tetraethoxysilane systems by liquid-state 29Si NMR

Colloids and Surfaces A: Physicochem. Eng. Aspects 289 (2006) 149–157

Study on the ammonia-catalyzed hydrolysis kinetics of single phenyltriethoxysilane and mixed phenyltriethoxysilane/ tetraethoxysilane systems by liquid-state 29Si NMR Xianyong Sun a,b , Yao Xu a,∗ , Dong Jiang a,b , Dongjiang Yang a,b , Dong Wu a , Yuhan Sun a , Yongxia Yang c , Hanzhen Yuan c , Feng Deng c a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China c State Key Laboratory of Magnetic Resonance & Atomic & Molecular Physics, Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China Received 1 December 2005; received in revised form 10 April 2006; accepted 11 April 2006 Available online 25 April 2006

Abstract In situ 29 Si liquid-state nuclear magnetic resonance was used to investigate the ammonia-catalyzed hydrolysis and condensation of the single phenyltriethoxysilane (PTES) systems and the mixed tetraethoxysilane (TEOS)/PTES systems dissolved in methanol. By varying the molar ratio of the PTES, water and ammonia in the initial solutions, the hydrolysis rate constants for PTES in single precursor systems were disclosed as well as the corresponding reaction orders by fitting the concentration curves of the intermediate species as functions of time. Due to the cooperation of inductive and steric effect, PTES shows a low reaction activity. Under ammonia catalysis, the hydrolysis reaction orders of TEOS and PTES in the mixed precursor systems all retained the first-order, which is similar to single precursor systems. The hydrolysis rate constants of TEOS and PTES in the mixed systems were larger than the values of TEOS and PTES in their single precursor systems, respectively. Another important result was: the reaction orders of both ammonia and water increased to different extent for TEOS and PTES in mixed systems. Hydrolysis and condensation kinetics showed more compatible hydrolysis–condensation relative rates between TEOS and PTES, which affected remarkably the final microstructure of silica particles. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrolysis; Condensation; Kinetics; 29 Si NMR; Siloxane

1. Introduction Organic–inorganic hybrid materials, also called “nanomers” [1], are now of great scientific and technological interest due to their novel properties. Sol–gel processing has been proved to be a greatly impactful method for mild synthesis of these hybrid materials with high purity and homogeneity [2–7]. One of the main attractions for the approach is that this wet way provides wide possibilities for materials processing, which is also called ‘soft chemistry’ [6]. Another important advantage is its potential for the tailed design and development of new materials with desired properties by structural manipulation at a molecular level [8,9]. ∗

Corresponding author. Tel.: +86 351 4049859; fax: +86 351 4041153. E-mail addresses: [email protected] (X. Sun), [email protected] (Y. Xu). 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.04.024

¨ StOber reaction [10], known as a typical process to fabricate this kind of materials, is a series of ammonia-catalyzed reactions of tetraethoxysilane [Si(OR)4 or TEOS; R = C2 H5 ] with water in low molecular weight alcohols to produce monodispersed spherical silica nanoparticles that range in size from 5 to 2000 nm. By introducing organic groups or polymers into the inorganic silica matrices, the properties of sol–gel derived materials can be tailored according to different requirements [8,9,11]. Up to now, a large amount of sol–gel derived hybrid materials (so-called ORMOSILs [12–14] or ORMOCERs [15] were firstly reported in early 1980s) have been prepared, aiming at the potential applications in optical devices [16–19], functional thin films [20–24], and membranes for gas separation [25–27]. Although a wealth of sol–gel synthetic approaches has been applied, a deep understanding of the underlying reactive kinetics in the sol–gel process is currently not fully developed. Even for a single precursor, TEOS for example, though the kinet-

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ics of sol–gel reactions has been formulated at various levels of sophistication [2,3,28–31], the reactive pathway is not yet well understood and no general rule can be used to guide the control of the sol–gel process. The physical and chemical properties of sol–gel derived hybrid materials depend largely on the reactive pathways of precursors and the evolution of sol–gel processes. The reaction kinetics is highly sensitive not only to the nature of the starting precursor but also to the pH value and the solvent polarity of the reaction medium [2,3,32]. Furthermore, when TEOS as a commonly used inorganic precursor is co-hydrolyzed together with an organic-substituted ethoxysilane monomer (Rx Si(OEt)4−x , where R can be methyl, ethyl, phenyl, vinyl, 3-aminopropyl, 3-methacryloxypropyl, or 3-glycidoxypropyl), more other factors will add the complexity of the initial reaction kinetics and the subsequent sol–gel process, i.e., the polarity, the chemical activity, the steric hindering, and the inductive effect of organic substituents [33–36] which lead to different relative reaction rates, thus deciding the homogeneity of the product. It has been proved that the final product properties are dependent on the early hydrolysis and condensation steps of ethoxysilanes [37], since they determine the subunits from which the further structures are made. Therefore, an investigation on the reaction kinetics of single organic-substituted monomer system and TEOS, and an organic-substituted monomer mixed system become crucial in understanding the nature of hybrid sol–gel process. Liquid-sate NMR can provide the information of soluble intermediate species at the molecular level, and 29 Si NMR has unique advantages in analyzing the hydrolysis and condensation process of siloxane [38–40]. The liquid-state 29 Si NMR signals of hydrolyzed intermediate species and oligomers can be easily observed under acidic condition. Ali´e and Pirard [41] studied the condensation in the mixture of TEOS and EDAS (NH2 CH2 CH2 NH(CH2 )3 Si(OCH3 )3 ) by 17 O NMR. They found the much faster hydrolysis of EDAS than TEOS due to the alkalescency provided by the two N atoms in EDAS. Therefore, the EDAS/TEOS cross-condensation was much prior to the TEOS self-condensation that occurred only after the exhaustion of EDAS. Brus [42,43] studied the relationship between the cross-condensation of hydrolyzed TEOS/MTES and the selfcondensation of hydrolyzed TEOS by 2D 1 H-29 Si and 1 H-1 H NMR spectra. They concluded that MTES self-condensation first occurred and the resultant cage-like polymers embedded into the continuous silica matrix subsequently produced by the TEOS self-condensation. Among the limited reports on the sol–gel kinetics of doubleprecursor systems, most of them were performed under acidic condition. The similar studies were seldom done under basic condition because of the difficulty in experiment [35,44]. The condensation species are not easy to be observed because of the faster condensation than the hydrolysis under basic condition, so that no condensed species can be detected even if in 29 Si NMR experiment. Despite the difficulties in the experiment, the ammonia-catalyzed hydrolysis process of a doubleprecursor system is still necessary and important to produce proper organic-modified silica nanocomposites through sol–gel process because the resulting materials will have microstructures

and performances different from those via acid-catalyzed route. The wide usage requires further investigation to elucidate the early stages of the basic-catalyzed process. In the present work, the ammonia-catalyzed hydrolysis kinetics of single PTES system and mixed TEOS/phenyltriethoxysilane (PTES) system in methanol is revealed with detailed rate constants being determined. 2. Experimental 2.1. Materials and components Reagent grade aqueous ammonium hydroxide (26% NH3 ), anhydrous methanol, deionized water, TEOS (Acros, 99% purity) and PTES (TCI, 98% purity) were used as received. The reaction components of single precursor (PTES) systems are presented in Table 1. The reaction components of mixed doubleprecursor systems are presented in Table 2. The solutions S1–S5 in Table 2 were prepared by ammonia-catalyzed hydrolysis of an equimolar mixture of TEOS and PTES, which was held constant at 0.5 M. The effect of ammonia content on the reaction rates can be derived from comparing S1 with S2 and S3. The effect of water content on the reaction rates can be derived from comparing S1 with S4 and S5. The effect of PTES/TEOS ratio on tetraethoxysilane can be derived from comparing S1 with S6 and S7. 2.2. In situ liquid state 29 Si NMR experiments In situ liquid state 29 Si NMR sample was prepared by mixing two solutions A and B at the temperature of 25 ◦ C. In a typical preparation process, Solution A: TEOS and PTES were dissolved in a half of the total methanol and stirred for 20 min. Solution B: deionized water and ammonia hydroxide were dissolved in the other half of the total methanol and was stirred for 5 min. The reaction was initiated by mixing solutions A and B. After 5 min stirring, the sample was transferred to a NMR sample tube (5 mm o.d.) and analyzed immediately. Chromium (III) acetylacetonate, Cr(acac)3 (1wt%) was added as the spin relaxation agent. Many studies [28,45] have proved that Cr(acac)3 had little effect on the reaction rate or the product distribution. The hydrolysis of single PTES precursor was carried out under the same reactive conditions as mentioned above. The hydrolysis and condensation rate constants were also calculated as a comparison with those of mixed systems. Table 1 Reaction components of single PTES precursor systems Sample

Molar ratio of PTES/H2 O/NH3

ak

(a) (b) (c) (d) (e)

0.5/1.5/0.236 0.5/1.5/0.157 0.5/1.5/0.118 0.5/1.0/0.236 0.5/2.0/0.236

6.52 6.49 6.53 6.50 6.43

Ph1

× 103

ak

± ± ± ± ±

23.98 25.27 24.96 24.08 25.19

0.33 0.32 0.33 0.33 0.32

Pc1

× 103 ± ± ± ± ±

1.4 1.3 1.3 1.2 1.3

a The errors of k and k are estimated from the 5% error of integration area, Ph Pc and subscripts h and c indicate the hydrolysis and the condensation, respectively.

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Table 2 Reaction components and rate constants of hydrolysis and condensation of mixed TEOS/PTES systems Sample

Molar ratio of TEOS/PTES/H2 O/NH3

kTh1 × 103 (TEOS)

S1 S2 S3 S4 S5 S6 S7

0.5/0.5/1.75a /0.1 0.5/0.5/1.75a /0.05 0.5/0.5/1.75a /0.2 0.5/0.5/3.5b /0.1 0.5/0.5/7.0c /0.1 0.5/0.25/1.375a /0.1 0.5/0.125/1.19a /0.1

18.2 18.8 18.8 17.9 17.8 18.0 17.6

Average Reference

± ± ± ± ± ± ±

0.92 0.94 0.95 0.90 0.89 0.90 0.88

18.0 ± 0.9 7.41 ± 0.4

kPh1 × 103 (PTES) 8.81 9.16 9.17 8.70 8.76 9.01 8.96

± ± ± ± ± ± ±

0.45 0.48 0.47 0.44 0.44 0.45 0.45

8.95 ± 0.45 6.50 ± 0.33

kTc1 × 103 (TEOS) 60.29 65.49 68.14 70.15 67.79 66.54 63.56

± ± ± ± ± ± ±

3.0 3.3 3.4 3.5 3.4 3.4 3.2

66.01 ± 3.3 157.2 ± 7.8

kPc1 × 103 (PTES) 44.58 37.86 36.16 37.19 42.33 40.52 39.28

± ± ± ± ± ± ±

2.3 1.9 1.8 1.9 2.2 2.1 2.0

39.7 ± 2.0 24.70 ± 1.3

MH2 O , MT and MP are the molar amount of H2 O, TEOS and PTES, respectively. The errors of kh and kc are estimated from the 5% error of integration area in NMR, and subscripts h and c indicate the hydrolysis and the condensation, respectively. Reference experiments were about the individual hydrolysis of single precursor under same reactive conditions as this paper. a M H2 O = 2MT + 1.5MP . b M H2 O = 4MT + 3MD . c M H2 O = 8MT + 6MP .

All in situ liquid-state 29 Si NMR experiments were carried out in duplicate on a UNITY INOVA-500 Spectroscopy. To achieve sufficient signal intensity, 168 scans were acquired for each spectrum with a 3 s pulse delay using a 90◦ pulse angle. The spectral frequency of 29 Si was 99.351 MHz. The resulting spectra were internally referenced to a tetramethylsilane (TMS) standard. The resonance peaks of the observed species were well resolved and could be integrated quantitatively. During the experiments, gelation did not occur and the transesterification was negligible [28]. All these NMR experiments were conducted at 25 ◦ C and the temperature was controlled to an error range of ±0.1 ◦ C. 3. Results 3.1. 29 Si NMR spectra and concentration of soluble Si species In order to assign the 29 Si NMR chemical shifts for different silicon species, the traditional notation was adopted [2]: T presents the trifunctional silicon in PTES and Q presents the tetrafunctional silicon in TEOS. Then the symbols Tnm and Qnm denote the products of hydrolysis or condensation of PTES and TEOS, respectively, where m and n are the number of siloxane bridges and the number of silanol surrounding the Si atom, respectively. The chemical shifts of soluble Si species appeared in the experiments are listed in Table 3. Fig. 1(a) shows the typical time-dependent 29 Si NMR spectra during the reaction of the mixed precursors. There were five peaks detected in the experiment. The two resonance signals at −55.97 ppm and −57.1 ppm are corresponding to T10 and T00 , respectively. The two resonance signals at −80.4 ppm and −81.3 ppm are corresponding to Q10 and Q00 in methanol, respectively. These above four signals have been detected in every experiment. A weak resonance signal at −78.3 ppm represents Q30 . Seen from Fig. 1(a), the hydrolysis of TEOS led to the formation of Q10 (mainly) and Q30 (weak),

Fig. 1. Time-dependent liquid-state 29 Si NMR spectra of: (a) TEOS/PTES systems, (b) single PTES systems, and (c) single TEOS systems.

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Fig. 2. Time-dependent soluble silicon species concentrations in typical TEOS/PTES system: (a) of sample S1, (b) of sample S3, (c) of sample S4, (d) of sample S6, (
) T00 , () Q00 , () T10 , () Q10 , (
) Q30 .

and Q20 was not observed probably because its signal intensity is below the detective limit of the liquid-state 29 Si-NMR spectrometer due to the fast condensation of intermediate Q20 under basic conditions [28]. Fig. 1(b) and (c) are the time dependent spectra of the single PTES systems and the single TEOS systems, respectively. Among Fig. 1(a)–(c), it can be found that the biggest difference is that some signals of dimmers appear in the spectra of the single precursor systems: T01 at −65.0 ppm in (b), Q01 at −88.4 ppm in (c). But no signal of similar dimers appears in Fig. 1(a) of the mixed precursors systems. The relative concentrations of the intermediate soluble Si species were determined by integrating the resonance peak at fixed individual frequency in NMR spectra. The integrated area of the initial Q00 peak of TEOS (0.5 M) without catalyst or water was taken as 100%. The time-dependence of monomer concentration (Q00 or T00 ) was obtained by fitting the experimental data with an exponential decay of first order, and the time-dependence of intermediate species concentration was obtained by fitting a multi-peaks fitting to guide the eyes. The concentration curves of single PTES systems are not shown. The disappearance of the Table 3 Liquid-state 29 Si NMR chemical shift δ and silicate structures Species

Structures

δ (ppm)

T01 T10 T00 Q30 Q10 Q00 Q01

(EtO)2 PhSi* O SiPh(OEt)2

−65.0

PhSi*(OEt)2 (OH)

−55.97

PhSi*(OEt)3

−57.1

Si*(OEt)(OH)3

−78.3

Si*(OEt)3 (OH)

−80.4

Si*(OEt)4

−81.3

(EtO)3 Si* O Si(OEt)3

−88.4

monomers from their initial level and the appearance of one or more intermediate species of typical TEOS/PTES systems are presented in Fig. 2 for each studied reaction mixture as functions of time. The concentration curves are the base for further calculation of rate constants. Comparing (a) of S1 and (b) of S3 in Fig. 2, the decrease of monomer becomes faster with the increasing ammonia content. The similar trend can be seen by comparing (a) and (c) of S4, but no obvious difference can be observed from the comparison of S4 and S5 (not shown). The increasing content of PTES in mixture speeds up the hydrolysis of TEOS monomer from the comparison of Fig. 2(a) of S1 and (d) of S6 and S7 (not shown), but no notable influence can be found on the hydrolysis rate of PTES monomer itself. Therefore, a qualitative conclusion can be drawn from Fig. 2: the hydrolysis of TEOS and PTES are both more sensitive to the ammonia content than to the water content, and the addition of PTES speeds up the hydrolysis of TEOS. 3.2. Hydrolysis kinetics 3.2.1. Hydrolysis mechanism assumption and reaction model Within the initial ammonia-catalyzed hydrolysis stages, the chemical reactions can be described as follows: NH3 + H2 O

Ionization of ammonia

NH+4 + OH−

(1)

(OH)x−1 SiO− + H2 O

(2)

−→

Si(OH)x + OH− Ionization of hydrolyzed monomers

−→

(CH3 )4−x Si(OR)x + H2 O Hydrolysis

−→ (CH3 )4−x Si(OH)(OR)x−1 + ROH

(3)

X. Sun et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 289 (2006) 149–157

(OR)x SiOH + OHSi(OR)x Water condensation

−→

(OR)x Si − O − Si(OR)x + H2 O

(4)

(OR)x−1 SiOH + Si(OR)x Alcohol condensation

−→

(OR)x−1 Si − O − Si(OR)x−1 + ROH

(5)

Seen from the above reactions, the hydrolysis mechanism is very complex when two precursors are used. The condensation of intermediate species runs nearly simultaneously with the process of the hydrolysis of the precursor. So based on this considering, Lee [46] and Harris [47] proposed a simple reaction model and a detailed reaction model, respectively. According to the detailed model, the hydrolysis of monomer is dependent on the concentration of the monomer itself, water and OH− , but [OH− ] is associated with the concentration of NH3 that is related to the surface of silica nucleus and hydroxyl of hydrolyzed intermediate species. So, in reality, it is not feasible to obtain precise rate constants with regard to the detailed model. In general, the hydrolysis will be accelerated and the divergence between the hydrolysis rate and the condensation rate will be reduced when ammonia and water contents increase. Consequently, part of hydrolyzed monomers Si*(OEt)(OH)3 have not enough time to condense and still exist in solution leading to the Q30 signal. Accordingly, a simple hydrolysis and condensation model of TEOS is adopted here as follows: kTh1

Q00 + H2 O−→Q10 + ROH Q10

kTh2

+ 2H2 O−→Q30

kTc1 Q10 + Q10 −→Q01

+ 2ROH

(6) (7)

+ H2 O

(8)

Q30 + Q30 −→Q41 + H2 O

(9)

kTc2

But from Fig. 1(a) no NMR signal of condensation species Q01 was detected. Considering that the condensation species larger than Q01 can be detected, the disappearance of Q01 can be attributed to the low concentration in the tested solution. The fast exhaustion of Q01 and T01 makes their concentrations lower than the detective limit of the NMR spectrometer. The main reason for the fast exhaustion is that Q01 quickly participates in the further nucleation and oversteps the detective scope of the liquid state NMR instrument. As to PTES, the hydrolysis and condensation model is similar to that of TEOS like follows: kPh1

T00 + H2 O−→T10 + ROH kPc1

T10 + T10 −→T01 + H2 O

153

The exponents α, β and γ here are the reaction orders belonging to PTES monomer, NH3 and H2 O, respectively. Since NH3 was used as the catalyst during the hydrolysis and condensation, its concentration remained constant; the change of water content was less than 5% of the initial level [46], so it was also regarded as invariable. If we define: 1 kPh = kPh1 [NH3 ]β [H2 O]γ

(13)

The rate expression (12) can be simplified to: rP = −

d[T00 ] α 1 = kPh [T00 ] dt

(14)

As can be seen from Fig. 2, the exponentially fitted timedependent concentration of T00 is a good approximation for the experimental data. In this case, it is assumed that the reaction (14) is of first-order, i.e., α = 1, so the rate expression (14) will converse to: rP = −

d[T00 ] 1 = kPh [T00 ] dt

(15)

Integration of Eq. (15) leads to: 1 t ln[T00 ] = ln[T00 ]0 − kPh

(16)

Thus, a plot of ln[T00 ] versus t should yield a straight line with 1 as the slope and ln[T0 ] as the intercept. Fig. 3 shows the −kPh 0 0 relation between ln[T00 ]0 and t. Those good linearity proves α = 1 indeed. Furthermore, taking the logarithm of Eq. (13) will offer: 1 = ln kPh1 + β ln[NH3 ] + γln[H2 O]. ln kPh

(17)

1 versus ln[NH ] with a slope β will proSo a linear plot of ln kPh 3 duce the reaction order β of NH3 when [H2 O] remains constant (samples S1, S2 and S3). This plot is shown in Fig. 4. 1 versus ln[H O], shown Similarly, a linear plot of ln kPh 2 in Fig. 5, can produce the reaction order γ of H2 O when [NH3 ] remains constant (samples S1, S4 and S5). Directly from Figs. 4 and 5, β = 0.833 and γ = 0.163 have been determined. 1 obtained from Fig. 3, β and γ in Eq. (17), k Applying kPh Ph1

(10) (11)

3.2.2. Hydrolysis kinetics of mixed TEOS/PTES system In reactions (10) and (11), both the reactive rates are dependent on [NH3 ] and [H2 O], so the rate expression of the initial hydrolysis of PTES is: rP = −

d [T00 ] α = kPh1 [T00 ] [NH3 ]β [H2 O]γ . dt

(12)

Fig. 3. Time-dependent PTES monomer concentration in TEOS/PTES systems. 1 of PTES The slope of straight line is the pseudo-first-order rate constant kPh hydrolysis.

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1 and k 1 vs. NH conFig. 4. Pseudo-first-order hydrolysis rate constants kPh 3 Th centration in single PTES and TEOS/PTES systems.

has been calculated and collected in Table 2. Finally the initial hydrolysis rate equation of PTES in the mixed TEOS/PTES precursor systems is: rP = 8.95 × 10−3 [T00 ][NH3 ]0.833 [H2 O]0.163

(18)

Under ammonia catalysis, when an ethoxyl group ( OC2 H5 ) in the monomer is replaced by a hydroxyl group ( OH), the steric hinderance surrounding the silicon atom decreases. As an electron-withdrawing substitute, OH helps to stabilize the negative charge on silicon when the OH− makes a nucleophilic attack on the silicon atom of the hydrolyzed monomer. Considering both steric and inductive factors, the hydrolysis rate increases with the extent of OH substitution. Taking in account that under basic condition the condensation rate of ethoxysilanes is faster than the hydrolysis rate, a conclusion can be drawn that the initial hydrolysis of monomer is the rate-limiting step of the total reactions [48]. Namely, the reaction (10) is the rate-limiting step for PTES hydrolysis, and the rate expression (18) is the rate expression of the total reaction for PTES.

1 and k 1 vs. H O conFig. 5. Pseudo-first-order hydrolysis rate constants kPh 2 Th centration in single PTES and TEOS/PTES systems.

Fig. 6. Time-dependent TEOS monomer concentration in TEOS/PTES systems. 1 of TEOS The slope of a straight line is the pseudo-first-order rate constant kTh hydrolysis.

From Eqs. (10) and (11), the disappearance of T00 is accompanied by the appearance of hydrolysis species T10 and condensation species. Since the intermediate T10 is so active that it has little time to accumulate, so after T10 content approaches its maximum, a steady-sate concentration is reached. According to the steady-state approximation [49,50], following Eq. (19) was acquired: d[T10 ] = kPh1 [T00 ]ss − kPc1 [T10 ]ss ≈ 0, dt

(19)

where [T10 ]ss and [T00 ]ss are the steady-state concentrations of [T10 ] and [T00 ], respectively. Hence the condensation rate constant kPc1 can be defined by: kPc1 = kPh1

[T00 ]ss [T10 ]ss

.

(20)

The kPc1 data are also listed in Table 2.

Fig. 7. Time-dependent PTES monomer concentration in single PTES systems. 1 of PTES The slope of straight line is the pseudo-first-order rate constant kPh hydrolysis.

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Applying the analysis process similar to that for PTES above in the hydrolysis kinetics of TEOS in mixed TEOS/PTES system 1 versus ln[NH ], and (reaction (6)), plots of ln[Q00 ] versus t, ln kTh 3 1 ln kTh versus ln[H2 O] are shown by Fig. 6, the TEOS branch of Fig. 4 and the TEOS branch of Fig. 5, respectively. Consequently, the hydrolysis rate equation of TEOS in mixed systems was obtained as: rT = 1.80 × 10−2 [Q00 ][NH3 ]0.833 [H2 O]0.480 .

(21)

Using the steady-state approximation [49,50], the condensation rate constant kTc1 in the double-precursor systems has also been defined and listed in Table 2. Because Q30 did not appear in every experiment, kTh2 and kTc2 that should be defined by the reactions (7) and (9), are not taken into account.

155

ethoxyl is a moderate electron acceptor [51]. Hence, the replacement of ethoxyl group by phenyl group would cause a decrease in positive charge on the Si atom, which leads to a downfield shift (more positive δ than non-substituted TEOS). On the other hand, methyl is classified as a weak electron donor. Thus, the net positive charge on the silicon of MTES is lower than that of PTES, resulting in a value of −41.8 ppm in our previous work [54]. It can be concluded, in good agreement with other studies [34,52], that the 29 Si-NMR chemical shift could be predicted based on the observation of inductive effects. Compared with the value of the PTES monomer, the hydrolysis product [T10 ] has a downfield shift. The fact could be explained that, when ethoxyl group is substituted for hydroxyl group, the positive charge on Si atom decreases owing to the higher ionic character of the hydroxyl group than the ethoxyl group as follows [53]:

(23)

3.2.3. Hydrolysis kinetics of single PTES system As to the PTES in the double-precursor systems, the similar analysis process is applied. From the plots of ln[T00 ] versus 1 versus ln[NH ], and ln k 1 versus ln[H O] (shown by t, ln kPh 3 2 Ph Fig. 7, the single PTES branch of Fig. 4 and the single PTES branch of Fig. 5), respectively, resulting in hydrolysis rate equation of TEOS like follows: rP = 6.50 × 10−3 [T00 ][NH3 ]0.461 [H2 O]0.071

(22)

The condensation rate constant kPc1 of PTES in the single PTES precursor systems has also been defined and listed in Table 1.

According to our previous work [54], the individual hydrolysis rate equations of the single TEOS and MTES system are like follows, respectively: rT = 7.41 × 10−3 [TEOS][NH3 ]0.333 [H2 O]0.227 , rM = 7.27 × 10

−3

[MTES][NH3 ]

0.723

0.144

[H2 O]

(24) .

(25)

Comparing the hydrolysis rate equations of the single PTES system, it can be observed that the reaction activity of PTES is below that of TEOS and MTES. Under basic conditions, water dissociates to produce nucleophilic hydroxyl anions in a rapid step, and then the hydroxyl anion attacks the silicon atom. Iler [55] and Keefer [56] proposed an SN 2-Si mechanism in which OH− displaces OR− with inversion of the silicon tetrahedron:

(26)

4. Discussion 4.1. Discussion on hydrolysis of single PTES precursor The hydrolysis rate of the single silica precursor is highly related to the electronic density on the Si atom, which is manifested by the chemical shift in the NMR spectra. Therefore, firstly the discussion is about the substituent effect on the chemical shift of the monomer and the corresponding hydrolysis product. The value of the 29 Si NMR chemical shift of PTES monomer is different from the values of other monomers (TEOS or MTES (CH3 Si(OEt)3 )) in our previous experiments [54]. Generally, the increase of the net positive charge on the silicon (Si+ ) leads to an upfield shift (more negative δ) [51]. The phenyl group is classified as a weak electron acceptor, while the

Both inductive effect and steric effect influence the reaction rate of silicon precursors. Considering only the inductive effect, the silicon requires little negative charge in the transition state, so the replacement of a moderate electron-accepting group ( OEt) by a weaker electron-accepting group (phenyl) or an electron-providing group (methyl) resulted in a reduction of the reaction rate. The reaction rate may follow an order of TEOS > PTES > MTES. On the contrary, when only the steric effect was taken into account, since the hydroxyl anion attacks the silicon atom from the rear, the substituents that reduce steric hindrance in the transition state will enhance the hydrolysis rate, the reaction activity may follow an order of MTES > TEOS > PTES. Combining both inductive and steric effects, the experimental hydrolysis rate follows an order of TEOS > MTES > PTES. The experimental result indicates that the steric effects predominate over inductive effect and become a dominant influence on the reaction activity [57].

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4.2. Comparison of hydrolysis and condensation between double-precursor system and single precursor system The main reason for studying the hydrolysis and the condensation of TEOS/PTES mixed system is to understand the hybrid sol–gel process on molecular level, and the subsequent microstructure and chemical homogeneity of derived hybrid materials. A clear analysis of the hydrolysis kinetics will provide us effective directions to synthesize hybrid materials in the light of special material design. So it is the first thing to disclose why and what about the difference between the hydrolysis kinetics of the mixed double precursor system and that of the single precursor system. Comparing Eq. (18) with (22) and Eq. (24) with (21), it is obviously found that the reaction orders of NH3 and H2 O become larger in the mixed double-precursor system than in the single-precursor system. This result indicates that the hydrolysis of TEOS and the hydrolysis of PTES become more sensitive to NH3 and H2 O in the mixed precursor system than in the single precursor system, due to the competitive effect of different reactants. In addition, the reactive order of NH3 is always larger than that of H2 O, which shows stronger sensitivity of hydrolysis to NH3 than to H2 O. It is found that kTh1 increases from 7.41 × 10−3 to 1.80 × 10−2 when the situation of TEOS is changed to the mixed precursor system from the single precursor system. In the mean time, kPh1 increases from 6.50 × 10−3 to 8.95 × 10−3 when the situation of PTES is changed to mixed precursor system. However, kTc1 decreases from 157.2 × 10−3 to 66.01 × 10−3 , and kPc1 increases from 24.94 × 10−3 to 39.7 × 10−3 . If we define two variables KT = kTc1 /kTh1 for TEOS and KP = kPc1 /kPh1 for PTES, the relative rate between the condensation and the hydrolysis can be seen more clearly. The obtained KT and KP are collected in Table 4. Interestingly, the divergence between KT and KP decreases from 21.2–3.8 to 3.7–4.4 when the situations of TEOS and PTES are changed to the mixed precursor system from the single precursor system, which shows a more compatible sol–gel process between TEOS and PTES, that should benefit the fabrication of the hybrid materials with higher homogeneity. Comparing the condensation rate of S7, S6 and S1 in Table 2, some useful information can be obtained. With the increasing content of PTES in the mixed systems, the condensation rate of TEOS decreases while the condensation rate of PTES increases. This can be understood by the reduction of the average functionality for silicon atoms of TEOS, a phenomenon that prevents the formation of the siloxane network. The case is reverse for PTES. Table 4 Relative rates of hydrolysis and condensation

Mixed precursor system Single precursor system

KT (TEOS)

KP (PTES)

3.7 21.2

4.4 3.8

KT = kTc1 /kTh1 , KP = kPc1 /kPh1 , where kTc1 and kTh1 are the condensation and hydrolysis rate constants of TEOS, respectively, and kPc1 and kPh1 are the condensation and hydrolysis rate constants of PTES, respectively.

In summary, the hydrolysis becomes more active, and the reactive behaviors of TEOS and PTES become more compatible in TEOS/PTES mixed precursor systems compared with the corresponding single-precursor systems. All these phenomena indicate a possible product with high homogeneity. 5. Conclusion In situ liquid state 29 Si NMR was used to follow the hydrolysis and condensation of single PTES systems and mixed TEOS/PTES systems. Both hydrolysis rate and condensation rate of PTES monomer has been disclosed as well as the reaction orders of reactants. Due to cooperation of inductive and steric effect, PTES shows a lower reaction activity than that of TEOS or MTES. Since the condensation of siloxane is much faster than the hydrolysis under basic condition, and condensation species participate in nucleation at once and become insoluble, it is difficult to detect liquid-state 29 Si NMR signal of condensation species. Therefore, only the kinetics of initial hydrolysis and overall condensation of TEOS/PTES mixed systems have been disclosed. The most valuable conclusion is that the hydrolysis and condensation of TEOS and PTES become more compatible in double-precursor systems than in single precursor systems. The true hybrid sol–gel process may be: TEOS hydrolyzes and self-condenses into silica nucleus, simultaneously hydrolyzed PTES modifies the surface of silica nucleus and resulted in the clusters through cross condensing with silica nucleus. Although no appropriate experimental technique can be utilized to directly obtain the condensation kinetics, in situ liquid-state 29 Si NMR provides a powerful tool to study initial hydrolysis kinetics which is crucial to the structure and homogeneity of final hybrid material. Acknowledgement The financial support from the National Key Native Science Foundation (No. 20133040) was gratefully acknowledged. References [1] M.A. Aegerter, M. Mennig, P. Muller, H. Schmidt, Verre 6 (2000) 30–37. [2] C.J. Brinker, O.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic, San Diego, 1990. [3] L.L. Hench, J.K. West, Chem. Rev. 90 (1990) 33–72. [4] A.K. Cheetham, C.J. Brinker, M.L. Mecartney, C. Sanchez (Eds.), Better Ceramics Through Chemistry V, Materials Research Society, Pittsburgh, 1994. [5] J.D. Mackenzie, E.P. Bescher, J. Sol–Gel Sci. Technol. 13 (1998) 371–377. [6] J.D. Mackenzie, J. Sol–Gel Sci. Technol. 27 (2003) 7–14. [7] H. Weller, Angew. Chem. Int. Ed. Engl. 32 (1993) 41–53. [8] P. Innocenzi, G. Brusotin, M. Guglielmi, R. Bertani, Chem. Mater. 11 (1999) 1672–1679. [9] G. Schottner, Chem. Mater. 13 (2001) 3422–3435. ¨ [10] W. StOber, A. Fink, E. Bohn, J. Coll. Interf. Sci. 26 (1968) 62–69. [11] G.H. Hsiue, W.J. Kuo, Y.P. Huang, R. Jeng, J. Polym. 41 (2000) 2813–2825. [12] H. Schmidt, J. Non-Cryst. Solids 73 (1985) 681–691. [13] D. Ravaine, A. Seminel, Y. Charbouillot, M. Vincens, J. Non-Cryst. Solids 82 (1986) 210–219. [14] C.Y. Li, J.Y. Tseng, K. Morita, C. Lechner, Y. Hu, J.D. Mackenzie, Sol–Gel Optics II. Proc. SPIE1758 (1992) 410–412.

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