
(b)
2t
Figure 19: (a) Idealised regular arrangement of aluminium ( ) and magnesium
atoms ( ) in the brucite-like metal hydroxide sheet of [Mg2+xAl1-x(OH)6] (CO3)(1x)/2.nH2O)
with α ≅ 0, the lattice parameter a = 0.305 (Belloto et al., 1996); and (b)
the hexagonal close packing structure of the stearate chains
4.6
Effect of Surfactant on Intercalation
Figure 20 shows the FT-IR spectra obtained by dispersing the LDH-CO3 in distilled
water at 80 °C in the presence of the surfactants SDS and Tween 60 in
comparison with LDH-CO3. The absorption bands obtained fit perfectly with all the
bands obtained in LDH-CO3. This result indicates that the surfactants did not
intercalate on their own.
65
%Transmittance
LDH-SDS
LDH-tween 60
LDH-CO 3
4000 3600 3200 2800 2400 2000 1600 1200
800
400
-1
Wavenumber, cm
In
this case a large amount of surfactant is required for better dispersion and purity
of the LDH intercalates.
Figure 20: FT-IR spectra of LDH-SDS and LDH-Tween 60 in comparison with
LDH-CO3
The results in Figure 20 are in agreement with the XRD pattern in Figure 21. The
observed XRD pattern is similar to the LDH-CO3 pattern – in fact, no difference
can be observed. Only two diffraction peaks due to LDH-CO3 are observed.
66
Intensity,a.u
LDH-CO 3
LDH-SDS
LDH-tween 60
0
5
10
15
20
25
30
2θ
θ /°(CoKα
α)
Figure 21: XRD pattern of LDH-SDS and LDH-Tween 60 synthesised at 80°C in
comparison with LDH-CO3
Figure 22 shows the TG/DTG curves for the LDH-SDS and LDH-Tween 60 samples
prepared at 80 °C. Three thermal events similar to those of LDH-CO3 are
observed. The observed residual masses at 700 °C are 58.33, 58.68 and 58.43%
for the LDH-CO3, LDH-SDS and LDH-Tween 60 samples respectively. These results
show that the surfactants SDS or Tween 60 did intercalate on their own. This is in
agreement with the FT-IR and XRD results obtained, shown in Figures 20 and 21
respectively.
67
0.3
Mass loss, % .
80
60
LDH-CO 3
0.2
LDH-SDS
0.1
LDH-tween 60
0
40
-0.1
20
-0.2
0
0
100
200
300
400
500
600
Derivative curve, %/°C .
100
-0.3
700
Temperature, °C
Figure 22: TG/DTG curves of the product obtained by dispersing the LDH in
water in the presence of the surfactants SDS (LDH-SDS) and Tween 60 (LDHTween 60), in comparison with LDH-CO3 heated from 25–700 °C in air
In contrast, when acetic acid is used the sulphate vibration modes are observed
(see Figure 23). All the SDS bands discussed in Section 4.4 are observed. These
indicate that the surfactant anions were intercalated in the LDH, instead of the
desired acetic acid anions. The results obtained in this case show that in the
presence of short-chain fatty acids such as acetic acid, which is also a weak acid,
the LDH prefers SDS anions and not acetate. Miyata and Kumura (1973) reported
that acetate intercalated in LDH.
In the study, the acetate anions were
successfully intercalated.
The present method works well with long chain carboxylic acids. However, high
anionic or non-ionic surfactant concentration is required. When less surfactant is
used in carboxylic acid intercalation, incomplete reactions are observed indicating
that prolonged periods of heating is required and the materials also retains large
68
amounts of impuruties. Carboxylate intercalation is successful at the melting point
or at temperatures higher than the melting point of the carboxylic acid.
% Transmittance
SDS
LDH acetate
4000
3400
2800
2200
1600
1000
400
Wavenumber, cm-1
Figure 23: FT-IR spectra of LDH-acetate obtained at room temperature in
comparison with the surfactant SDS
4.7
Effect
of
Reaction
Temperature
on
Carboxylate
Anion
Intercalation
The effect of reaction temperature on the degree of intercalation for lauric and
stearic acid is shown in Figure 24. The current method results in an incomplete
reaction at temperatures below the melting point of the carboxylic acid, i.e. below
60 and 53 °C for stearic and lauric acid respectively. However, Itoh et al. (2003)
and Kanoh et al. (1999) reported the intercalation of water-soluble sodium
stearate at temperatures as low as 5 °C. The apparent degrees of intercalation
exceeded the theoretical limit for intercalation at temperatures above 60 °C. This
is attributed to the presence of sodium in the interlayers of LDH.
69
The results obtained by Kanoh et al. (1999) indicate that the structure of stearate
intercalated LDH changes reversibly to form a monolayer or bilayer intercalated
LDH structure, depending on the intercalation temperature. This was supported by
the monolayer LDH-stearate structure obtained at lower temperatures for 1 AEC,
and by the bilayer structures obtained at higher temperatures, above 70 °C.
However, bilayer intercalated LDH was also formed at lower temperatures when
excess sodium salts, 3 AEC, were used (Kanoh et al., 1999). The calculated size of
the stearate anion is 2.25 nm (Kanoh et al., 1999). Anbarasan et al. (2008)
reported monolayer intercalated LDH-stearate at 70 °C with a d-spacing of
2.67 nm. This result was obtained by dispersing LDH in water in the presence of
stearic acid.
Bilayer and monolayer intercalated stearate anions in Zn-Si-LDH with interlayer
spacings of 3.72 and 3.12 nm, and 3.71 and 2.89 nm for Zn-Sn-LDH, were
reported for samples prepared by stirring at room temperature for 72 hours
(Saber and Tagaya, 2003 a, 2003 b and 2007). Similar results were also obtained
for myristate intercalated Zn/Si-LDH and Zn/Sn-LDH (Saber and Tagaya, 2007).
Carlino and Hudson (1995) reported mono- and bilayer caprate and sebaccate
intercalated LDH obtained using coprecipitation and the thermal reaction method
at 150 °C.
The present results suggest only a bilayer intercalated carboxylic acid LDH
structure. Inomata and Ogawa (2006) obtained bilayer stearate and oleate
intercalated LDH at room temperature and at 60 °C using the rehydration method
under hydrothermal conditions. However, the results showed incomplete reaction,
indicating that a prolonged period of heating was required at room temperature
and at 60 °C. Temperature proves to be the most important parameter to control
in order to obtain highly crystalline LDH intercalates. It therefore appears that the
packing of the chains in the interlayers of LDH depends on the route taken to
prepare the LDH intercalates. This observation is based on the different results
obtained by different authors at different temperatures.
70
4
Carboxylate/LDH .
LDH-laurate
LDH-stearate
3
2
1
0
50
60
65
70
75
80
Intercalation temperature, °C .
85
Figure 24: Effect of reaction temperature on the degree of lauric and stearic acid
intercalation
4.8
Differential Scanning Calorimetry (DSC)
Figure 25 shows the DSC melting endotherms for pure and technical-grade stearic
acid, magnesium stearate and LDH stearate synthesised using SDS. The
technical-grade stearic acid shows a low melting enthalpy compared with the 99%
grade because it contains significant amounts of palmitic acid, as shown in
Table 7. The technical-grade stearic acid was used in the preparation of
LDH-stearate. This compound shows two endotherms: the one centred at
ca. 107 °C corresponds to the dominant endothermic event. The main endotherm
is positioned at about 50 °C higher than the parent stearic acid. These reflect the
effect of the two-dimensional constraints imposed by the rigid inorganic sheet on
the stearate bilayers. The magnesium stearate shows the highest melting range.
Its melting enthalpy is higher than that observed for the stearic acids, but lower
than that observed for LDH-stearate. In this compound all the stearic acid radicals
are fully ionised and directly co-ordinated to the magnesium atoms in an
octahedral site (Vold and Hattiangdi, 1949; Bracconi et al., 2003). In contrast, in
LDH stearate only a portion of the stearic acid molecules in the LDH galleries are
expected to be ionised.
71
0
Heat flow, W/g .
-1
-2
magnesium
LDH-stearate stearate
-3
-4
-5
stearic acid
(technical)
-6
stearic acid
(99%)
-7
-8
40
50
60
70 80 90 100 110 120 130
Temperature, °C
Figure 25: DSC melting endotherms for technical-grade stearic acid, pure stearic
acid (99%), LDH-stearate prepared at 80 °C using SDS and magnesium stearate.
The measured enthalpies were -185, -221, -241 and -173 kJ/kg respectively
Table 7: Differential scanning calorimetry (DSC) results for selected compounds
Compound
Stearic acid
Stearic acid
LDH-
Magnesium
(technical
(99% pure)
stearate
stearate
grade)
Endotherm peak, °C
55.6
(SDS)
72.8
107.2 &
121.0
117.8
Enthalpy, J/g
-158
-173
-241
-221
Figure 26 shows a hot-stage microscopic image at different temperatures and a
DSC heating scan of the LDH-stearate prepared using Tween 60. The two melting
endotherms are also observed, as in the sample prepared using SDS. However, in
this case the second endotherm is more pronounced than that of LDH-stearate as
shown in Figure 27. The crystals appear superficially intact at temperatures
72
exceeding the second endothermic peak. At 140 °C both the LDH-stearate
samples are fully molten.
27
Heat flow, mW .
18
9
111°C
127°C
0
59°C
140°C
-9
-18
-27
50
70
90
110
Temperature, °C
130
150
Figure 26: DSC melting endotherm and hot-stage microscopic image of LDHstearate prepared at 80 °C using Tween 60
The same behaviour is observed with LDH-stearate (SDS). Figure 27 shows a
microscopic image of LDH-stearate (SDS) at 100 to 170 °C. The sample starts to
melt at 136 °C and at 170 °C it is fully molten. Inomata and Ogawa (2006)
obtained an endotherm and a shoulder at 78 and 87 °C respectively for LDHstearate. This behaviour was reversible since on cooling these peaks were
observed at lower temperatures, namely 66 and 74 °C. In contrast, in the present
data no recrystallisation was observed when cooling to lower temperatures. The
observed thermal behaviour is non-reversible. In this case the presence of the
interlayer recrystallisation water may be required for recrystallisation.
73
LDH-stearate (SDS) 100 °C
LDH-stearate (SDS) 152 °C
LDH-stearate (SDS) 136 °C
LDH-stearate (SDS) 170 °C
Figure 27: Microscopic images o f LDH-stearate (SDS) taken at 100, 136, 152
and 170 °C
Magnesium stearate shows an endothermic peak at 121 °C and LDH-stearate at
117.8 °C, as seen in Figures 25 and 26. Due to the close similarities between
these two samples, it could be suggested that the LDH stearate contains
magnesium stearate.
In order to check whether or not the LDH-stearate sample contains magnesium
stearate, a mixture of the two was prepared at 80 °C. Figure 28 compares the
74
XRD patterns obtained for LDH-stearate, the mixture of LDH-stearate and
magnesium stearate, and magnesium stearate recrystallised in Tween 60.
Similar reflections are observed in the LDH-stearate and recrystallised magnesium
stearate samples. However, the mixture of magnesium stearate and LDH-stearate
shows completely different reflections. Two twin reflection peaks are observed,
indicating the presence of two different phases. These double diffraction peaks are
observed at 2θ = 4.39, 4.37° and 6.54, 6.22° respectively. However, none of
these peaks is observed in the LDH-stearate sample and the recrystallised
magnesium stearate sample. This polyphasic XRD pattern proves that LDHstearate is not a mixture with magnesium stearate. Thus, it is a stearate
intercalated LDH.
Log Intensity,a.u
Mg stearate and LDH-stearate mixture
Recrystallized Mg stearate
LDH-stearate
1
4
7
10
13
16
19
2θ
θ /° (Co Kα
α)
Figure 28: XRD pattern of LDH-stearate in comparison with the mixture of
stearate-LDH and magnesium stearate
Figure 29 shows the DSC curve for LDH-stearate prepared at 80 °C using SDS.
Only one endothermic peak and a shoulder are observed. The main endotherm is
centred at 91 °C and there is a shoulder at 102 °C. Similarly to LDH-stearate,
non-reversible thermal behaviour is also observed in this case. No recrystallisation
75
is observed on cooling. The theoretical melting point of lauric acid is 53 °C, which
is lower than that observed for LDH-laurate.
Heat flow, W/g
1
0
-1
-2
Laurate-LDH
-3
30
60
90
Temperature, °C
120
Figure 29: DSC curve for LDH-laurate prepared at 80°C using SDS
4.9
Temperature-scanned XRD
Figures 30 to 32 show the XRD spectra of stearate-LDH (Tween 60) and
stearate-LDH (SDS) as a function of temperature. There is a notable shift in the
diffraction peak positions at temperatures above ca. 85 °C. This is an indication of
a phase change. The first observed shift coincides with the onset of the first
melting endotherm observed in the DSC scans in Figures 30 and 31. The observed
phase change is associated with a decrease in the LDH d-spacing from ca. 5.1 nm
to about 4.7 nm and at the same time there is an increase in the separation
between the alkyl chains from 0.406 nm (2θ = 25.42°) to about 0.425 nm (2θ =
24.6°). This change in peak positions results in a peak broadening, which suggests
a transition towards disorder. This is due to the loss of water and melting of the
intercalated chains. This behaviour is non-reversible and coincides with the results
obtained on DSC.
76
Figure 32 indicates that the layer starts to contract above 85 °C. Above 135 °C the
material becomes amorphous, possibly due to reaction between the free
carboxylates and the magnesium and aluminium hydroxide groups. Borja and
Dutta (1992) attributed this disordering behaviour of the bimolecular film to the
formation of kinks and gauge blocks in the alkyl chains. However, the LDH-CO3
impurity peak remains visible and only disappears slowly at higher temperatures.
T, °C
Intensity, a.u.
50
60
70
80
90
100
110
120
130
140
150
Cool
0
5
10
15
20
25
25
30
2θ
θ /° (CoKα
α)
Figure 30: Effect of temperature on the X-ray diffraction spectra of LDH-stearate
synthesised at 80 °C using Tween 60 (scans taken at 5 °C/min intervals)
The effect of temperature on the corresponding peak intensities for stearate-LDH
(SDS) is shown in Figure 31. This confirms that the onset temperature for the
transition is at ca. 85 °C and disappears at ca. 125 °C.
77
Relative intensity, a.u. .
1.0
d = 5.10 nm
0.8
0.6
d = 4.7 - 4.5 nm
d = 0.406 nm
0.4
d = 0.425 nm
0.2
0.0
55
75
95
115
135
Temperature, °C
Figure 31: Effect of temperature on the intensity of the selected X-ray diffraction
peaks of LDH-stearate (SDS) (scans taken at 5 °C/min intervals)
Recovery of this material is possible when the upper temperature to which the
sample is heated is limited to 100 °C and it is subsequently cooled down to 30 °C,
as shown in Figure 32. In contrast, Inomata and Ogawa (2006) reported a
reversible behaviour of the stearate-LDH by in situ XRD in the temperature range
16 to 140 °C. The original d-spacings were also recovered on cooling.
78
Intensity, a.u.
T, °C
30
40
50
60
70
80
90
100
90
80
70
60
50
40
30
Heating
Cooling
0
5
10
15
20
25
30
2θ
θ / ° (CoKα
α)
Figure 32: Changes in the X-ray diffraction spectra on heating LDH-stearate
(SDS) to 100 °C and cooling it to 30 °C (scans taken at 5 °C/min intervals)
Similar behaviour is observed in the LDH-laurate sample shown in Figure 33. In
this case the phase transition starts at ca. 65 to 95 °C. This result coincides with
the DSC results given in Figure 29, which show the melt endotherm at ca. 97 °C.
In this case the LDH layers start to contract in this range. At temperatures above
95 °C, the amorphous phase starts to form. The diffraction peak in plane d003
broadens at 115 °C, indicating the phase change to amorphous. However, the
LDH impurity peaks remain unchanged.
79
T, °C
25
35
45
55
65
75
85
95
105
115
125
135
Intensity, a.u.
Laurate-LDH
Cool
0
5
10
15
20
25
25
30
2θ
θ /° (CoKα
α)
Figure 33: Changes in X-ray diffraction spectra of LDH-laurate on heating to 135
°C and cooling to ambient (scans taken at 5 °C/min intervals)
4.10
Particle Morphology
Figure 34 shows SEM images of pure fatty acid intercalated LDH in comparison
with LDH-CO3. The LDH-CO3 consisted of numerous smaller crystals inter-grown in
a ‘sandrose’ arrangement, as shown in Figure 34(a) (Adachi-Pagano et al., 2000).
After the reaction with stearic acid (as shown in Figure 28 b), this structure was
replaced by the low-aspect-ratio flakes which were significantly larger – as much
as 20 µm across. The change in crystal size morphology indicates that the
intercalation was accompanied by a recrystallisation process. The stearate-LDH
crystals are larger than those of the precursor before (Figure 34 b) and after
(Figure 36 c) extraction with ethanol.
Dimotakis and Pinnavaia (1990) reported that intercalation occurs in a topotactic
manner. In contrast, the present reactions did not proceed in a topotactic manner.
This is an expected behaviour considering that the stearate monolayers provide a
template for Mg-Al-LDH growth (He et al., 2004). Well-defined crystal morphology
80
is obtained in stearate-LDH (Tween 60). However, flaky rod-like crystal
morphology is obtained for LDH-laurate. The reason for this is not well
understood. The other fatty acid intercalated-LDH SEM images are shown in
Appendix E.
(a)
(c)
(b)
(d)
(e)
Figure 34: SEM images showing: (a) the ‘sandrose’ morphology of LDH-CO3
crystals; (b) the flake-like habit of LDH-stearate (SDS); (c) the delamination of the
LDH-stearate crystals after extraction with ethanol; (d) the LDH-stearate crystals
obtained with Tween 60; and (e) the LDH-laurate crystals
81
82
5
CONCLUSION
Molten carboxylic acid reacts with LDH-CO3 when dispersed in an aqueous
medium under atmospheric conditions to form a bilayer intercalated LDH. The
interlayer carbonate anions can be easily replaced by the carboxylate anions in the
LDH galleries. The greater stability of the bilayer intercalated LDH-stearate
compared with LDH-CO3 arises from the stabilising effect of the hydrophobic
interactions between the chains (Choy et al., 1999; Takagi et al., 1993).
The carboxylate anions are intercalated in different forms, as evident from FT-IR.
The stearate and behenate anions exist in the forms –RCOOH and –RCOO- in
between the brucite-like LDH sheets. This is evident from the absorption peaks at
1558, 1536 and 1538 cm-1 for both samples (Borja and Dutta, 1992). However, in
the LDH-octanoate, laurate and myristate samples only the ionised carboxylate
form is present, as evident by the FT-IR absorption bands at 1559, 1558, and
1555 cm-1 respectively. The close agreement between the XRD and FT-IR analyses
of the present results and those previously obtained (Borja and Dutta, 1992;
Kanoh et al., 1999) suggests successful carboxylate intercalation.
The addition of an anionic surfactant (SDS) positively influenced the intercalation
process. It acts as a dispersant for the LDH-CO3 particles and keeps the unreacted
stearic acid in emulsion, thereby facilitating its removal from the final product. The
purer carboxylate intercalated LDH is obtained with less mixing and purification
effort, as is evident from the sharp diffraction peaks obtained in the XRD results.
The method works well with long-chain carboxylic acids, i.e. C12 to C22. However,
at low reaction temperatures with short-chain carboxylic acids – C2, C4, C6 and
C10 – the LDH layers preferred to intercalate the anionic surfactant instead of the
desired carboxylate anion. Kanoh et al. (1999) attributed this to the
hydrophobicity of the sodium salts used in the attempt to intercalate these anions
at low temperatures. In contrast, the present method employs neat acids and
even octanoic acid was intercalated as a bilayer with ease. Temperature is
83
therefore the most important parameter to control during the intercalation
process. In the present method the intercalation was successful at temperatures
higher than the melting point of the carboxylic acids. Bilayer intercalated LDH
products were previously obtained by exchanging the Cl- in LDH-Cl with fatty acids
in ethanol and with sodium carboxylates in aqueous media (Borja and Dutta 1992;
Kanoh et al., 1999). Inomata and Ogawa (2006) also obtained bilayer intercalated
LDH-stearate using the reconstruction method. In the present method all the
carboxylate anions (laurate, myristate, stearate and LDH-behenate) were
intercalated as bilayers.
The surfactant-mediated intercalation method is an environmentally friendly option
compared with the methods reported in the literature. The anionic surfactant
(SDS) solution should be recyclable and no volatile or flammable organic solvents
are necessary to free the product from the excess stearic acid. However, XRF
analysis reveals that the use of SDS also leads to sodium carboxylate in addition to
the carboxylic acid forms. This problem can be avoided by replacing SDS with a
non-ionic surfactant, such as Tween 60.
The LDH carboxylate intercalates obtained with either surfactant showed similar
thermal behaviours. Specifically, two phase transitions are observed at elevated
temperatures. At temperatures that are higher than the melting point of the
corresponding free acids, the alkyl chains assume a disordered liquid-like state
within the clay galleries. In addition, an increase in temperature results in a
decrease in the interlayer spacing. This state is reversible to some extent.
However, at even higher temperatures, the material becomes completely
amorphous and behaves like a true melt. Cooling does not lead to the recovery of
the well-ordered crystalline state. Inomata and Ogawa (2006) reported a
reversible thermal behaviour of LDH-stearate.
The effects of temperature on the material properties of intercalated LDH are very
important when the materials are to be used in polymeric materials. This is
because the degree of dispersion is very important and depends on the
84
intermolecular interaction between the polymer and the modified clay surface.
This has an influence on the performance of the final material (Inomata and
Ogawa, 2006). The temperature effects on the properties of LDH-carboxylate
obtained in the present method have two important implications. First, it appears
that, once the LDH-carboxylate is fully molten, the presence of interlayer water
may be required for recrystallisation. Secondly, exfoliation is the main requirement
for conventional polymer melt-blending. However, exfoliation of stearate
intercalated LDH is unlikely to proceed when conventional melt-blending
techniques are used because this sample melts below typical polymer processing
temperatures. Furthermore, crystals are only stable if they form a stacking
structure of at least 20 sheets (He et al., 2002).
85
6
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105
APPENDIX A:
EXPERIMENTAL METHODS
A.1
Synthesis procedure for monocarboxylate intercalated LDH
Table
A.1
below
gives
the
detailed
procedure
followed
to
synthesise
monocarboxylate intercalated LDH. All the mixtures were heated for 9 hours per
day and allowed to cool down overnight at room temperature. This was repeated
four times. In some instances the heating was done from high temperature for
one cycle and then to the melting point of the acid in the three remaining cycles.
Table A.1: Synthetic procedure followed for intercalation of fatty acids in LDH
LDH-acetate
40 g SDS, 4.12AEC acetic acid (21.02 g), 20 g LDH-CO3 stirred in
(C2)
1 500 ml of distilled water for 2 days. pH of the mixture was 10.01. Final
pH = 10.01
Experimental yield (acetate-LDH) = 24.61 g
Expected yield (calculated from acetic acid and LDH-CO3) = 41.02 g
60% product yield was recovered.
LDH-
1) 40 g SDS, 4.12AEC butyric acid (30.84 g), 20 g LDH-CO3 stirred in
butyrate
1 500 ml of distilled water at room temperature for 2 days. pH of the
(C4)
mixture was 10.01
Experimental yield (LDH-butyrate) = 14.74 g
Expected yield (calculated from LDH-CO3 and butyric acid) = 50.84 g
29% product yield was recovered.
2) The experiment was repeated at 80 °C using 4.5AEC, which amounted
to 33.83 g butyric acid. 29% product yield was recovered
LDH-
40 g SDS, 4.12AEC hexanoic acid (40.66 g), 20 g LDH-CO3 dissolved in
hexanoate
1 500 ml distilled water at room temperature and allowed to stir for
(C6)
2 days. Final pH = 10.03
Experimental yield (LDH-hexanoate) = 4.46 g
Expected yield (calculated from LDH-CO3 and hexanoic acid) = 60.66 g
7.35% product yield was recovered.
Octanoate-
1) 40 g SDS, 4.12AEC octanoic acid (50.6 g), 20 g LDH-CO3 stirred in
106
LDH (C8)
1 500 ml of distilled water at room temperature for 2 days. Final pH =
10.01
Experimental yield (LDH-octanoate) = 7.77 g
Expected yield (calculated from LDH-CO3 and octanoic acid) = 70.60 g
11% product yield was recovered.
2) The experiment was repeated using 4.5AEC amounting to 55.38 g of
octanoic acid. 35% product yield was recovered.
3) The experiment was repeated at 80 °C and 33% product yield was
recovered.
LDH-
1) 40 g SDS, 4.12 AEC decanoic acid (60.29 g), 20 g LDH-CO3 stirred in
decanoate
1 500 ml of distilled water at 50 °C for 3 days, with partial addition of
(C10)
decanoic acid and 1 day without acid addition. Final pH = 10.00
Experimental yield (LDH-decanoate) = 10.92 g
Expected yield (calculated from LDH-CO3 and decanoic acid) = 80.29 g
13.6% product yield was recovered.
2). The experiment was repeated using 4.5AEC (66.14 g) of decanoic acid
and heating was from 37 to 32°C. 21% product yield was recovered.
LDH-laurate
1) 40 g SDS, 4.12AEC lauric acid (70.4 g), 20 g LDH-CO3 stirred in
(C12)
1 500 ml of distilled water at 60 °C for 3 days, with partial addition of
lauric acid and 1 day without acid addition. Final pH = 10.01
Experimental yield (LDH laurate) = 28.93 g
Expected yield (calculated from HT and lauric acid) = 90.40 g
32% product yield was recovered.
2) A similar procedure was followed with heating from 53 to 48 °C and
37% product yield was recovered.
3) The experiment was repeated using 4.5AEC (76.92 g) lauric acid at 65,
70 and 80°C. Product yields of 35, 45 and 39% respectively were
recovered.
107
LDH-
1) 40 g SDS, 4.12 AEC myristic acid (80.63 g), 20 g LDH-CO3 stirred in
myristate
1 500 ml of distilled water at 60 °C for 3 days, with partial addition of
(C14)
myristic acid and 1 day without acid addition. Final pH = 10.05
Experimental yield (LDH-myristate) = 15.63 g
Expected yield (calculated from LDH-CO3 and myristic acid) = 100.63 g
15.5% product yield was recovered.
2) The experiment was repeated using 4.5AEC (87.67g) myristic acid with
heating from 59 to 54 °C. 32% product yield was recovered.
LDH-
1) 40 g SDS, 4.12AEC stearic acid (100 g), 20 g LDH-CO3 stirred in
stearate
1 500 ml of distilled water at 80 °C for 3 days, with partial addition of
(C18)
stearic acid and 1 day without acid addition. Final pH = 10.00
Experimental yield (LDH-stearate) = 57.60 g
Expected yield (calculated from LDH-CO3 and stearic acid) = 120 g
48% product yield was recovered.
2) The experiment was repeated with the mixture being heated from 75
to 70 °C. 20% product yield was recovered.
3) The experiment was also repeated at 50, 60, 65, 70, 75 and 85 °C
using 4.5AEC (109.24 g stearic acid) and at 80 °C using SDS. Product
yields of 40, 22, 40, 43, 71 and 64% respectively were recovered.
LDH-LDH-
1) 40 g SDS, 4.12AEC behenic acid (119.21 g), 20 g LDH-CO3 stirred in
behenate
1 500 ml of distilled water at 90 °C for 3 days, with partial addition of
(C22)
behenic acid and 1 day without acid addition. Final pH = 10.02
Experimental yield (LDH-LDH-behenate) = 41.76g
Expected yield (calculated from LDH-CO3 and behenic acid) = 139.21 g
37% product yield was recovered.
2) The experiment was repeated using 4.5AEC (130.78 g) behenic acid.
The mixture was heated from 85 to 80 °C. 93% product yield was
recovered.
108
APPENDIX B:
B.1:
THERMAL ANALYSIS
Expected TG mass loss after the first and last thermal events
Thermal decomposition steps: [Mg1-xAlx(OH)2] (CO3)x/2.0.5 H2O, where x = 0.3114
Molecular mass = 248.88 g/mol
After the first thermal event: dehydrated LDH: [Mg1-xAlx(OH)2] (CO3)x/2
Molecular mass = 219.95 g/mol
Expected %residual mass (LDH dehydrated) = 100 ×
= 100 ×
MM(dehydrated LDH)
MM(LDH)
219.95 g/mol
248.48 g/mol
= 88.38%
Last thermal event: MgO and Al2O3
Molecular mass = 140.10g/mol (LDO, based on the assumption that the residue
contains only MgO and Al2O3)
Expected %residual mass (at 700°C) = 100 ×
MM(dehydrated LDH)
MM(LDH)
= 100 ×
140.10 g/mol
248.48 g/mol
= 56.26%
109
B.2:
Thermogravimetric
curves
and
the
derivative
fatty
acid
intercalated LDH
0.0
80
60
-0.5
40
Acetate-LDH
20
0
0
Derivative mass, %/°C .
Residual mass, % .
100
-1.0
100 200 300 400 500 600 700
Temperature, °C
Figure B1: TG/DTG curves of acetate intercalated LDH synthesised at room
temperature
110
0.0
80
Butyrate-LDH at
room temperature
60
-0.5
40
Butyrate-LDH
at 80°C
20
0
0
Derivative mass, %/°C .
Residual mass, % .
100
-1.0
100 200 300 400 500 600 700
Temperature, °C
Figure B2: TG/DTG curves of butyrate intercalated LDH synthesised at room
temperature and at 80 °C
0.0
80
60
40
Hexanoate-LDH
20
0
0
Derivative mass, %/°C
.
Residual mass, % .
100
-0.5
100 200 300 400 500 600 700
Temperature, °C
Figure B3: TG/DTG curves for hexanoate intercalated LDH at room temperature
111
0.0
80
60
Room
temperature
40
20
Octanoate-LDH
0
0
80°C
Derivative mass, %/°C .
Residual mass, % .
100
-0.5
100 200 300 400 500 600 700
Temperature, °C
Figure B4: TG/DTG curves of octanoate intercalated LDH synthesised at room
temperature and at 80 °C
0.5
Decanoate-LDH at
50°C
80
Decanoate-LDH
from 37-32°C
60
0.0
40
20
0
0
Derivative mass, %/°C .
Residual mass, % .
100
-0.5
100 200 300 400 500 600 700
Temperature, °C
Figure B5: TG/DTG curves of decanoate intercalated LDH synthesised at 50 °C
and from 37 to 32 °C
112
Derivative weight loss, %/°C
100
90
80
70
60
54-48°C
60°C
65°C
70°C
80°C
50
40
30
20
10
0
0
200
400
600
Temperature/°C
Figure B6: TG curves of LDH-laurate prepared at 54–48, 60, 65, 70 and 80 °C
Derivative weight loss,%/°C
0
54-48°C
-0.5
60°C
65°C
70°C
80°C
-1
0
200
400
Temperature/°C
600
Figure B7: Derivative of weight loss curves of LDH-laurate obtained at 54-48, 60,
65, 70 and 80 °C
113
0.5
Myristate-LDH at
60°C
80
60
0.0
40
20
Derivative mass, %/°C .
Residual mass, % .
100
Myristate-LDH at 59-54°C
0
0
100
200
300
400
500
600
-0.5
700
Temperature, °C
Figure B8: TG/DTG curves of LDH-myristate prepared at 59–54 and at 60 °C
100
%Residual mass loss
90
50°C
80
70
60°C
60
70°C
50
75°C
40
75-70°C
30
80°C
20
80°C tween
60
85°C
10
0
0
200
400
Temperature/°C
600
Figure B9: TG curves of LDH-stearate prepared at 50, 60, 70, 75, 75–70, 80 and
85oC using SDS and at 80 °C using Tween 60
114
Derivative weight loss, %/°C
0
-0.5
50°C
60°C
70°C
75°C
-1
75-70°C
80°C
80°C tween 60
85°C
-1.5
0
200
400
600
Temperature/°C
Figure B10: Derivative of weight loss curves of LDH-stearate prepared at 50, 60,
100
0.5
80
0.0
60
-0.5
Behenate-LDH at 90°C
40
-1.0
Behenate-LDH at 85-80°C
20
-1.5
0
-2.0
700
0
100
200
300
400
500
600
Derivative mass, %/°C .
Residual mass, % .
70, 75, 80 and 85 °C
Temperature, °C
Figure B11: TG/DTG curves of LDH-behenate prepared at 90 and 85–80 °C
115
0
Derivative mass loss,
%/°C
Residual mass, % .
100
80
60
40
LDH-SDS
20
LDH-tween 60
0
0
200
400
-0.045
600
Temperature, °C
Figure B12: TG/DTG curves of the product obtained by dispersing the LDH in
distilled water in the presence of the surfactants SDS (LDH-SDS) and Tween 60
(LDH-Tween 60) at 80 °C
116
APPENDIX C:
FT-IR
Acetate-LDH
% Transmittance
Butyrate-LDH
4000
Hexanoate-LDH
Decanoate-LDH
3600
3200
2800
2400
2000
1600
1200
800
400
-1
Wavenumber, cm
Figure C1: FT-IR spectra of short-chain carboxylates: acetate, butyrate,
hexanoate and decanoate acid intercalated LDH, obtained at room temperature
and at 50 °C for decanoate
117
% Transmittance
80°C
Room
temperature
4000
3400
2800
2200
1600
1000
400
Wavenumber, cm-1
Figure C2: FT-IR spectra of LDH-octanoate prepared at room temperature and at
80 °C
118
80°C
% Transmittance
70°C
65°C
60°C
53-48°C
4000
3600
3200
2800
2400
2000
1600
1200
800
400
Wavenumber, cm-1
Figure C3: FT-IR spectra of LDH-laurate prepared at 80, 70, 65 and 60 oC and
from 53–58 °C
119
85°C (SDS)
80°C (tween 60)
% Transmittance
80°C (SDS)
75°C (SDS)
70°C (SDS)
60°C (SDS)
4000
3600
3200
2800
2400
2000
1600
1200
800
400
Wavenumber, cm-1
Figure C4: FT-IR spectra of LDH-stearate prepared at 60, 70, 75, 80 and 85 °C
using SDS and at 80 °C using Tween 60
120
Intensity, a.u
APPENDIX D: XRD RESULTS
LDH-acetate
0
5
10
15
20
25
30
2θ
θ /°
Intensity, a.u
Figure D1: XRD pattern of LDH-acetate prepared at room temperature
LDH-butyrate
0
5
10
15
20
25
30
2θ
θ /°
Figure D2: XRD pattern of LDH-butyrate prepared at 80 °C
121
Intensity, a.u
LDH-octanoate
0
5
10
15
20
25
30
25
30
2θ
θ /°
Intensity, a.u
Figure D3: XRD pattern of LDH-octanoate prepared at 80 °C
LDH-laurate
0
5
10
15
20
2θ
θ /°
Figure D4: XRD pattern of LDH-laurate prepared at 80 °C
122
Intensity, a.u
LDH-myristate
0
5
10
15
20
25
30
2θ
θ /°
Intensity, a.u
Figure D5: XRD pattern of LDH-myristate prepared at 60 °C
0
LDH-stearate
5
2θ
θ /°
10
15
20
Figure D6: XRD pattern of LDH stearate prepared at 80 °C using SDS
123
Intensity, a.u
LDH-behenate
0
5
10
15
20
25
30
2θ
θ /°
Intensity, a.u
Figure D7: XRD pattern of LDH-behenate prepared at 90 °C
LDH-stearate and magnesium stearate mixture
0
5
10
15
20
2θ
θ /°
Figure D8: XRD pattern of a mixture of LDH-stearate and magnesium stearate
prepared at 80 °C
124
APPENDIX E:
SEM RESULTS
Figure E1: SEM image of LDH-acetate
Figure E2: SEM image of LDH-butyrate
125
Figure E3: SEM image of LDH-octanoate
Figure E4: SEM image of LDH-behenate
126
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