Toward Functionalized Surfaces through Surface Esterification of

Jun 25, 2002 - Three aspects of the esterification reaction of silica surfaces with alcohols that are relevant for the synthesis of functional materia...
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Langmuir 2002, 18, 5749-5754

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Toward Functionalized Surfaces through Surface Esterification of Silica Gabriel C. Ossenkamp,* Tim Kemmitt,† and Jim H. Johnston‡ Industrial Research Limited, Applied Inorganic Chemistry, P.O. Box 31-310, Lower Hutt, New Zealand, and Victoria University of Wellington, School of Chemical and Physical Sciences, P.O. Box 600, Wellington, New Zealand Received December 12, 2001. In Final Form: March 17, 2002 Three aspects of the esterification reaction of silica surfaces with alcohols that are relevant for the synthesis of functional materials have been studied: surface coverage, hydrolytic stability, and the introduction of functional groups. The chemisorption of alcohols on silica can be described by the Langmuir adsorption model, which allows precise control of the surface coverage. It was shown that the hydrolytic stability of the materials obtained could be significantly improved by reacting the esterified silica with hexamethyldisilazane. The use of alcohols containing mercapto groups gave silica with surface-tethered mercapto groups. These groups are available for further chemical reactions, but the hydrolytic stability of the functional material decreases in comparison to silica esterified with the corresponding nonfunctional alcohol.

Introduction 1

The surface of silica carries silanol groups, tSiOH, which can be chemically modified to influence the chemical and physical properties of the silica.2 This is generally achieved through organic molecules that react with the silanol groups to produce a covalently bonded organic layer, tSiOR. The success of such materials for a given application depends on close control of the surface properties and the chemistry of the bonded layers. Organic chemistry permits the introduction of functional groups into the covalently bonded layers, which gives rise to a bounty of opportunities to tailor the functionalized surfaces to suit the requirements of desired applications. This has enabled surface-modified silica to be used in applications ranging from chromatographic supports to coupling agents for polymer fillers.2 Silica is a popular choice for the solid support material due to its low price, chemical and mechanical stability, and availability in a wide range of specific surface areas and morphologies, each with different surface chemistry.3 Modification of the surface of silicon wafers is also of interest for a range of emerging technology applications utilizing the electronic properties of the assembly of organic molecules and the underlying silicon chip. This can be achieved through chemical modification of the silanol groups on oxidized silicon surfaces.4 There is also considerable interest in the functionalization of glass surfaces for display devices and ophthalmic lenses, also requiring reactions with silanols.5 The most common reagents for surface modification of silica and siliceous substances are based on silanes, R4-nSiXn with n ) 1-3 and X ) Cl, OMe, OEt. The group(s) X are hydrolyzed during the reaction with the silica surface, and the group(s) R may carry further functional * Corresponding author. E-mail: [email protected]. † Industrial Research Limited. ‡ Victoria University of Wellington. (1) Kiselev, A. V. Kolloidn. Zh. 1936, 2, 17. (2) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and chemical modification of the silica surface; Elsevier: Amsterdam, 1995. (3) The Surface properties of Silicas; Legrand, A. P., Ed.; John Wiley & Sons: London, 1998. (4) Stewart, M. P.; Buriak, J. M. Adv. Mater. 2000, 12, 859. (5) Peng, B.; Ruehe, J.; Johannsmann, D. Adv. Mater. 2000, 12, 821.

groups to produce a functionalized material with tailored properties.6 The reaction of such silanes with silica surfaces suffers from the possibility of side-reactions that are difficult to control and are very sensitive to the reaction conditions. When n > 1, competing inter-silane condensation reactions often lead to formation and deposition of oligomeric clusters on the surface that disturb the intended monolayer coverage.7,8 The esterification of silanols with alcohols (eq 1) is wellknown and also leads to covalent attachment of organic molecules to the silica surface.9-13

tSi-OH + R-OH (1) h Si-OH‚‚‚HO-R h Si-OR + H2O (1) The mechanism of the reaction is dependent on the structure of the silica substrate, as silanol groups can react as in eq 1, but it has also been shown that alcohols can open siloxane bridges on the silica surface.14,15 The concentration of siloxane bridges on the surface of the precipitated silica used in this study is however comparatively low,12 so that this reaction is of minor importance compared to the direct reaction of silanols. Little thermodynamic data exists in the literature for the esterification reaction: Ballard et al.11 measured ∆H ) 50.7 kJ/mol, whereas Plueddeman6 estimates K ≈ 10, both values implying that hydrolysis is much more likely than for silanes. This fact is often mentioned in the literature, (6) Plueddeman, E. P. Silane Coupling Agents; Plenum Press: New York, 1991. (7) Vallant, T.; Kattner, J.; Brunner, H.; Mayer, U.; Hoffmann, H. Langmuir 1999, 15, 5339. (8) Van Der Voort, P.; Vansant, E. F. Pol. J. Chem. 1997, 71, 550. (9) Dzhigit, O. M.; Kiselev, A. V.; Mikos-Avgul, N. N.; Shcherbakova, K. D. Dokl. Akad. Nauk SSSR 1950, 70, 441. (10) Iler, R. K. U.S. Patent 2,657,149, 1953. (11) Ballard, C. C.; Boroge, E. C.; Iler, R. K.; St. John, D. S.; McWorther, J. R. J. Phys. Chem. 1961, 65, 20. (12) Iler, R. K. The chemistry of silica; Wiley: New York, 1979. (13) Shioji, S.; Kawaguchi, M.; Hayashi, Y.; Tokami, K.; Yamamoto, H. Adv. Powder Technol. 2001, 12, 343. (14) Mertens, G.; Fripiat, J. J. J. Colloid Interface Sci. 1973, 42, 169. (15) Acosta Saracual, A. R.; Pulton, S. K.; Vicary, G.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2285.

10.1021/la015732z CCC: $22.00 © 2002 American Chemical Society Published on Web 06/25/2002

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but very few explicit studies exist.13,16,17 We recently reported on the improved synthesis and factors influencing the hydrolytic stability of some surface-esterified silicas.18 The tSi-OR bond has also been synthesized through intermediate activation of the silica surface through chlorination with SOCl219,20 and through reaction with Si(NEt2)421 prior to the reaction with alcohols. Furthermore, more reactive compounds such as ketals and orthoesters22 and orthoformates23 have been employed to form surface-esterified silica. This paper examines three aspects of the esterification reaction: surface coverage, hydrolytic stability, and the introduction and availability of surface-tethered functional groups. Together these factors determine the suitability of the esterification reaction for the synthesis of functional materials. First, the adsorption isotherms for the esterification reaction are reported for a number of different alcohols. This gives insights into the nature of the chemisorbed layers, which is of interest in comparison to silanes, and allows the adjustment of the density of surface-bonded organic groups. Adsorption isotherms have been studied for the physisorption of alcohols to silica,15,24,25 but to the best of our knowledge they have not been reported for the chemisorption reaction. Second, we report on additional strategies that can be employed to improve the hydrolytic stability of the esterified silica, which is relevant to the long-term stability of the material. Finally, we demonstrate the possibility of using mercapto-containing alcohols in the esterification reaction to produce silica with surfacetethered mercapto groups. The esterification reaction has previously been employed to synthesize functional materials based on silica26,27 and oxidized silicon,28 but these studies were not concerned with the mercapto group. It is shown that the surface-tethered mercapto groups are available for further chemical reactions. The hydrolytic stabilities of the functionalized materials are investigated. Experimental Section Synthesis of Alcohols. 1-Octanol (A) (Lancaster), 1-decanol (B) (BDH), and 1-hexadecanol (C) (BDH) were obtained commercially and used without further purification. 2,2′-Bis(hydroxymethyl)-1,3-propanediol-monohexadecanoate (D) was prepared29 and purified30 according to the literature. 8-Mercapto1-octanol (E) and 10-mercapto-1-decanol (F) were synthesized

(16) Ahmed, A.; Gallei, E.; Unger, K. Ber. Bunsen-Ges. Phys. Chem. 1975, 79, 66. (17) Utsugi, H.; Horikoshi, H.; Matsuzawa, T. J. Colloid Interface Sci. 1975, 50, 154. (18) Ossenkamp, G. C.; Kemmitt, T.; Johnston, J. H. Chem. Mater. 2001, 13, 3975. (19) Eckert-Lill, C.; Lill, N. A.; Endres, W.; Rupprecht, H. Drug Dev. Ind. Pharm. 1987, 13, 1511. (20) Eckert-Lill, C.; Lill, N. A.; Rupprecht, H. Colloid Polym. Sci. 1987, 265, 1001. (21) Yam, C. M.; Tong, S. S. Y.; Kakkar, A. K. Langmuir 1998, 14, 6941. (22) Guidotti, B. R.; Herzog, E.; Bangerter, F.; Caseri, W. R.; Suter, U. W. J. Colloid Interface Sci. 1997, 191, 209. (23) Guidotti, B. R.; Caseri, W. R.; Suter, U. W. Langmuir 1996, 12, 4391. (24) Gavrilov, K.; Kutsosvkiy, Y.; Paukshtis, E.; Okunev, A.; Aristov, Y. Mol. Cryst. Liq. Cryst. 1994, 248, 159. (25) Borello, E.; Zecchina, A.; Morterra, C.; Ghiotti, G. J. Phys. Chem. 1967, 71, 2945. (26) Auteri, F. P.; Belford, R. L.; Robinson, B. H.; Clarkson, R. B. Colloids Surf., A 1993, 81, 25. (27) Nechifor, A. M.; Philipse, A. P.; de Jong, F.; van Duynhoven, J. P. M.; Egberink, R. J. M.; Reinhoudt, D. N. Langmuir 1996, 12, 3844. (28) Charles, M.; Colin, B.; Delaire, T.; Jaffrezic, N.; Mandrand, B.; Martelet, C.; Saby, C. Innov. Technol. Biol. Med. 1993, 14, 324. (29) Richert, M.-T.; Paquot, C. Oleagineux 1967, 22, 613. (30) Richert, M.-T.; Paquot, C. Oleagineux 1969, 24, 413.

Ossenkamp et al. according to the literature.31 The corresponding disulfides, 8-(8hydroxy-octyldisulfanyl)-octan-1-ol (G) and 10-(10-hydroxy-decyldisulfanyl)-decan-1-ol (H), were synthesized from the mercaptoalcohol as follows: 0.15 mol mercapto-alcohol was dissolved in 200 mL of ethanol, and the solution was stirred with 0.15 mol NaOH until dissolved. Then a solution of 0.075 mol I2 in 200 mL of ethanol was added dropwise while stirring. The mixture was left stirring for 15 h, and the precipitate was filtered off and recrystallized from a 1:1 mixture of hexane isomer mixture and ethyl acetate. Synthesis of Alkoxylated Silica. To a suspension of 3 g of silica (Degussa Sipernat 320; BET N2 surface area, 175 m2/g) in 50 mL of xylene isomer mixture (Scharlau, analytical grade), the calculated amount of alcohol was added. A continuous solidliquid extractor containing about 0.5 g of CaH2 (Aldrich) and a condenser with a CaCl2 drying tube was mounted on the reaction flask, and the mixture was refluxed, carefully regulating the heating rate to obtain a continuous and stable flow of xylene through the extractor. After refluxing for 18 h, the sample was filtered and washed with hot xylene and ethyl acetate several times. Finally, samples were dried at 0.1 Torr at room temperature for several hours before being heated to 130 °C for >5 h. Modification with Hexamethyldisilazane (HMDS). One gram of dried (130 °C/0.1 Torr/>5 h) alkoxylated silica was suspended in 50 mL of dry ether (Scharlau), and 3 mL of HMDS (Koch-Light Laboratories Ltd) was added. The reaction mixture was refluxed for 30 min and then evaporated to dryness. It was then heated at 130 °C at 0.1 Torr for at least 3 h to remove excess reagents. Oxidation of Surface-Bonded SH Groups. Precipitated silica (330 mg) with chemisorbed 8-mercapto-1-octanol, Si-E, was dispersed in 20 mL of xylene. I2 (25 mg) and two drops of NEt3 were added, and the mixture was stirred for 30 min. The solids were filtered off, washed with xylene and ethyl acetate, and then dried in vacuo. Hydrolysis. The hydrolysis was followed by immersing about 50 mg of surface-esterified silica in 50 mL of boiling water. Either the time required for the sample to sink from the surface of the water was monitored, or the solution was filtered after a given time, the solids were washed and dried, and the carbon content was determined by microanalysis. NMR Spectroscopy. All spectra were recorded on a Varian Inova 200 spectrometer at room temperature with an operation frequency of 50.2935 MHz for 13C. Samples were placed in a 7 mm ZrO2 rotor and spun at 4.9 ( 0.1 kHz. 13C magic-angle spinning (MAS) NMR single-pulse excitation spectra were obtained using a 13C 90° pulse length of 5 µs and high-power proton decoupling during the acquisition time of 30 ms. The recycle time was set to 0.5 s, and typically 8000 scans were accumulated. Spectra were referenced externally to the methyl resonance of hexamethylbenzene, set to 17.4 ppm. A 1 Hz linebroadening was employed. Generation of Illustrations of Modified Silica. β-Cristobalite (111) with a Si-Si distance of 3.2 Å was chosen as a model for the silica surface.32 Silicon atoms on the surface were saturated to achieve four bonds through the addition of a silanol group perpendicular to the surface. A computer program was written that randomly substituted these silanol groups with alltrans octyl and trimethylsilyl (TMS) groups with given densities. The coordinates for all atoms were calculated based on normal bond lengths (C-O, 1.24 Å; C-H, 1.08 Å; C-C, 1.54 Å; Si-O, 1.64 Å; Si-C, 1.90 Å), and all bond angles were set to 109.45°. The space filling models were generated by processing the atomic coordinates with the aid of the POV-Ray ray-tracing package (POV-Ray 3.1 for Windows) using the following van der Waals radii: Si, 2.1 Å; C, 1.75 Å; O, 1.5 Å; H, 1.25 Å.

Results and Discussion The first step in the esterification reaction (eq 1) is physisorption of the alcohol to the silica surface via hydrogen bonds between the silanols and the alcohol, followed by chemical reaction with the silanol group. This (31) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657. (32) Stevens, M. J. Langmuir 1999, 15, 2773.

Surface Esterification of Silica

Figure 1. 13C-{1H} MAS NMR spectra of 1-octanol on precipitated silica. The octanol C1 resonance corresponding to physisorbed and chemisorbed alcohol is marked with “p” and “c”, respectively. (a) Physisorbed 1-octanol, (b) raw surfacealkoxylation product, and (c) surface-alkoxylated silica after removal of physisorbed excess alcohol through heating in vacuo. Chart 1. Alcohols Employed in This Study

transition from physisorbed to chemisorbed alcohol can be followed by solid-state 13C-{1H} MAS NMR, as shown for silica surface-esterified with 1-octanol (Si-A, 1.7 µmol/ m2, Chart 1) in Figure 1. Comparison of the spectrum of the reaction product before heating in vacuo (Figure 1b) with that of 1-octanol physisorbed to silica (Figure 1a) reveals a new peak with a chemical shift about 1 ppm higher than that of C1 in the physisorbed alcohol (peak at 65 ppm). Heating of this raw reaction product in vacuo removes the peak with the lower chemical shift in Figure 1b resulting in the spectrum of the pure esterified product in Figure 1c. Thus, physisorbed and chemisorbed 1-octanol can be distinguished by the increase in chemical shift by about 1 ppm. This increase in chemical shift for the C1 carbon is similar to the difference in chemical shifts between tetraalkoxysilanes and the corresponding alcohol,33 corroborating the results of the desorption experiment. A precipitated silica was chosen as the substrate in these studies, since precipitated silicas are widely used, for example, as fillers for polymer systems, where they are generally employed together with a silane coupling agent. Due to their manufacturing process, precipitated silicas commonly contain impurities, notably Na2SO4 and iron. This may influence the surface chemistry of the precipitated silica, compared to chemically purer silicas, such as pyrogenic silica. However, pyrogenic silicas have a different surface chemistry and entirely different morphology compared to precipitated silica. Thus, it was decided to (33) Liepins, E.; Zicmane, I.; Lukevics, E. J. Organomet. Chem. 1986, 306, 167.

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use a standard precipitated silica to study aspects of the esterification reaction in a system that is also used in practice. The surface-esterified silica was produced by refluxing precipitated silica in a solution of the alcohol in xylene, as described previously.18 This allows azeotropic removal of the water formed in the esterification reaction, and the condensed solvents are allowed to percolate through CaH2 to remove the water before re-entering the reaction vessel. This removal of water pushes the reaction toward higher surface coverages and ensures consistent water concentrations in the system. These reaction conditions make investigations of the esterification reaction reproducible and the examination of adsorption isotherms convenient. By variation of the alcohol concentration, the adsorption isotherms for the chemisorption of various alcohols (B-E and G, Chart 1) were investigated. This is of interest for comparison with some of the silane compounds used for surface modification of silica, since these can suffer from side-reactions that may lead to formation of complex surface layers. Since alcohols cannot undergo such sidereactions, the surface-esterification reaction would be expected to lead to the formation of more uniform monolayers. Besides being of interest for elucidation of the nature of the chemisorption reaction, the adsorption isotherms are of practical importance as they allow the synthesis of materials with a given surface coverage. The results of these investigations are shown in Figure 2. Figure 2a depicts the relationship between the initial alcohol concentration and the alkoxy group density achieved, which is of interest for the practical synthesis of materials with tailored surface coverages. The solvent volume varies slightly during the reaction, due to fluctuations of the amount of solvent residing in the continuous extractor. Therefore, all experiments were carried out with the same amount of silica and solvent and the alcohol concentrations are expressed as the amount of alcohol available per unit surface area of silica at the start of the reaction. The adsorption isotherms reflect the balance between the affinity of the alcohol for the silica surface and the solubility of the alcohol in the solvent. The higher the number of hydroxyl groups per alcohol, the lower the concentration of alcohol that is necessary to achieve a given surface density. This is most clearly demonstrated by the esterification with the trihydroxy compound D. On the other hand, there is little difference between the adsorption isotherms for the simple alcohols B and C. Following the treatments of the analogous chemisorption of fatty acids on alumina,34,35 these data are analyzed within a simple Langmuir model. Only formation of monolayers without interaction between the alcohol molecules is considered, and it is assumed that all adsorption sites are identical. These conditions lead to the familiar Langmuir equation (eq 2).36

[ROH] ) (FmK)-1 + Fm-1[ROH] F

(2)

Here [ROH] is the equilibrium alcohol concentration in the solution, calculated from the starting concentration employed and the density achieved. F is the density of alkoxy groups achieved, Fm is the density of alkoxy groups at monolayer coverage (maximum density), and K is a (34) Karaman, M. E.; Antelmi, D. A.; Pashley, R. M. Colloids Surf., A 2001, 182, 285. (35) Stradella, L.; Venturello, G. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 11. (36) Wedler, G. Chemisorption: An Experimental Approach; Butterworth: London, 1976.

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Figure 2. (a) Adsorption isotherms for the chemisorption of 10-mercapto-decan-1-ol F (9), 1-hexadecanol C (3), 1-decanol B (b), 10-(10-hydroxy-decyldisulfanyl)-decan-1-ol H ()), and 2,2′-bis(hydroxymethyl)-1,3-propanediol-monohexadecanoate D (+). (b) The corresponding plots according to the linearized Langmuir equation. Table 1. K, Gm, and A Derived from Adsorption Isotherms of Various Alcohols alcohol (m2/mol)

B

K Fm (/10-6 mol/m2) A (/Å2)

(113 ( 45) × 2.7 ( 0.3 62 ( 7

C 103

D

(143 ( 41) × 1.8 ( 0.1 92 ( 5

103

(790 ( 12) × 2.5 ( 0.2 66 ( 5

E 103

G

(124 ( 26) × 1.8 ( 0.1 92 ( 5

(185 ( 55) × 103 3 ( 0.2 55 ( 4

103

Table 2. Surface Coverages Reported for Esterification of Silica with Various Alcohols alcohol

substrate

method

temperature (/°C)

coverage (/µmol/m2)

reference

1-butanol 1-octanol 1-decano l 1-tetra-decanol 1-hexa-decanol

ppt silica mesoporo us silica, FSM

silica + alcohol in autoclave reflux of silica + alcohol in benzene silica + alcohol + hexane in autoclave silica + alcohol + hexane in autoclave silica + neat alcohol

118 80 235 233 200

3.22 2.49 3.91 3.15 3.37

11 44 45 45 46

ppt silica

constant describing the strength of interaction between the alcohol and the surface. From Fm, the area A per adsorbed molecule can be calculated. The adsorption isotherm data are plotted according to the linear eq 2 in Figure 2b. The data agree well with a linear least-squares fit, which also provided K and Fm. The agreement between the model and the experimental data suggests that the simple Langmuir model provides an adequate description of the system and hence that the alcohols are chemisorbed in monolayers. The deduced values of K, Fm, and A are given in Table 1. The values of Fm obtained generally agree with the maximum coverages reported by other authors for simple alcohols under similar reaction conditions (Table 2). It is, however, interesting to note the higher Fm of D compared to Fm of C. The molecules have the same length of the hydrocarbon chain and differ only in the hydroxy end group bonding to the silica surface. Since the trihydroxy end group of D must be larger than the monohydroxy end of C, it is obvious that not only simple geometric considerations determine Fm. Hence, the value of A does not simply reflect the space requirements of the molecule. The common potential drawback of the tSi-OC bond is its inherent hydrolytic instability, implied by the equilibrium formation reaction (eq 1). Although many papers concerned with surface-esterified silica mention this hydrolytic instability, few systematic studies exist.13,16,17 Recently, we studied the hydrolytic properties of surface-esterified silica and some means of improving it.18 We showed that branched and polymeric alcohols provide improvements over simple long-chain alcohols, but these materials too suffer from hydrolysis. Since the

hydrolysis of a surface-esterified silica may limit its longterm stability for certain applications, it is of interest to attempt further optimization of the hydrolytic stability of such materials. The hydrolysis of the surface-esterified silica can be conveniently studied by placing it in boiling water. Initially, the hydrophobic surface-modified silica floats on the water, but as hydrolysis progresses and more and more silanol groups become exposed, the water contact angle is reduced until the material eventually sinks. This provides a good qualitative measure of the hydrolytic stability of the surface-esterified materials, which can be complemented by following the decrease in carbon content as measured through carbon microanalysis. Thus, a silica surface-esterified with 1-octanol (Si-A) to a surface coverage of 1.7 µmol/m2 sinks within 20 min in boiling water, at which stage its surface coverage has been reduced to about 0.5 µmol/m2 (Figure 3). The hydrolysis continues until all alcohol has been removed. It was found that the hydrolytic stability of this surfacemodified silica could be considerably improved by reacting the surface-esterified silica with HMDS. This reagent condenses rapidly under mild conditions with unmodified silanol groups to give hydrolytically stable tSiOSiMe3 groups.37 This gave a material (Si-A/TMS) with a surface density of 1.7 µmol/m2 octyl groups and 1.1 µmol/m2 SiMe3 groups, as determined by the increased carbon content. The hydrolysis of Si-A/TMS in boiling water is compared with that of Si-A in Figure 3. Si-A/TMS does not sink (37) Chmielowiec, J.; Morrow, B. A. J. Colloid Interface Sci. 1983, 94, 319.

Surface Esterification of Silica

Figure 3. The hydrolysis of 1-octanol chemisorbed to precipitated silica, Si-A (O), and of 1-octanol chemisorbed on silica and further reacted with HMDS, Si-A/TMS ([).

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Figure 4. Idealized space filling models of β-cristobalite (111) surface-esterified (left) with 1.7 µmol/m2 1-octanol in the alltrans conformation and (right) with 1.7 µmol/m2 1-octanol in the all-trans conformation and 1.1 µmol/m2 trimethylsilyl groups.

until after 100 min, and initially the hydrolysis is very slow. The rate of hydrolysis after its onset is much slower in Si-A/TMS compared to Si-A. The few studies of the mechanism of hydrolysis reported in the literature were carried out under too different conditions to allow direct comparison: Ahmed et al. studied the gas-phase hydrolysis of silica gel esterified with 1-octanol between 190 and 250 °C. They concluded that the reversal of the siloxane bridge opening reaction, eq 3, is an important reaction pathway for the hydrolysis at elevated temperatures:16

tSi-OH + tSi-OR h tSi-O-Sit + ROH (3) Utsugi et al. simply presumed the reversal of the silanol condensation reaction (eq 1) in their study of the hydrolysis at 90 °C.17 A recent study by Shioji et al. reports the activation energies for the gas-phase hydrolysis of various surface-esterified alcohols to be about 42 kJ/mol, irrespective of the alcohol, but they do not distinguish between these two reaction mechanisms.13 Given the lower temperatures for hydrolysis employed in this study and the reported evidence of the catalytic effect of Lewis bases on the hydrolysis of surface-esterified silica under these conditions,18,38 paralleling the well-known effects of Lewis bases in the hydrolysis of tetraalkoxysilanes that forms the basis of much of the sol-gel chemistry of silica,39 the reversal of eq 1, that is, nucleophilic attack by water on the silicon of the Si-OR bond, appears as the likely mechanism of hydrolysis. In light of this mechanism of hydrolysis, the considerable improvement in hydrolytic stability of the esterified alcohol after the reaction with HMDS is most likely a combination of several effects: steric hindrance of the attack of water by the neighboring SiMe3 groups, the decrease in surface energy produced by introducing the SiMe3 groups and their effect on the ability of water to form hydrogen bonds with the surface. An idealized impression of the threedimensional structure of Si-A compared to Si-A/TMS is given in Figure 4, showing that there is considerably less crowding in the absence of the TMS groups. Thus, this combination of the esterification reaction and a silylation reaction could make possible a considerable saving of expensive functionalized silanes. To demonstrate the suitability of the esterification reaction for the synthesis of materials with a tailor-made functional surface, silica was esterified with the mercaptoalcohol E (Chart 1). Analogous silane-based materials have been employed as coupling agents for silica in tire rubber applications40 and for the harvesting of aqueous heavy (38) Utsugi, H.; Matsuzawa, T.; Akoshima, A. Shikizai Kyokaishi 1975, 48, 372. (39) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: New York, 1990. (40) Luginsland, H.-D. Kautsch. Gummi Kunstst. 2000, 53, 10.

Figure 5. 13C-{1H} MAS NMR spectra of (a) of 8-mercaptooctan-1-ol chemisorbed to precipitated silica, Si-E, (b) 8-mercapto-octan-1-ol chemisorbed to precipitated silica and oxidized with I2, Si-B/Ox, and (c) 8-(8-hydroxy-octyldisulfanyl)-octan1-ol chemisorbed to precipitated silica, Si-G.

metals.41 Both of these applications depend on the availability of the surface-tethered mercapto for further reactions. The esterification reaction with the mercaptoalcohol proceeded straightforwardly and gave Si-E, with a surface density of 1.4 µmol/m2. The 13C-{1H} MAS NMR of this material is depicted in Figure 5a. To study the availability of the terminal mercapto groups in this silica for further chemical reactions, it was suspended in a solvent and oxidized with I2 to give Si-E/Ox. For comparison, silica was also esterified with the disulfide G corresponding to E, which was the product expected from the oxidation reaction. The NMR spectra of Si-E and Si-G are also shown in Figure 5. The spectra of SiE/Ox and Si-G are indeed very similar, with Si-E/Ox showing both the shift from -CH2SH (δ ) 36 ppm) to -CH2-S2- (δ ) 40 ppm) and the corresponding change in the shape of the alkyl chain resonance. Thus, it is clear that the mercapto groups in Si-E have been converted to the disulfide through the oxidation with I2. The difference between the spectra of Si-E/Ox and Si-G is the wider -CH2O- resonance at ∼65 ppm in Si-G. Taking the increase in chemical shift of about 1 ppm for the chemisorption of an alcohol to silica demonstrated above into account, it can be inferred that not all of the hydroxyl groups in the symmetric disulfide in Si-G are bonded to the silica surface. This could be caused by a fraction of (41) Vieira, E. F. S.; Simoni, J. D.; Airoldi, C. J. Mater. Chem. 1997, 7, 2249.

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demonstrated by the fact that the functionalized compounds Si-F and Si-H do not float on water. The highest hydrolytic stabilities have been achieved using a polymeric alcohol.18 The provision of a polymeric alcohol with side groups carrying a desired functionality presents some synthetic challenges but should lead to surface-esterified functionalized silicas with highly improved hydrolytic stability. For some applications, the mere hydrogen bonding between the silica surface and such a polymer might be enough to provide the desired stable tailor-made surfaces. Investigations of these factors are under way in our laboratory and will be reported on in due course. Figure 6. The hydrolysis of 10-mercapto-decan-1-ol chemisorbed to precipitated silica, Si-F (O), 10-(10-hydroxy-decyldisulfanyl)-decan-1-ol chemisorbed to precipitated silica, Si-H ([), and 1-decanol chemisorbed to precipitated silica, Si-B (+).

disulfide molecules in Si-G being bonded to the silica surface in an I-shaped manner, with one hydroxyl group not reacted with the silica surface, and the remaining molecules being bonded with both hydroxyl groups to the silica surface in a U-shaped manner. The widths of the -CH2O- resonance in Si-E and Si-E/Ox are similar, indicating that the oxidation with I2 did not influence the bonding of the alcohols to the surface. Hence, all disulfide molecules in Si-E/Ox are bonded to the silica surface in a U-shaped manner. Silicas modified with such disulfides are of interest in the tire rubber industry, and we have recently explored some novel approaches to the synthesis of similar oligosulfide-bridged surface-esterified species.42 The hydrolytic stabilities of silica esterified with some longer-chain analogues of E and G, Si-F and Si-H (Chart 1), in boiling water are depicted in Figure 6. For comparison, the hydrolysis of silica esterified with the nonfunctionalized parent compound B is also shown. Both the surface-ester of the ω-mercaptan and the disulfide derived from B hydrolyze quicker than the nonfunctional parent compound B. This may be understood on the basis of the higher surface energy of the mercapto- and disulfidefunctionalized surfaces compared to the hydrocarbon surface (the critical surface tension of a surface treated with mercaptopropyltrimethoxysilane is 41; that of propyltrimethoxysilane is 28.543), facilitating the approach of water to the silica surface. This difference is also (42) Ossenkamp, G. C.; Kemmitt, T.; Johnston, J. H. J. Colloid Interface Sci., in press.

Conclusions Three factors important for the synthesis of functional materials by means of the esterification of silica have been examined. The adsorption isotherms for the chemisorption of a variety of alcohols can be interpreted within the Langmuir adsorption model, suggesting that the chemisorption occurs to form monolayers. The adsorption isotherms also provide important guidance for the synthesis of silica surface-esterified with a tailored surface density of alkoxy groups. The hydrolytic stability of the silica esterified with 1-octanol was significantly improved by treating the esterified silica with hexamethyldisilazane. Silica was esterified with alcohols containing mercapto groups, and it was demonstrated by oxidation of the mercapto groups with I2 that the surface-tethered groups are available for further chemical reactions. The hydrolytic stability of the esters formed with functional alcohols is lower than that of the parent nonfunctional alcohols. Acknowledgment. G.C.O. thanks Industrial Research Limited and Victoria University of Wellington for a doctoral fellowship. This work was funded by the New Zealand Foundation for Research in Science and Technology, Contract No. C0X8001. LA015732Z (43) Arkles, B. Chemtech 1977, 7, 766. (44) Kimura, T.; Kuroda, K.; Sugahara, Y. J. Porous Mater. 1998, 5, 127. (45) Utsugi, H.; Horikoshi, H. Chem. Soc. Jpn. J. Chem. Ind. Chem. 1972, 12, 2244. (46) Zaborski, M.; Vidal, A.; Ligner, G.; Balard, H.; Papirer, E.; Burneau, A. Langmuir 1989, 5, 447.