Effects of Silanol Density, Distribution, and Hydration State of Fumed

G. Vaccaro , S. Agnello , G. Buscarino and F. M. Gelardi. The Journal of Physical .... John M. Galloway , Mayfair Kung , Harold H. Kung. Surface Scien...
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Langmuir 2000, 16, 5008-5016

Effects of Silanol Density, Distribution, and Hydration State of Fumed Silica on the Formation of Self-Assembled Monolayers of n-Octadecyltrichlorosilane Rongwei Wang and Stephanie L. Wunder* Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122 Received December 14, 1999. In Final Form: March 8, 2000 The saturation adsorption of octadecyltrichlorosilane (OTS) on two fumed silicas [Aerosil 380 (A380), Aerosil OX50 (OX50)] was investigated as a function of their hydration states. The amount of water adsorbed onto the fumed silica was in the order superhydrated A380 > as is A380 ∼ superhydrated OX50 > as is OX50. The amount of OTS adsorbed onto the Aerosil samples was in the order superhydrated OX50 > superhydrated A380 . as is A380 ∼ as is OX50. This ordering was explained by the effects of the underlying silica substrate: (i) the effective surface area available for adsorption was greater for the less aggregated OX50 than for A380; (ii) the increased number of non-nearest-neighbor isolated silanols on OX50 provided a greater number of sites for direct hydrogen bonding of OTS; (iii) increased water adsorption resulted in increased OTS adsorption as reported in the literature;1 (iv) however, some “mobile” water not tightly bound to the silica surface of A380 could be rinsed off, along with attached OTS molecules. The alkyl chains were ordered conformationally and laterally for the superhydrated fumed silica, where the amount of adsorbed OTS was greater than about 60%. In the case of the as is samples, where the amount of adsorbed OTS was approximately 34%, the OTS on OX50 was liquidlike, but the OTS on A380 was conformationally ordered but with no lateral order. This was attributed to the effects of the underlying substrate: (i) OX50 was more hydrophobic, with more siloxane bridges on its surface, and had a greater proportion of isolated silanols; (ii) A380 was more hydrophilic owing to the greater amount of adsorbed water. Therefore, at the same surface coverage, there were (i) fewer large islands of OTS on A380, in which the hydrophobic alkyl chains pointed away from the hydrophilic surface, and (ii) a larger number of small islands of OTS on OX50 that could nonspecifically adsorb on the hydrophobic sites on the surface.

Introduction Self-assembled monolayers (SAMs) have been of significant interest in a variety of research and application areas, including electronics,2 nonlinear optics,3 biological interfaces,4-6 and catalysis.7 SAMs can be formed by the spontaneous adsorption of molecules from solution onto a solid substrate and are viable alternatives to LangmuirBlodgett films that can be used to tailor the physical and chemical properties of solid surfaces. They have stability and resistance to mechanical, thermal, and environmental attacks,8 owing to strong covalent bonds to various metallic surfaces and cross-polymerization between adjacent OTS molecules. There are the two main groups of organic compounds used for the formation of SAMs, which include alkanethiols or dialkyl disulfides9 on gold or silver and alkyltrichloro/alkyltrialkoxysilanes10,11 on hydroxylated surfaces (glass, silicon wafers, titanium dioxide, alumina, etc.). Among the latter compounds, octadecyltrichlorosilane (OTS) is the most extensively used silanizing reagent * Phone: 215-204-5046. Fax: 215-204-1532. E-mail: slwunder@ unix.temple.edu. (1) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (2) Sugi, M. J. Mol. Electron. 1985, 1, 2. (3) Kuriyama, K.; Oisi, Y.; Kajiyama, T. Rep. Prog. Polym. Phys. Jpn. 1994, 37, 553. (4) Uchida, M.; Tanizaki, T.; Oda, T.; Kajiyama, T. Macromolecules 1991, 24, 3238. (5) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (6) Dulcey, C. S.; et al. Science 1991, 252, 551. (7) Tundo, P. J. Chem. Soc., Chem. Commun. 1977, 18, 641. (8) Kojio, K.; Ge, S.; Takahara, A.; Kajiyama, T. Langmuir 1998, 14, 971. (9) Mar, W.; Klein, M. L. Langmuir 1994, 10, 188. (10) Plueddemann, E. P. Silane Coupling Agents; Plenum: New York, 1982. (11) Engelhardt, H.; Orth, P. J. Liq. Chromatogr. 1987, 10, 1999.

for forming SAMs on silica-based substrates or metals with oxide coatings.12-15 Although the silanization of silica surfaces with trifunctional alkylsilanes was designed for chromatographic applications 50 years ago,16 the formation and behavior of stable and structurally controlled monolayers remain unclear because of a lack of understanding of the underlying interactions of OTS with hydroxylic substrates. Using attenuated total reflection-infrared spectroscopy (ATR-IR) in their earlier study of OTS SAMs, Sagiv and co-worker12 claimed that the spontaneous adsorption of OTS on the substrates was irreversible. As a result, the original understanding of SAM formation was a threestep mechanism.17-19 First, the trichloro moieties of OTS molecules are hydrolyzed on the hydroxylic substrate surface. Second, the hydrolyzed OTS molecules are strongly attracted via hydrogen bonds to the surface silanols and neighboring OTS molecules. Finally, covalent bonds between the hydrolyzed OTS molecules and surface silanols or adjacent OTS molecules are formed. Sagiv17 even further suggested that the hydrolysis of OTS on the substrate surface instead of in bulk solution prevented a three-dimensional condensation of the silanol groups and (12) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (13) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 101, 201. (14) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (15) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (16) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Sci. 1946, 1, 513. (17) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (18) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (19) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607.

10.1021/la991635i CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000

Saturation Adsorption of OTS on Fumed Silicas

favored the formation of well-defined planar monolayers. Later, work by Tripp and Hair1,20 showed that there was no direct reaction of OTS with surface silanols or adsorption with the first water layer bound to fumed silica surfaces; instead, the adsorption of OTS occurred with the subsequent layers of water. The effect of water on the formation of SAMs has been studied on both planar18,19,21,22 and spherical substrates.1 Angst and Simmons15 found a close-packed monolayer of OTS on a fully hydrated surface of oxidized silicon wafer, as well as lower coverage on a dry silicon wafer. Although both research groups qualitatively confirmed that fully hydrated surfaces favor the formation of SAMs, the quantitative relationship between the amount and type of physisorbed water and/or the type and distribution of silanol groups on the substrate surface and the amount and structure of adsorbed OTS is still not clearly established. In our previous study,23 the state of order of the alkyl chains of OTS was correlated with the hydration state of OX50 fumed silica. For OX50 previously heat-treated at 450 °C (dehydrated), 150 °C (dehydrated), evacuated at room temperature (dehydrated), or “as is” (less than a monolayer of water), Raman spectroscopic evidence indicated only disordered meltlike structures for the alkyl chains. Only for “superhydrated” OX50 was there evidence of long trans sequences and lateral packing of the chains, although the latter never achieved the hexagonal closepacked structure observed for polyoctadecylsiloxane (POS). In the present study, the silanization of two fumed silicas, A380 and OX50, with different surface areas, 380 and 50 m2/g, respectively, will be studied. In particular, the role of the inherent surface properties of the silica substrate, such as the type and distribution of silanol groups, as well as the type and amount of adsorbed water, will be investigated. The distribution of silanol groups, e.g., isolated versus hydrogen bonded (H-bonded), is known to affect the sites of water adsorption24 and thus the amount and type of physisorbed water. The degree of aggregation/agglomeration of the silica, determined by its primary particle size and state of hydration, results in effective surface areas different from the nominal surface areas. These factors can affect the amount of OTS adsorbed on the silica surface and the state of order of the alkyl chains. The amount of water and OTS adsorbed on fumed silicas, determined by TGA, will be correlated with the structure of the physisorbed water and alkyl chains of OTS using spectroscopic evidence. FTIR spectroscopy provides information on the hydration state of fumed silicas. Raman spectroscopy provides information on the intrachain order, that is, the ratio of trans to gauche bonds in the alkyl chain, as well as evidence of interchain packing, and will be used, as it has been in studies of membrane structures,25 to simultaneously determine the lateral packing and conformational order of alkyl chains attached to the fumed silica surface. Experimental Section Materials. Fumed silicas were obtained from Degussa AG and had nominal surface areas and average primary particles sizes of 50 ( 15 m2/g and 40 nm for OX50 and 380 ( 30 m2/g and (20) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215. (21) LeGrange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (22) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (23) Wang, R.; Guo, J.; Baran, G.; Wunder, S. L. Langmuir 2000, 16, 568. (24) Gallas, J. P.; Lavalley, J. C.; Burneau, A.; Barres, O. Langmuir 1991, 7, 1235. (25) Gaber, B.; Peticolas, W. L. Biochim. Biophys. Acta 1977, 465, 260.

Langmuir, Vol. 16, No. 11, 2000 5009 7 nm for A380, respectively. Anhydrous pentane (95% purity) were obtained from Aldrich. The chemicals were used as received. Sample Preparation. The fumed silica particles were either used as received and denoted as “as is” or exposed to water vapor. The latter was effected by bubbling high-purity argon gas through doubly distilled water and passing it through Aerosil powder for varying lengths of time. This procedure left the fumed silica powdery, i.e., not clumped. When equilibrium water levels were obtained, that is, when the FTIR spectra showed no further changes, the silica was denoted as “superhydrated”; this occurred after ∼3 days for A380 and ∼4 days for OX50. The method used to hydrate the silica surface corresponds to what has been called “fully hydrated” in the literature.26 When the hydrated silicas were analyzed for TGA or FTIR analysis, or the superhydrated silica was used for further silanization, they were transferred quickly, to ensure that the water content of the particles did not change. Different batches of “as received” fumed silica had variable hydroxyl group density and water content. The same batch had variable hydroxyl group density and water content depending on the storage conditions. Adsorption was initiated by the addition of between 5 and 10 g of OTS to ∼100 mL of anhydrous pentane containing 0.5-2 g of fumed silica. The procedure was carried out in a glovebox that was purged with argon and evacuated in excess of five times and kept under a positive pressure of argon. The reaction mixture was stirred for 4 h at a temperature of 25 ( 2 °C and transferred to centrifuge tubes in the glovebox. The stoppered tubes were centrifuged outside the glovebox and moved back to the glovebox to decant the supernate. The samples were washed with fresh anhydrous pentane to remove unreacted OTS. This procedure was repeated four times until no OTS-based products were detected in the supernate. The fumed silica samples were further rinsed with pentane in a Bu¨chner funnel and finally dried under reduced pressure at room temperature overnight. Raman Spectroscopy. Raman spectra were collected using a computer-controlled double monochromator (SPEX 1403) with 1800 groove/mm gratings and a thermoelectrically cooled photomultiplier tube. The samples were excited with ∼30 mW of the 514.5 nm line of an argon-ion laser. All of the spectra were recorded in a backscattering geometry with an 80× microscope objective and at 2 cm-1 steps with 1 s integration interval and 5 cm-1 resolution, and signals from >20 scans were averaged. FTIR Spectroscopy. FTIR spectra were recorded at room temperature with 10 scans and a resolution of 1 or 4 cm-1 using a Mattson Research Series FTIR spectrometer. Plain fumed silica was thinly spread on polished NaCl plates, and spectra were obtained without purging and were referenced to the unpurged background. This was necessary since the dry air purge rapidly dehydrated the silica. Thermogravimetric Analysis (TGA) Measurement. TGA was performed on a TA Instruments Hi-Res TGA 2950 Thermogravimetric Analyzer using a ramp rate of 10 °C/min. The temperature was increased from room temperature to 600 °C for plain fumed silica. The silanated samples were held at 100 °C for 0.5 h, and then the temperature was raised to 800 °C.

Results and Discussion Characterization of OX50 and A380 Surfaces. It is well-known that there are four main silanols (single, geminal, vicinal, H-bonded, or bridged) on the silica surface and the silanol density is almost independent of the specific surface area.27,28 The number and type of silanols can be determined quantitatively by various methods, including FTIR spectroscopy.29,30 The IR spectra of silicas exhibit a sharp band at 3747 cm-1 that is due to non-nearestneighbor (or noninteracting) isolated silanols. Burneau (26) Morrow, B. A.; McFarlan, A. J. Langmuir 1991, 7, 1695. (27) Fierro, J. L. G. Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1990; Vol. 57. (28) Michael, G.; Ferch, H. Technical Bulletin Pigments No. 11; Degussa AG: Frankfurt/M., 1993. (29) Schneider, M.; Boehm, H. P. Kolloid Z Z. Polym. 1963, 187, 128. (30) Mathias, J.; Wannemacher, G. J. Colloid Interface Sci. 1988, 125, 61.

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et al.31 suggested that nearest-neighbor isolated silanols absorb around 3735∼3746 cm-1, by an analysis of the far-IR spectrum. It is difficult to distinguish between isolated (non-nearest-neighbor and nearest-neighbor) and geminal silanols in the mid-infrared spectrum because of the similarity in their properties.31 A shoulder band at lower wavenumber, at approximately 3720 cm-1 (3690∼3735 cm-1), is due to the terminal silanol group of an H-bonded pair or chain of H-bonded silanols. The small broad band centered near 3650∼3670 cm-1 has been attributed to H-bonded silanols that are perturbed owing to interparticle contact and have been called internal.31 The perturbed silanols (about 10∼20% of total silanols) are largely inaccessible to many reactants in gas-phase reactions.26,28 The band due to H-bonded pairs or chains of silanols is centered at ∼3520 cm-1. When physisorbed water is present, there is a broad peak centered at around 3400∼3500 cm-1 that partially overlaps the H-bonded silanols. The dependence of the structure of the water-silica interface on the surface silanol distribution has recently been reviewed.32 Water is believed to adsorb on hydrophilic sites of the silica surface, not on the siloxane bridges (SisO-Sis, where Sis is a surface silicon atom). Burneau et al.31 classified free surface silanols into three principal groups: terminal silanols, to which water strongly adsorbs, nearest-neighbor isolated silanols (also called weakly perturbed silanols), which together can bond a single water molecule, and truly isolated, non-nearest-neighbor silanols, which are least likely to adsorb water because the calorimetric heat of a single hydroxyl-water bond is approximately 6 kcal/mol, less than the heat of liquefaction, 10.5 kcal/mol.33 In addition, geminal silanols could be sites for water adsorption, but water would desorb easily from these sites because the H-bonds between the water molecules and surface silanols are not linear. The physisorbed water has been described as existing in multilayers on the silica surface, with the first hydration stage more strongly H-bonded to surface silanols, and the second hydration stage of “mobile” water further from the interface. The second adsorption stage has been described as corresponding to the clustering of water around the first adsorption centers eventually leading to a liquidlike layer; this liquidlike water layer is not observed in the case of Aerosil fumed silica.28 Figures 1 and 2 show the FTIR spectra of single batches of A380 and OX50, respectively, as a function of aging, evacuation conditions, thermal treatment, and exposure to water vapor. The top row in Figure 1 shows the FTIR spectra of A380 just received (Figure 1a), after having been kept under ambient conditions for >2 years (Figure 1d), and superhydrated, after exposure to saturation water vapor levels (Figure 1g). The presence of adsorbed water can be seen as a shoulder band at ∼3240 cm-1, and increases as expected with exposure to water vapor. The band due to H-bonded pairs or chains of silanols is observed at ∼3423 cm-1 and the terminal silanol in the H-bonded pairs or chains is observed at 3665 cm-1. The relative intensity of the isolated silanol peak at 3747 cm-1 decreases with increasing amount of adsorbed water and is not observed at all in the case of superhydrated A380. The second and third rows show the FTIR spectra of these same samples after evacuation at room temperature for 1 h and after heat treatment at 150 °C for 1 h, respectively. (31) Burneau, A.; Barres, O.; Gallas, J. P.; Lavalley, J. C. Langmuir 1990, 6, 1364. (32) Legrand, A. P. The Surface Properties of Silicas; John Wiley & Sons: New York, 1998. (33) Klier, K.; Zettlemoyer, A. C. J. Colloid Interface Sci. 1977, 58, 216.

Wang and Wunder

Figure 1. FTIR spectra of the same batch of A380: columns from left to right are spectra of as received samples, as is samples stored for >2 years, and superhydrated samples. The top row is spectra of the untreated samples, the middle row is spectra of the samples after evacuation at room temperature for 1 h, and the bottom row are the spectra of samples after evacuation at 150 °C for 1 h (resolution, 4 cm-1).

Figure 2. FTIR spectra of the same batch of OX50: columns from left to right are spectra of as received samples, as is samples stored for >2 years, and superhydrated samples. The top row is spectra of the untreated samples, the middle row is spectra of the samples after evacuation at room temperature for 1 h, and the bottom row are the spectra of samples after evacuation at 150 °C for 1 h (resolution, 1 cm-1).

After these treatments, the shoulder water band at ∼3240 cm-1 is no longer observed. The spectra for a sample evacuated at room temperature and at 150 °C appear very similar but are not identical. The relative intensity of the 3747 cm-1 band to the 3423 cm-1 band increases for the silica treated at the higher temperature. This may be due to small amounts of water still adsorbed on the silica evacuated at room temperature or to some dehydroxylation of the silica heat-treated at 150 °C. The most striking comparison is the significant differences in the spectra of 150 °C heat-treated samples prepared from the same silica with different initial water contents. The relative intensity of the 3747 cm-1 band to the 3423 cm-1 band decreases from left to right, i.e., with increasing initial hydration state of the samples. These results clearly indicate that there is an increase in the total number of surface silanols for silica exposed to water vapor for increased periods of time. The increased surface silanol density results in greater numbers of silanols that are close enough for H-bond formation. This data is consistent with literature results. For as is fumed silica, the total silanol group density of about 2.5 SiOH/nm2 is too low for the formation

Saturation Adsorption of OTS on Fumed Silicas

of many intramolecular hydrogen bonds,28 and the bridged silanols are primarily intermolecular, i.e., between primary particles. In the case of well “hydrated” fumed silica, there are 4.9-5.0 SiOH/nm2,26 increasing the probability of intramolecular H-bond formation. Similar trends are observed for the OX50 silica, as shown in Figure 2. Superhydrated OX50 (Figure 2g) never adsorbs water enough to completely eliminate the isolated silanol peak at 3747 cm-1. The amount of adsorbed water is less for all of the OX50 samples (Figure 2a,d,g) compared with the A380 samples (Figure 1a,d,g). The spectra of dehydrated OX50 (Figure 2c,f,i) and “dehydrated” A380 (Figure 1c,f,i) show differences in the distribution of silanols on their respective surfaces, as indicated by the relative intensity of the peaks associated with each type of silanol. The ratio of isolated silanols to (inaccessible + H-bonded silanols) is always greater for OX50 compared with A380 samples. The ratio of H-bonded silanols to (inaccessible + H-bonded silanols) is always greater for A380 compared with OX50. Although the inaccessible silanol peak has been shown to increase in intensity with increased pressure used to form a freely standing disk,34 the spectra shown here were obtained without externally applied pressure. These results are in agreement with the data of Mathias et al.,30 who showed that the isolated silanol group density decreased with increasing specific surface area (i.e., decreasing particle size) of fumed silica, with a value of 1.7 SiOH groups/nm2 for OX50 and 0.9 SiOH groups/nm2 for A380 (data taken from graphs in their paper). In addition, the density of H-bonded or perturbed silanols was shown to increase with decreasing particle size, with a value of approximately 0.4 SiOH groups/nm2 in the case of OX50 and 1.6 SiOH groups/nm2 for the A380 silica (calculated from their data). As is A380 has significantly more adsorbed water than does as is OX50. This is consistent with literature reports indicating that non-nearest-neighbor isolated silanols, more prevalent on the OX50 surface, are the least likely sites of water sorption,31 and weakly perturbed silanols (such as terminal and nearest-neighbor isolated silanols), more prevalent on the A380 surface, are the best sites of water adsorption.32 Although the amount of water initially adsorbed on the fumed silica depends on the storage conditions, representative batches of A380 always had more physisorbed water initially than did representative batches of OX50. The amount of physisorbed water increases with exposure time to water vapor for both samples, and it took more time for the OX50 samples to reach the same hydration state as for the A380 samples. In particular, the superhydrated OX50 (Figure 2 g) is similar in appearance to the as is A380 (Figure 1d), except for the inaccessible silanol peak. FTIR spectra give qualitative information on the hydration state of the silica surface, but TGA provides a quantitative measure of the amount of physisorbed water. Figure 3 plots thermograms of as is and superhydrated A380 and OX50. The amount of physisorbed water on the fumed silicas, presented in Table 1, decreased in the order superhydrated A380 > as is A380 ∼ superhydrated OX50 > as is OX50, and these results are in agreement with the IR data. With increasing temperature, the weight loss curves all show an initial sharp decline followed by a more gradually changing weight loss region. In the case of A380 samples, the change in slope occurs at ∼78 °C, and for OX50, it occurs at ∼50 °C. These temperatures are independent of the hydration state of the Aerosil fumed (34) Armistead, C. G.; Tyler, A. J.; Hambleton, F. H.; Mitchell., S. A.; Hockey, J. A. J. Phys. Chem. 1969, 73, 3947.

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Figure 3. TGA plots of (a) superhydrated A380; (b) as is A380; (c) superhydrated OX50; (d) as is OX50.

silicas. The difference in the temperatures at which the change of slope occurs may be due to the nature of the H-bonding of water with the surface silanols. For A380, there are more weakly interacting nearest-neighbor single silanols, and therefore more water molecules on A380 can be H-bonded to two adjacent SiOH groups. Since the breaking of two H-bonds is expected to take more energy than required to break a single H-bond, more H-bonded water molecules on A380 are removed at higher temperature. This result further indicated that the OX50 surface has less affinity for water as compared with that of A380 owing to its large portion of non-nearest-neighbor isolated or geminal silanols. Derivative TGA curves (not shown) exhibit broad minima at approximately 150 °C for all the samples, and maxima at 182 °C for the OX50 silica and at 210∼225 °C for the A380 samples. Although fumed silica loses 90% of the adsorbed water molecules under reduced pressure at room temperature,1,35 in a TGA experiment the water is removed at ambient pressure and increasing temperature. The minima in the TGA curves can be attributed to the approximate temperature at which the physisorbed water has come off. The least tightly bound water, referred to as “mobile water”, comes off at lower temperature (below ∼50, 78 °C for OX50, A380, respectively), and water more tightly bound to the surface silanols is removed between these temperatures and 150 °C. The temperatures of 182 and 210-225 °C correspond to the maxima in the temperatures at which dehydroxylation occurs for OX50 and A380, respectively. The data are consistent with literature reports1,35,36 that physisorbed water can be removed below 167 °C; above this temperature surface silanols start to condense and evolve water. Morrow27 observed no difference in the IR spectra of silicas dehydrated at 25 and 150 °C, demonstrating that dehydroxylation does not occur below the higher temperature. The higher temperature for condensation of the silanols for A380 compared with OX50 silica may result from the different silanol group distributions in the two silicas. As observed in Figure 1, the A380 silica has a large proportion of H-bonded silanols. In a chain of H-bonded silanols, the interior H-bonded silanols are less accessible than the terminal silanols26 and thus are expected to condense at a higher temperature. Similarly, there is a larger percentage of weakly perturbed silanols (nearest-neighbor isolated silanols) on A380; the further apart the silanols, the higher the temperature of dehydroxylation. Truly isolated silanols (non-nearest-neighbor isolated silanols), (35) Voort, P. V. D.; Gillis-D’Hamers, I.; E. F. Vansant, E. F. J. Chem. Soc., Faraday Trans. 1990, 86, 3751. (36) Tripp, C. P.; Hair, M. L. J. Phys. Chem. 1993, 97, 5693.

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Wang and Wunder

Table 1. Water Content, Surface Silanol Density, and Order Parameters of OTS on Fumed Silica

sample A380 OX50

dehydrated as is superhydrated as is superhydrated

total water density,a H2O/nm2

mobile water density,b H2O/nm2

H-bonded water density,c H2O/nm2

silanol density,d OH/nm2

OTS coverage, %

Ttrans, %

Plateral, %

4.7 8.7 2.3 4.0

4.2 8.0 1.5 2.7

0.5 0.7 0.8 1.3

2.7 4.0 2.0 3.3

17 34 61 32 71

20 61 88 41 85

4 11 30 9 23

a Calculated by total weight loss from initial temperature to 150 °C. b Calculated by weight loss below the temperature at the sharp inflection point of TGA curves. c Calculated by weight loss from the sharp inflection point to 150 °C. d Calculated by weight loss above 150 °C up to 600 °C.

more prevalent on OX50, do not start to condense until 800-1000 °C.26,35 Table 1 presents estimates of the amount of total physisorbed water, mobile water, and H-bonded water. The estimates were obtained by attributing the weight loss below 150 °C to the total amount of physisorbed water, and the relative contributions of mobile versus H-bonded water by assuming that mobile water came off below the temperature at which the change in slope occurred in the TGA curves, and more tightly bound water came off between this temperature and 150 °C. For as is silica with physisorbed water, the amount of water has been reported to be 2.5-4.0 H2O/nm2.1 The value for A380 is slightly above this range and for OX50 is slightly below this range. The quantity of physisorbed water on the superhydrated A380 or OX50 is about 1.8 times the amount of water on the respective as is samples, and this number is the same arrived at by Tripp.37 The TGA results quantitatively confirmed that there were the differences in water adsorption between A380 and OX50. There were more mobile water molecules adsorbed on as is A380 than as is OX50, and superhydrated A380 held more mobile water than did superhydrated OX50. In addition, although as is A380 and superhydrated OX50 had similar total amounts of physisorbed water, the amount of mobile water was greater on the A380 sample. This can be seen by a comparison of Figure 3b with Figure 3c, where the fast initial water loss is greater for the as is A380 sample. The greater mobile water densities for the A380 samples compared with the OX50 samples arise because there were initially more nucleation sites for water adsorption on A380, resulting in further adsorption of less tightly bound water. There is more H-bonded water on OX50 compared with A380, but the water is more weakly bound (i.e., comes off at a lower temperature, ∼50 °C) for OX50 compared with A380 (comes off at a higher temperature, ∼78 °C). The weight loss above 150 °C represents the removal of water from the condensation of adjacent silanols. Since two SiOH groups condense to form one water molecule, the number of SiOH groups reported in Table 1 is twice the measured number of H2O loss. The silanol densities for both as is Aerosil samples are close to those (2.5 OH/ nm2 for A380 and 2.2 OH/nm2 for OX50) measured by Mathias et al.,30 although values up to 3.5 OH/nm2 have been reported.26 The silanol densities for both superhydrated Aerosil samples are lower than the values (4.9 or 5.0 OH/nm2) reported in the literature.27,38 One reason for the discrepancy is that in the present experiments, in which the samples were heated to 600 °C, not all of the isolated silanols have been removed. It has been reported26,27 that after evacuation at 400 or 450 °C, the surface silanol density falls to about 1.2-1.5 OH/nm2. The silanol group density for the samples in the present study should be increased by this amount, and thus is in agreement with the previously reported data. (37) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1961. (38) Zhuravlev, L. T. Langmuir 1987, 3, 316.

Figure 4. TGA plots of degradation of octadecyl chains on (a) superhydrated OX50; (b) superhydrated A380; (c) as is A380; (d) as is OX50.

Characterization of OTS Adsorbed on A380 and OX50 with TGA. Figure 4 shows TGA plots of weight losses of adsorbed OTS molecules on as is and superhydrated fumed silicas. The theoretical mass of attached octadecyl groups per unit area for 100% coverage is calculated to be 2.1 mg/m2 assuming a maximum value of 5 adsorbed OTS molecules/nm2. Using this theoretical value, the percent coverages of OTS on the four silica surfaces are presented in Table 1. In addition, the percent coverage for dehydrated A380 silica is presented, and it is the lowest observed. Both superhydrated OX50 and A380 have greater amounts of adsorbed OTS than do the as is samples. Comparison of as is A380 and as is OX50 shows that the OTS coverages are comparable, although the amount of water on the as is A380 silica is twice that on the as is OX50. Comparison of superhydrated A380 with superhydrated OX50 indicates that although the amount of water on superhydrated A380 is nearly double that on superhydrated OX50, the amount of adsorbed OTS molecules was greater on the latter. A comparison of superhydrated OX50 with as is A380 shows that the surface coverage of OTS for OX50 is twice that of A380, although the surface densities of physisorbed water on the samples are similar. These results indicate that several interrelated factors must be considered in order to explain the complex relationship between the amount of adsorbed OTS and the underlying silica substrate. These effects all derive from the amount and type of silanols on the silica surface; these in turn affect the type and amount of adsorbed water and the state of aggregation/agglomeration of the primary particles of the Aerosil fumed silica. A schematic of the proposed structures for fumed silica is presented in Figure 5. Fumed silicas are almost always found as aggregates/agglomerates, which result from intermolecular H-bonding and sintering (Si-O-Si formation during the initial hydrolysis/condensation at high temperature) between the primary particles. The nominal surface area of the silicas is thus reduced by their state

Saturation Adsorption of OTS on Fumed Silicas

Figure 5. Schematic of proposed structure for fumed silica.

of agglomeration; the number of inaccessible silanol groups increases and the area available for silanization decreases. The increased surface area of the smaller size particles provides increased intermolecular H-bonding sites, and the state of aggregation/agglomeration therefore increases with decreasing particle size. Concomitantly, the effective area available for silanization is greater for OX50 compared with A380. For superhydrated Aerosil samples, there is increased silanol density, so that there can be more intramolecular H-bonding. During the silanization procedure in anhydrous pentane, the initially hydrophilic, agglomerated silica disperses more easily after adsorption of the hydrophobic OTS. Increased silanol density on the superhydrated (i.e., fully hydrated) silica also provides a greater number of sites for H-bonding of hydrolyzed OTS with the surface. Isolated silanols, more prevalent on OX50 compared with A380, provide direct H-bonding sites for the attachment of OTS; water is least likely to bind to non-nearest-neighbor isolated silanols, and they are therefore available for H-bonding to OTS. Last, water is necessary for the formation of good monolayers to both hydrolyze the OTS and provide mobility for self-assembly. However, water that is too loosely associated with the silica can also condense with OTS molecules; in this case, both the water and the attached OTS can be easily rinsed off. All of these factors can contribute to the complex nature of the adsorption of OTS to the silica surface. For OTS attached to dehydrated fumed silica, the amount of adsorption is low, below 20%. This is consistent adsorption onto a silica surface with a low density of surface silanol and for where the primary particles are aggregated/agglomerated. Increased surface coverage is observed for as is silicas compared with dehydrated silica, owing to the presence of adsorbed water, as expected. There was similar surface coverage for the as is A380 and as is OX50, although there was twice as much adsorbed water on the former sample. Both as is silicas have similar silanol densities, but as is OX50 has a greater percentage of non-nearest-neighbor isolated silanols. Since the latter directly H-bonds to OTS, there are more primary sites of attachment of OTS to the as is OX50 surface, better anchoring the OTS molecules. For as is A380, the initial layer of water on as is A380 may not react with OTS as observed by Tripp et al.;1,20 this may arise because this water is already H-bonded to the surface silanols. In

Langmuir, Vol. 16, No. 11, 2000 5013

Figure 6. Raman spectra of C-H stretching region of OTS adsorbed on (a) superhydrated A380; (b) superhydrated OX50; (c) as is A380; (d) as is OX50.

addition, the effective surface area of as is A380 is less than that of as is OX50 owing to agglomeration. The similar surface coverages for both samples suggest that there is cancellation of these opposing effects. The increased OTS adsorption for superhydrated A380 and OX50 compared with their as is counterparts is expected on the basis of both the higher silanol densities and water content. Both factors result in better dispersion and thus increased effective surface areas for the superhydrated samples. The higher silanol densities provide more sites for H-bonding of OTS to the surface. The surface coverage is greater for the superhydrated OX50 sample, despite the greater water content on A380. As was the case for the as is samples, the higher percentage of nonnearest-neighbor isolated silanols for superhydrated OX50 leads to a greater number of sites of direct H-bonding of OTS to the surface, and the greater mobile water content for A380 may lead to some removal of OTS during rinsing. In addition, the effective surface area is expected to be greater for superhydrated OX50 than for superhydrated A380 on the basis of the larger primary particle size of OX50. The doubled OTS coverage of superhydrated OX50 compared with as is A380, despite similar amounts of adsorbed water, indicates the importance of the increased silanol density of superhydrated versus as is samples, and the higher numbers of non-nearest-neighbor isolated silanols on OX50 in anchoring the OTS to the silica surface. The increased silanol density of superhydrated OX50 and its larger primary particle size compared with A380 will result in both better dispersion and thus increased effective surface area for the OX50. The highest OTS surface coverage, 71%, was observed for the superhydrated OX50 silica. Although TEM pictures28 show these particles as the least aggregated fumed silica, the FTIR spectra indicate the existence of a population of inaccessible silanols. The difference in OTS coverage between these two samples further demonstrates that the surface property of silica determines the structure of the watersilica interface, which ultimately affects OTS adsorption. Characterization of OTS Adsorbed on As Is and Superhydrated Fumed Silicas with Raman Spectroscopy. Figures 6 and 7 are Raman spectra of the C-H

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Figure 7. Raman spectra of the skeletal stretching region of OTS adsorbed on (a) superhydrated A380; (b) superhydrated OX50; (c) as is A380; (d) as is OX50.

and skeletal stretching regions of OTS adsorbed on as is and superhydrated fumed silicas, respectively. The C-H and skeletal stretching regions of phospholipid spectra can be used to estimate the lateral crystallike order between the chains and the all-trans bonds in the hydrocarbon chains, respectively.25 After investigating the C-H regions of the spectra of pure solid hexadecane and of hexadecane dissolved in a solid perdeuteriohexadecane matrix, Gaber and Peticolas25 pointed out that both crystal (lateral) interaction and “rotomer-broadening” effects account for about 50% of the total intensity change upon melting. They developed two order parameters as measures of the order due to intrachain structure and lateral crystalline interactions. In the present study, the trans parameter is accordingly defined as

Ttrans )

(I1124/I1062)observed (I1124/I1062)POS

(1)

where (I1124/I1062)POS ) 0.725 is calculated for polymerized OTS (POS) using Raman data.23 I1124/I1062 is the ratio of the two all-trans bands in an alkane spectrum. In the melt, there is a band at 1090 cm-1 that results from the introduction of gauche conformational isomers. The 1124 cm-1 vibration is unaffected by the band associated with gauche conformers, while the I1062 sits on the low-frequency side of the 1090 cm-1 band. Thus, the intensity of the 1062 cm-1 vibration increases with respect to the 1124 cm-1 band with increasing disorder. The lateral parameter is defined as

Plateral )

ICH2(sample) - ICH2(liq hexadecane) ICH2(crystalline) - ICH2(liq hexadecane)

(2)

where ICH2 stands for I2885/I2850. The values ICH2(liq hexadecane) ) 0.7 and ICH2(crystalline) - ICH2(liq hexadecane) ) 1.5 are directly quoted from Gaber and Peticolas’ study. Plateral is only used as an approximate and semiquantitative parameter, but it will provide us some insight into the amount of lateral packing of the OTS chains on the silica surface. Both order parameters can be thought of as probabilities. Ttrans is denoted as a measure of the all-trans chain probability (Ttrans ) 1 for solid POS; Ttrans ) 0 for no order). The value of Ttrans for an alkane melt at room temperature is approximately 0.25. Plateral is referred to as a measure of the packing probability (Plateral ) 1 for the most possible

packing state, as in crystalline hexadecane; Plateral ) 0 for no packing, as in liquid hexadecane). The values of Ttrans and Plateral are presented in Table 1. The data for dehydrated A380 indicate that the trans/ gauche ratio in the skeletal region, as well as the lateral packing shown in the CH stretching region, is liquidlike. The values of Plateral for as is A380 and OX50 are low, and the spectra appear liquidlike in the CH stretching region. However, the results clearly show that the alkyl chains on as is A380 are more ordered than on as is OX50, although the percent coverages for both are very similar. Ttrans for as is A380 is significantly greater, reflecting the long sequences of trans segments for this sample shown in the Raman spectra of Figure 7c. One possible explanation for the difference in intrachain order between as is A380 and as is OX50 is the nature of the Aerosil surfaces, which results in different distributions of OTS molecules. In the case of as is A380, which has a larger underlying mobile water layer and few isolated SiOH groups, the formation of large islands of OTS, as has been suggested by Sagiv,39 is favored. OTS molecules H-bonded to available isolated SiOH groups form the nucleus of these islands. The greater underlying water layer makes the silica surface more hydrophilic for A380 than for OX50, so that the alkane chains point away from the surface and adopt long trans sequences to maximize their hydrophobic interactions. In the case of OX50, there are more non-nearest-neighbor isolated SiOH groups and a smaller mobile water layer. Once hydrolyzed, the OTS molecules H-bond with the isolated SiOH groups, and selfassembly occurs around these more numerous points of attachment. At the same surface coverage, smaller island formation occurs. In addition, with much less adsorbed water, the silica surface is more hydrophobic for OX50 than for A380, so that the alkyl chains can adsorb on hydrophobic siloxane sites on the surface. Both factors contribute to the conformational and packing disorder observed for the as is OX50. Compared with the as is Aerosil samples, both superhydrated Aerosil samples have better lateral packing and more all-trans sequences in the alkyl chains. Ttrans and Plateral values are similar, although the coverage for OX50 is greater than for A380. In fact, Ttrans and Plateral values are slightly greater for the A380, which has less surface coverage of OTS. These results strongly suggest that the structure and packing of the chains is characteristic of the surface available for silanization, since as discussed above, the A380 is expected to have less accessible surface area owing to increased particleparticle interactions. The high order of both superhydrated Aerosil samples arises from their increased silanol density and increased water content compared with the as is Aerosil samples. The ability of the hydrolyzed alkyl chains to self-assemble on the mobile water layer results in chains oriented away from the hydrophilic surface with good lateral packing. Ttrans and Plateral values are still smaller than those observed for POS, for which Ttrans ) 100% and Plateral ) 43%. The decrease in these values for OTS on silica compared with POS may be attributed to packing constraints proposed by Stevens.40 Analysis of the Effects of Experimental Conditions on the Formation of OTS SAMs on Aerosil Fumed Silicas. As the most common alkylchlorosilane for the preparation of SAMs, OTS adsorbed on a variety of substrates has been extensively studied.12,41,42 Several (39) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054. (40) Stevens, M. J. Langmuir 1999, 15, 2773. (41) Thompson, W. R.; Pemberton, J. E. Langmuir 1995, 11, 1720. (42) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532.

Saturation Adsorption of OTS on Fumed Silicas

factors have been shown to affect the quality of SAMs on the surfaces of substrates. They are the (1) concentration of OTS; (2) time for the adsorption process; (3) temperature for the formation of SAMs; (4) amount of adsorbed water on the surface of the substrate (as discussed above). Using ATR-IR and contact angle measurements, Sagiv and co-workers17 found that the adsorption isotherms of OTS on flat Si surfaces reached a plateau value when the solution concentration of OTS was 10-3 M. At this concentration the OTS monolayers formed on the surface were tightly packed and highly oriented. They further concluded that the adsorption onto the Si substrates from solutions below the critical concentration at which the plateau adsorption occurs produces incomplete monolayers with disordered molecular conformation. Although they claimed that the adsorption of OTS was irreversible owing to the intralayer polymerization and covalent anchoring of the film on the surface, they primarily attributed the formation of a densely packed monolayer to the high surface pressure that was established between the solution and solid surface above a critical concentration of OTS. In the present study, the concentration of OTS in anhydrous pentane was ∼10-2 M and the total amount of OTS added to the solution was usually ∼10 × the theoretical value needed to form a monolayer on the surface (assuming 1 OTS molecule occupies 20 Å2). The ratio of the amount of OTS per gram of Aerosil fumed silica was 7-25 mmol/g, which is higher than values typically cited in the literature.1 Thus, the concentration of OTS used in the present case is sufficiently high to allow saturation adsorption in the plateau region. The rate of formation of monolayers with well-oriented long alkyl chains on a silicon surface was studied by Wasserman and his colleagues43 using tetradecyltrichlorosilane as a silanizing agent. Their results from ellipsometric measurements of monolayer thickness clearly showed that a complete monolayer was formed on Si/SiO2 (exposed to air for 10 min), after 1 h in air at 30% relative humidity, while 5 h were required in a dry atmosphere. They believed that the difference in rate originated essentially from different amounts of water adsorbed on the polar surface of the Si/SiO2 substrate. Recently, Dugger’s group44 used X-ray photoelectron spectroscopy (XPS) to investigate the relationship of saturation coverage of OTS on the surface to solution contact time. Their results indicated that the saturation coverage was reached on oxidized silicon after only 2 h. Their experiments were carefully conducted in a glovebag or glovebox under a dry nitrogen atmosphere. Although contact times less than 30 min have been used in other OTS adsorption studies,12,15,20,45,46 the contact time in the present study was 3-4 h because the experiments were performed in a glovebox under a dry argon atmosphere. The effect of temperature on the silanization was first studied by Silberzan et al.18 using contact angle measurements. They claimed that an OTS monolayer on the oxidized silicon achieved its optimal quality in 2 min at 18 °C while even a 24-h contact time did not give rise to a satisfactory result at 30 °C. Unfortunately, there was a lack of spectroscopic evidence to verify the quality of the monolayers. With the aid of IR and ellipsometry, both Allara’s and Rondelez’s groups47 jointly reported that there (43) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (44) Rye, R. R.; Nelson, G. C.; Dugger, M. T. Langmuir 1997, 13, 2965. (45) Flinn, D. H.; Guzonas, D. A.; Yoon, R. H. Colloids Surf. 1994, 87, 163. (46) DePalma, V.; Tillman, N. Langmuir 1989, 5, 868. (47) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577.

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was a critical temperature for the formation of OTS monolayers on SiO2/Si substrates. They determined that this critical temperature for OTS was 28 ( 5 °C. They also concluded that films prepared below this critical temperature exhibited a heterogeneous structure with closely spaced islands of densely packed all-trans alkyl chains, while the one prepared above the critical temperature showed incomplete coverage and a disordered structure for the alkyl chains. In a separate study, Rondelez et al.48 further concluded that the critical temperature depends on alkyl chain length but is independent of the solvents used for the adsorption. Accordingly, room temperature (25 ( 2 °C) was reasonably chosen for our experiments. Nonetheless, although saturation adsorption conditions were used in the formation of the monolayers, sufficient time was allowed for their formation at a temperature below the critical temperature needed for self-assembly, superhydrated silica was employed, and the adsorption of OTS was below the theoretical value. Other factors must thus be involved in the formation of OTS monolayers on Aerosil fumed silicas. The results from the present study show that full coverage is not expected for Aerosil fumed silicas owing to their state of aggregation/agglomeration that reduces their effective surface area. The aggregation/ agglomeration is more severe for A380 compared with OX50 owing to its smaller primary particle size. About 18% of the hydroxyls are inaccessible even toward small probe molecules such as D2O or ND3,26 and with increasing size of the probe molecule, the number of silanols that can react decreases.26 The number of “sterically” accessible silanols can be estimated on the basis of the adsorption of donor molecules such as morpholine and is found to be less than the number based on deuterium exchange (0.9 SiOH/nm2 versus 2.5 SiOH/nm2).30 An important finding of the present investigation is that the amount and organization of the attached OTS depends on details of the underlying silica substrate such as the type and distribution of silanols, which in turn affect the structure of adsorbed water. The silanol density and distribution for low surface area Si/SiO2 and bulk fused silica used in previous investigations of OTS adsorption are not known, although the pretreatments used to prepare the surface increase the total silanol density. Values of between 4.5 and 12 SiOH/nm2 may therefore be expected; the larger numbers have previously been found for precipitated silicas.26 The silanol type and distribution both directly and indirectly affect the binding of OTS to the silica. Non-nearest-neighbor isolated silanols that do not H-bond with water can directly H-bond to OTS, providing anchoring sites for the silanating agent. Nearest-neighbor isolated silanols and terminal silanols in chains of vicinal H-bonded silanols preferentially H-bond with water, thus making these sites inaccessible for direct H-bonding to OTS. For dehydrated fumed silica, OTS molecules can H-bond to randomly placed nonnearest-neighbor isolated silanols, and the alkyl chains can nonspecifically adsorb to hydrophobic regions on the surface. These results are not in agreement with those of Le Grange et al.,21 who observed patches of ordered chains on flat dehydrated Si/SiO2 and fused silica surfaces. In the case of hydrated fumed silicas, water is believed to cluster around sites to which water is already adsorbed. A380, which has a greater number of sites for adsorption of the initial water layer, therefore, has a greater total amount of adsorbed water and more mobile water. OTS attached to mobile water clusters may be rinsed off. The Raman data of both superhydrated Aerosil samples are very similar and indicate that the alkyl chains have high (48) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367.

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for self-assembly is the hydrophobic interactions of the alkyl tails, with the underlying water providing the mobility for this self-assembly. For Aerosil fumed silica, there was never a truly liquidlike layer on the surface; after hydrolysis there should be no water left at all. Therefore the self-condensation of adjacent OTS molecules adsorbed on the silica surface may occur before or simultaneous with self-assembly of the alkyl tails. The self-condensation may result in dimers, trimers, and higher molecular weight species that may be difficult to remove from the silica surface, and result in the irreversibility of adsorption observed.14,39 Conclusions

Figure 8. Schematic of the adsorption of OTS on fumed silica surfaces.

trans content and good lateral packing, despite the differences in their surface coverage (61% A380, 71% OX50); larger effective surface areas are expected for fumed silicas with larger primary particle size. These results suggest that there is similar packing of the chains on the effective surface available for silanation for both Aerosil samples. The differences in the alkyl chain order for as is A380 and as is OX50, both of which have similar surface coverage (∼30%), can be explained by the effects of the underlying silica substrate. Larger islands of OTS assemble on the more hydrophilic A380 surface that has fewer isolated silanols, and smaller islands of OTS assemble on the more hydrophobic OX50 surface that has a higher isolated silanol density. A schematic of the adsorption of OTS on fumed silica surfaces is presented in Figure 8. In the mechanism generally accepted for the self-assembly of OTS molecules, the first step is the hydroxylation of the chlorides of OTS by surface water on the silica. The amount of water on any of the silica surfaces investigated (see Table 1) is less than required to hydrolyze all of the chlorine groups of OTS (3 chlorines × 5 OTS ) 15 molecules/nm2) at full surface coverage. However, since full surface coverage of OTS does not occur, water on the silica surface not silanated may migrate to the sites of OTS adsorption and hydrolyze the remaining Cl groups. The energy to remove a water molecule from a silica substrate is less than that required to vaporize H2O (10 e ∆Hads e 20 kcal/mol).33,49 In the anhydrous pentane used for the experiment, surface water is more likely to move laterally along the surface. In the case of the wettest sample, superhydrated A380, 40% of the surface is not silanated. The amount of water available for the OTS molecules on the 60% of the surface that is covered, if all of the water on the rest of the surface migrates, is 1.4 × 8.7 = 12 H2O/nm2. This is still less than required for complete hydrolysis. Generation of water to hydrolyze all the Cl on the OTS thus requires condensation between adjacent hydroxyl groups, since there is not expected to be condensation between the hydrolyzed OTS and the surface silanols.20 As has been suggested by Allara,50 for Si/SiO2 planar substrates, the driving force (49) Iler, R. K. The Chemistry of Silica Solubility, Polymerisation, Colloid and Surface Properties, and Biochemistry; John Wiley and Sons: New York, 1979.

Using TGA analysis, the amounts of water and OTS adsorbed on Aerosil fumed silicas have been monitored. Both superhydrated and as is A380 fumed silicas adsorb more water molecules than their OX50 counterparts, and A380 was more easily and quickly hydrated than OX50. These results are consistent with the different distribution of silanol groups on the two silica surfaces due to the different temperatures used in their production.30 In particular, the ratio of non-nearest neighbor isolated silanols to nearest neighbor silanols and terminal silanols in chains of vicinal H-bonded silanols is greater for OX50 compared with A380. This difference in silanol distribution results in less adsorbed water for the OX50 since water adsorption is least favorable on isolated silanol sites. In addition, there is a greater percentage of mobile water on the water adsorbed onto the A380 surface. Saturation adsorption of OTS was found to decrease in the order superhydrated OX50 > superhydrated A380 . as is A380 ∼ as is OX50. The differences in the saturation adsorption of OTS on the Aerosil samples arise from different effective surface areas, silanol densities, and distributions, and the possibility that OTS attached only to mobile water can be easily rinsed off. Raman data of OTS adsorbed on dehydrated A380 indicate complete interchain and intrachain disorder; the spectra are indicative of alkane melts. Comparison of the as is samples indicates that comparable (about 32-34%) OTS coverage results in disordered structures for OX50 but high trans content alkyl chains coupled with poor lateral order for the A380 sample. This has been explained as originating from the more hydrophilic nature of the A380 surface coupled with the decreased number of isolated SiOH and less effective surface area for A380 compared to OX50. OTS forms smaller islands centered on the isolated silanols on the more hydrophobic surface of OX50, allowing the chains room to disorder. Larger clusters of OTS adsorb on the smaller exposed area of A380, resulting in chains that orient away from the hydrophilic surface. Raman spectra of OTS adsorbed on superhydrated Aerosil samples indicate that well-ordered SAMs are formed for both OX50 and A380 despite differences in their surface coverage; increased surface coverage is expected for the Aerosil fumed silica with a larger primary particle size. Acknowledgment. The authors gratefully acknowledge the support of NIH Grant AR45472. LA991635I (50) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135.