2012 1

2012 1
Int. J. Mol. Sci. 2012, 13, 6279-6291; doi:10.3390/ijms13056279
International Journal of
Molecular Sciences
ISSN 1422-0067
Synthesis and Characterization of Hybrid Materials Consisting
of n-octadecyltriethoxysilane by Using n-Hexadecylamine as
Surfactant and Q0 and T0 Cross-Linkers
Ismail Warad 1,*, Omar Abd-Elkader H 2,3, Saud Al-Resayes 1, Ahmad Husein 4,
Mohammed Al-Nuri 4, Ahmed Boshaala 5, Nabil Al-Zaqri 1 and Taibi Ben Hadda 6
Department of Chemistry, Science College, King Saud University, P.O. Box 2455, Riyadh 11451,
Saudi Arabia; E-Mails: resayes@ksu.edu.sa (S.A.-R.); nabil_alzaqri@yahoo.com (N.A.-Z.)
Electron Microscope Unit, Zoology Department, College of Science, King Saud University,
Riyadh 11451, Kingdom of Saudi Arabia; E-Mail: omabdelkader7@yahoo.com
Electron Microscope & Thin Films Department, Physics Division, National Research Center,
Dokki 12622, Cairo, Egypt
Department of Chemistry, Science College, AN-Najah National University, P.O. Box 7, Nablus
00972, Palestine Territories; E-Mails: hamaydah2500@yahoo.com (A.H.);
mabnuri@yahoo.com (M.A.-N.)
Chemistry Department, Faculty of Science, Benghazi University, P. O. Box 1308, Benghazi, 22385,
Libya; E-Mail: ahmedboshaala@yahoo.co.uk
Materials Chemistry Laboratory, Faculty of Sciences, University of Mohammed Premier,
Oujda-60000, Morocco; E-Mail: taibi.ben.hadda@gmail.com
* Author to whom correspondence should be addressed; E-Mail: warad@ksu.edu.sa;
Tel.: +96-61-4675992; Fax: +96-61-4675992.
Received: 12 April 2012; in revised form: 3 May 2012 / Accepted: 10 May 2012 /
Published: 21 May 2012
Abstract: Novel hybrid xerogel materials were synthesized by a sol-gel procedure.
n-octadecyltriethoxysilane was co-condensed with and without different cross-linkers using
Q0 and T0 mono-functionalized organosilanes in the presence of n-hexadecylamine with
different hydroxyl silica functional groups at the surface. These polymer networks have
shown new properties, for example, a high degree of cross-linking and hydrolysis. Two
different synthesis steps were carried out: simple self-assembly followed by sol-gel
transition and precipitation of homogenous sols. Due to the lack of solubility of these
Int. J. Mol. Sci. 2012, 13
materials, the compositions of the new materials were determined by infrared spectroscopy,
C and 29Si CP/MAS NMR spectroscopy and scanning electron microscopy.
Keywords: sol-gel; solid state NMR; cross-linkers; stationary phases
1. Introduction
Alkyl stationary phases are widely used in liquid chromatography (LC). Much effort has been made
to prepare and describe the chromatographic properties [1]. In this field, the application of stationary
phases based on silica gel is very popular. For the successful employment of silica, it is of great
importance that the silica beads show a narrow particle size distribution and spherical shape. Most
HPLC separations are carried out under reversed phase (RP) conditions [2]. The surface modification
of well-defined silica beads with T-silyl functionalized organic systems is the major route to create
phases for reversed-phase liquid chromatography (RPLC) [3–5]. All the RP prepared by the different
conventional methods (solution or surface polymerized modification and monomeric synthesis)
mentioned above do not exhibit complete cross-linked T and Q units. For long-term stability of
stationary phases it is very important to prepare one with a high degree of cross-linked ligands. This
may be achieved using the sol-gel process.
Sol-gel processing of polysiloxanes to prepare hybrid inorganic-organic materials (HIOM) [6–9] is
quite a promising technique in field of chromatography [10,11]. It has been widely investigated to
prepare potential matrices for reporter molecules in chemistry sensors [12]. More recently, such
materials have also received attention as catalyst supports, enzymes and catalytically active transitions
metal complexes [13].
Recently, many investigations were extended to nanostructure mesoporous silicas which are based
on inorganic-organic hybrid polymers [14–19]. These materials possess extremely high surface areas
and accessible pores. Moreover, the pore size can be tuned with different sizes in the nanometer range
by choosing template systems or with a co-solvent [15]. For the generation of stationary phases,
T-functionalized silanes of the type Fn-Si(OR)3 were sol-gel processed with and without
co-condensation agents, which play an important role in controlling the density and the distance of the
reactive centers; in general Fn represents either alkyl spacer alone or end by a metal complex [20].
These reactive centers are distributed across the entire carrier matrix and play an important role
in catalysis [21–38].
In this work, hybrid materials were synthesized by a simple one-step self assembly followed by
co-condensation of different silane cross-linkers with n-octadecyltrialkoxysilane as the
chromatographic selector in the presence of n-hexadecylamine surfactant. Application of different
alkoxysilane co-condensation agents, such as Si(OEt)4, Si(OMe)4 (Q0) and MeSi(OMe)3 (T0) during
the sol-gel processes enable the building of polymer frameworks with different hydroxyl silica
functional groups and new properties compared to surface modified silica gel.
Int. J. Mol. Sci. 2012, 13
2. Results and Discussion
2.1. Sol-Gel Processing
The properties of the sol-gel processed products strongly depend on the reaction conditions, such as type
of solvent, temperature, time, catalysts, concentration of the monomers and type of the cross-linkers [9,10].
To ensure comparable results, uniform reaction conditions throughout hydrolysis and the sol-gel
transition have been maintained. Alcohol is necessary to homogenize the reaction mixture. All
poly-condensations were performed in a mixture of ethanol with a certain amount of water as catalyst
for the sol-gel process (see Experimental section). Long alkyl chains commonly used for liquid
chromatography were introduced by using the CH3(CH2)17Si(OEt)3, n-octadecyltriethoxysilane (X), in
the presence of n-hexadecylamine as template, which concomitantly serves as material to allow
mesoporous hybrid compounds. During the sol-gel process, a fixed ratio of X, template and
co-condensation agents [1:9:9], respectively, were used. Several types of co-condensation agents used,
such as Si(OEt)4 (TEOS), Si(OMe)4 (TMOS) and MeSi(OMe)3, are shown in Scheme 1.
Scheme 1. Synthesis of xerogels X0–X3: Self-assembly followed by sol-gel process at
room temperature using several cross-linkers and the amine as template.
i) no co-condensing agent
R = C16H33
C15 C15
ii) With one of the
following condensing agent
1 Si(OEt)4 Q
2 Si(OMe)4 Q
3 MeSi(OMe)3 T
C15 = C15H31
X1 = [1 X: 9 Template: 9 Si(OEt)4]
1- Hydrolsis
2- Extraction (EtOH)
to remove the template
Int. J. Mol. Sci. 2012, 13
After sol-gel processing, the amine was removed from the mixture by Soxhlet extraction with
ethanol. Four xerogels (X0–X3) were collected and are illustrated in Table 1.
Table 1. Sol-gel processes, yields and labeling of the materials.
Cross-Linkers Types
Yield %
77. 6
Silyl Fragments
T2 and T3
T2, T3, Q3 and Q4
T2, T3, Q3 and Q4
T2 and T3
Two kinds of stationary phases were obtained: (i) xerogels X0 were synthesized by co-condensation
of X and template with zero concentration (0) of cross-linker, and (ii) xerogels X1–X3 were
synthesized by co-condensation of X, template with monofunctional Q0 and T0 cross-linkers such as
Si(OEt)4, Si(OMe)4 and MeSi(OMe)3, respectively.
2.2. Solid-State NMR Spectroscopic Investigations
Due to cross-linking effects, the solubility of the polymeric materials X0–X3 is rather limited.
Therefore, solid-state NMR spectroscopy was used as a powerful technique for their characterization.
Si CP/MAS NMR spectroscopy
Silane functionality and bonding chemistry can easily be determined by 29Si CP/MAS NMR
spectroscopy. The universal chemical shift and symbols for these 29Si function groups are collected in
Scheme 2.
Scheme 2. The universal 29Si chemical shifts, symbols and orders of silyl species.
Int. J. Mol. Sci. 2012, 13
Scheme 2. Cont.
The average chemical shifts for T2 (δ = –58.8), and T3 (δ = −65.1), Q3 (δ = −101.9), Q4
(δ = −109.5) species are significantly changed by the incorporation of the different types of
co-condensation agents and are in agreement with values reported in the literature for comparable
systems [37]. Since all silicon atoms are in direct proximity of protons the Hartmann–Hahn [29–33]
match could efficiently be achieved.
The signal assignment of silyl fragments can be summarized as follows: a higher degree of
cross-linking of silicon species and/or an increase of oxygen neighbors leads to an upfield shift in
NMR spectra. Difunctional species (Dn) appear in the region of −7 to −20 ppm, tri-functional species
(Tn) from −49 to −66 ppm, and signals from the native silica (Qn) from −91 to −110 ppm. The high
condensation degrees which were obtained through the sol-gel processes and monitored by 29Si NMR
is mainly resonated to the employment of T2, T3, Q3 and Q4 as cross-linkers [28–35].
Figure 1. 29Si CP/MAS NMR spectra of X0–X3 materials which were prepared by using
Si(OEt)4, Si(OMe)4 and MeSi(OMe)3 as cross-linkers.
All the spectra of xerogels (X0–X3) show signals for substructures corresponding to T and Q
functions only, the D or M silyl fragment has not been observed, as seen in Figure 1. X0–X3 showed T
silyl fragment only in general 3 order predominated over 2, which supports the full cross-linked
Int. J. Mol. Sci. 2012, 13
organosiloxane species, demonstrating the incorporation of the function groups within the framework
walls of the mesostructures. T2 silyl fragment indicted that one Si-OH function group of the sector or
cross-linkers MeSi(OMe)3 was not incorporated in the sol-gel process. X0 and X3 revealed only T silyl
fragment, and this was expected since no Q co-condensation agents were used. X1 and X2 showed
both T and Q with 2 and 3 order, because both the sector T silyl fragment and Q silyl fragment of the
co-condensation agents were investigated in the sol-gel process; Q predominated over T due to the
concentrations used [9:1] respectively. The respective NMR spectra of the different materials’ phases
and the native silica are shown in Figure 1.
2.3. 13C CP/MAS NMR Spectroscopy
More detailed information on bonded phase architecture was obtained from the 13C CP/MAS NMR
spectra of the supported matrices X0–X3, and the corresponding signal assignments are summarized in
Figure 2. Characteristic peaks at approximatelyδ = 14.0 ppm are assigned to the carbon atom of CH3
and the carbon atom of the silicon adjacent methyl groups (SiCH2) in the Si–O–Si substructure
δ = 22.8 ppm is assigned to CH2CH3 and SiCH2CH2 are significant for all the stationary phases. Weak
signals of 13C were assigned to residual Si–OR (R = Me or Et) functionalities at around δ = 17.0 and
57.9 ppm, attributed to non-hydrolyzed EtO (X1) or MeO (X2 and X3) silica functional groups, which
point to a high degree of hydrolysis compared to the literature [7,9].δ = −3.6 ppm is assigned to SiMe
in case of using MeSi(OMe)3. The NMR investigation of such stationary bonded phases exhibits two
signals for the main chain methylene carbons at 30.0 and 32.2 ppm (gauche/trans conformations,
respectively) as shown in Figure 2 [5].
The samples X1, X2 and X3 show two peaks related to the main chain (CH2)14 at 30.0 and
32.0 ppm which represent two different conformations of the alkyl chain. The signal at 32.8 ppm
characterizes trans conformations, and the signal at 30.0 ppm reveals the existence of gauche
conformations. The effect of different motilities is only visible in the 13C CP/MAS NMR spectra of the
xerogels containing alkyl chains with 18 carbon atoms at least; the alkyl chain order is found to
increase with increasing chain length from C18 to C34 [36].
The NMR investigation of C18 bonded phases here exhibits only one signal for the main chain:
methylene carbons at 32.8 belong to the trans conformation when no co-condensation agent was
investigated (X0). When TESO or TMSO (Q0) was introduced to the sol-gel, the presence of
the Me-Si function group in the cross-linker plays a key role in the conformation; the trans
conformation was encouraged. The main reason of such a phenomenon is not yet clear, it could be
related to the un-hydrolysis Me-Si steric factor of remaining Me on the X3 polysiloxane surface
affecting the C18 chain not to bend away to form a gauche conformation, thus enhancing the C–H
Wander Val forces to form sort of stabilizing straight line chains as in Scheme 3. This proposal is
compatible with the decrease in the trans conformation at the expense of the alternative gauche
conformation increasing upon increasing the temperature [9,28–30].
Int. J. Mol. Sci. 2012, 13
Figure 2. 13C CP/MAS NMR spectra of X0-X3 materials.
Scheme 3. Schematic illustration of trans and gauche alkyl chain arrangements.
Here we report a new method at room temperature to control the gauche/trans conformation ratio of
the alkyl chains in the stationary phase by using fixed concentrations of several type of T0 and Q0
Int. J. Mol. Sci. 2012, 13
2.4. IR Investigations
The IR spectra of the desired materials, X0, X1, X2 and X3 in particular, show several peaks that
are attributed to stretching and bending. The broad intensive stretching vibrations at 2980–2840 cm−1
and bending vibrations at 1150–950 cm−1 belonging to (vCH) of the SiCH2(CH2)16CH3 functional
groups were the main IR active functional groups. A typical example of the IR behavior of X3 s
illustrated in Figure 3b.
In order to confirm the full sol-gel reaction, the structural vibration behaviors of these compounds
against the infrared spectra of X as the starting material and X3 as xerogel before and after the sol-gel
processes were investigated and compared, such as the typical examples in Figure 3.
The broad intensive stretching vibrations at 2980–2840 cm-1 of (vCH) belong to the SiOCH2CH3
functional groups of X starting material as in Figure 3a totally disappeared after the sol-gel process to
prepare the complex X3 as seen in Figure 3b, which strongly confirms the completeness of the sol-gel
process formation.
Figure 3. Infra-red spectra (a and b) of X and X3, before and after sol-gel, respectively.
2.5. Surface Structure of the Materials
The surface area data (BET) of such materials determined by the sol-gel procedure was found to
equal 1000–1500 m2/g, 0.5–1.3 cm3/g pore volume, and 13–20 Ǻ pore size [37].
SEM micrographs of the X2 and X3 powder are given in Figure 4. Morphological features of the
samples show the difference between spherical shapes, colloidal and porous structure and irregular
particles mainly with the size 0.5–2 µm. All powders consisted of hard sub micrometer agglomerates,
which are composed of fine crystallites. Generally, these agglomerate particles are hard to break down
even with a long ultra-sonication time.
Int. J. Mol. Sci. 2012, 13
Figure 4. Scanning electron micrograph (SEM) of X2.
3. Experimental
3.1. General Methods
All reactions were carried out in an inert atmosphere (argon) by using standard high vacuum and
Schlenk-line techniques unless otherwise noted. n-octadecyltriethoxysilane was purchased from ABCR
GmbH, Germany. Prior to use, n-hexane and Et2O were distilled from both LiAlH4 and
sodium/benzophenone, respectively. All other chemicals were obtained from Merck KGaA Darmstadt,
Germany, and were used without further purification.
3.2. Method of Characterization
C CP/MAS NMR spectra were recorded on a Bruker ASX 300 spectrometer (Bruker GmbH,
Rheinstetten, Germany) at a spinning rate of 4000 Hz with 7 mm double bearing rotors of ZrO2 and
proton of 90°pulse length and 7 μs was used. The contact time and delay time were 3 ms and 3 s,
respectively with the line broadening of 30 Hz.
Si CP/MAS NMR spectra were also collected on a Bruker ASX 300 NMR spectrometer.
Representative samples of 200–250 mg were spun at 3500 Hz using 7 mm double bearing ZrO2 rotors.
The spectra were obtained with a cross-polarization contact time of 5 ms. The pulse interval time was
1 s. Typically, 1.5 k FIDs with an acquisition time of 30 ms were accumulated in 1 kb data points and
zero-filling to 8 kb prior to Fourier transformation. Line broadening of 30 Hz and respective spectral
width were found for all spectra at about 20 kHz.
Scanning electron microscopy was performed on a JEOL JSM-6380 LA scanning electron
microscope (SEM).
3.3. General Procedure for the Sol-Gel Processes
2.2 × 10−2 mmol of n-hexadecylamine (template) in 50 mL of absolute ethanol was completely
dissolved. The addition of 2.4 × 10−3 mmol of n-octadecyltriethoxysilane (selector) to the template
Int. J. Mol. Sci. 2012, 13
solution and stirring for 3 h is an important step for self-assembly. 2.2 × 10−2 mmol of the
corresponding cross-linker was added and stirred for 10 h, at 35 oC with a molar substrate [1:9:9]
[Selector:Template:Cross-linker] respectively. An excess (5 g) of distilled water was added drop wise
to a stirred solution. This mixture was stirred for 24 h at room temperature until a gel was formed, then
the solvent was removed by reducing the pressure. For the removal of n-hexadecylamine the crude
xerogels were placed in a Soxhlet extractor containing 300 mL ethanol and the mixture was refluxed
for 3 days. Subsequently the gels were washed three times with n-hexane and ether (100 mL)
respectively, and dried under vacuum for 12 h.
Polysiloxanyloctane (X0). A mixture of 1 g (2.4 × 10−3 mmol) of n-octyltrimethoxysilane and 5.2 g
(2.2 × 10−2 mmol) n-hexadecylamine were sol-gel processed in ethanol and water to yield a colorless
swollen gel. After purification and drying 0.6 g of white powder was formed. 13C CP/MAS NMR δ = 14.0
(SiCH2 and CH3), 22.8 (SiCH2CH2 and CH2CH3), 32.8 [(CH2)14 trans conformation]. 29Si CP/MAS
NMR = −57.8 (T2), −67.1 (T3).
Polysiloxanyloctane (X1). A mixture of 1 g (2.4 × 10−3 mmol) of n-octyltrimethoxysilane, 5.2 g
(2.2 × 10−2 mmol) n-hexadecylamine and 5.5 g (2.2 × 10−2 mmol) of TEOS were sol-gel processed in
ethanol and water to yield a colorless swollen gel. After purification and drying 5.2 g of white powder
was formed. 13C CP/MAS NMR: δ = 13.2 (SiCH2 and CH3), 16.6 (OCH2CH3), 22.8 (SiCH2CH2 and
CH2CH3), 29.9 [(CH2)14 gauche conformation], 32.0 [(CH2)14 trans conformation], 59.3 (OCH2). 29Si
CP/MAS NMR: = −56.2 (T2), −66.8 (T3), −101.9 (Q3), −109.5 (Q4).
Polysiloxanyloctane (X2). A mixture of 1 g (2.4 × 10−3 mmol) of n-octyltrimethoxysilane, 5.2 g
(2.2 × 10−2 mmol) n-hexadecylamine and 5.5 g (2.2 × 10−2 mmol) of TMOS were sol-gel processed in
ethanol and water to yield a colorless swollen gel. After purification and drying 4.2 g of white powder
was formed. 13C CP/MAS NMR: δ = 13.2 (SiCH2 and CH3), 16.6 (OCH2CH3), 22.8 (SiCH2CH2 and
CH2CH3), 29.9 [(CH2)14 gauche conformation], 32.0 [(CH2)14 trans conformation], 57.8 (OCH2).29Si
CP/MAS NMR:  = −57.2 (T2), −67.8 (T3), −101.5 (Q3), −109.9 (Q4).
Polysiloxanyldodecane (X3). A mixture of 1g (2.4 × 10−3 mmol) of n-octyltrimethoxysilane, 5.2 g
(2.2 × 10−2 mmol) n-hexadecylamine and 5.2 g (2.2 × 10−2 mmol) of MeSi(OMe)3 were sol-gel
processed in ethanol and water to yield a colorless swollen gel. After purification and drying 4.5 g of
white powder was formed. 13C CP/MAS NMR: δ = −3.6 (CH3Si), 14.0 (SiCH2 and CH3), 17.2
(OCH2CH3), 22.8 (SiCH2CH2 and CH2CH3), 29.9 [(CH2)14 gauche conformation], 32.8 9
[(CH2)14 trans conformation], 57.4 (OCH2). 29Si CP/MAS NMR:  = −57.2 (T2), −67.8 (T3).
4. Conclusion
The sol-gel process offers new hybrid materials X0, X1, X2 and X3. A suitable pathway is the sol-gel
processing of n-alkyl like n-octadecyltriethoxysilane with an aliphatic amine like n-hexadecylamine as
template molecules and different cross-linkers such as Si(OEt)4, Si(OMe)4 (Q0) and MeSi(OMe)3 (T0)
carried out in ethanol at room temperature. The structure of all xerogels (X0–X3) were determined by
solid state 13C and 29Si NMR spectroscopy, infrared spectroscopy and SEM. Additionally, the mobility
of the alkyl chains and the dynamic behavior of the polymer matrix depended strongly on the type of
cross-linkers. At room temperature, pure trans was observed without cross-linker while trace amount
Int. J. Mol. Sci. 2012, 13
of trans was detected when Q0 cross-linkers were introduced. Both trans and gauche conformations in
equal amounts were recorded when T0 cross-linkers were used.
The project was supported by King Saud University, Deanship of Scientific Research, College of
Science Research Center.
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