Surface Geometric Structure of Chemically Modified Silica Studied by

Surface modification was performed by the autoclave method with 1-dodecanol to control the surface wettability. The preferential dispersion test prove...
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Langmuir 2000, 16, 3281-3287

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Surface Geometric Structure of Chemically Modified Silica Studied by Direct Atomic Force Microscopy (AFM) Imaging and Adsorption Method Masayoshi Fuji,* Kotoe Machida, Takashi Takei, Tohru Watanabe, and Masatoshi Chikazawa Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji,Tokyo 192-0397 Japan Received August 20, 1999. In Final Form: December 13, 1999 The relationship between wettability and surface geometric structure of modified silica particles and glass plates was investigated. Surface modification was performed by the autoclave method with 1-dodecanol to control the surface wettability. The preferential dispersion test proved that wettability varied at a surface modification ratio of 20%, which coincided with the changing point of the geometric structure of modifier chains determined by the adsorption method. The geometric structure was also evaluated by atomic force microscopy (AFM), and we could obtain the hexagonal packing of chains of the modifiers both in water and air at high surface modification ratio. Imaging of the surface of the nanosized particles on a glass plate was accomplished in water by taking advantage of the hydrophobic attractive force, which was proved by adhesion force measurements.

Introduction Surface modification of fine particles by various alkylsilanes and alcohols is used industrially to vary the surface wettability from hydrophilic to hydrophobic and to control the properties of particles, such as aggregation, dispersion, and adhesion. Based on these industrial applications, a number of studies1-4 have been carried out to clarify the relationship between the wettability and surface properties. For example, Wei et al.5 measured the wettability of modified glass after long storage times in different media and investigated the stability of modified surfaces. However, in most of the cases, much attention had been paid to the macroscopic properties, and the contribution of the geometric structure of the modifier chains to the wettability was not fully analyzed. On the other hand, the development of atomic force microscope (AFM) has made possible the direct imaging of modified surfaces, and many studies6-10 have been made with this powerful tool during the last 10 years. Flinn et al.11 examined the adsorption of octadecyltrichlorosilane on fumed silica using AFM and resolved the kinetics of the silylation process. In an other example, O’Shea et al.7 compared the results of Fourier transform infrared (FTIR) analysis and the AFM image of trichlorosilane molecules on the silica substrate and clarified the formation mech* Author to whom all should be sent. E-mail: fuji-masayoshi@ c.metro-u.ac.jp. Telephone and fax: +81-426-77-2850. (1) Israelachvili, J. N.; Michelle L. G. Langmuir 1989, 5, 288. (2) Laslowski, J.; Kitchener, J. A. J. Colloid Interface Sci. 1969, 29(4), 670. (3) Tsutumi, K.; Takahashi, H. Colloid Polym. Sci. 1985, 263, 506. (4) Barthel, H. Colloids Surf. 1995, 101, 217. (5) Wei, W.; Bowman, R. S.; Wilson, J. L.; Morrow, N. R. J. Colloid Interface Sci. 1993, 157, 154. (6) Schonherr, H.; Vancso, G. J. Langmuir 1997, 13, 1567. (7) O’Shea, S. J.; Welland, M. E.; Rayment, T. Langmuir 1993, 9, 1826. (8) Tsao, Y.-H.; Yang, S. X.; Evans, D. F.; Wennerstrom, H. Langmuir 1991, 7, 3154. (9) Banga, R.; Yarwood, J. Langmuir 1995, 11, 4393. (10) Fujii, M.; Sugisawa, S.; Fukuda, K.; Kato, T.; Shirakawa, T.; Seimiya, T. Langmuir 1994, 10, 984. (11) Flinn, D. H.; Guzonas, D. A.; Yoon, R.-H. Colloids Surf. 1994, 87, 163.

anism of the self-assembling monolayer. However, because of the limits of the applicability of AFM, most studies have been restricted to flat surfaces, and obtaining morphological information of colloidal particles had to overcome the problems of probe-induced particles.12-15 In this paper, we studied surface geometric structure effects on the wettability from the aspect of molecular order using alkoxylated silica particles and glass plates. The geometric structure of modifier chains was evaluated by adsorption of either nitrogen or neopentane, and the wettability was examined by the preferential dispersion test. In addition, AFM was used for imaging of the modified surface and the measurement of adhesion forces between tip and modified glass plates. The direct imaging of nanometer-sized particles was obtained in water by the new sample-stabilization techniques that can provide fixation of a particle on a glass plate. Experimental Section Materials. Nonporous amorphous silica particles with a surface area of 195 m2‚g-1 (Aerosil 200; Nippon Aerosil, Ltd.) and slide glass (IWAKI, Ltd.) were used. The mean diameter of silica particles was 12 nm, and the transmission electron microscopy (TEM) image of Aerosil200 is shown in Figure 1. 1-Dodecoxy alcohol of reagent grade and hexane were purchased for surface modification from Kanto Chemical, Inc. Surface Treatment. Alkoxylation was achieved by the autoclave method at 235 °C and 30 atm for 1.0 h.16 In reactions, the silica particles and glass plates were set in the same batches. After modification, the samples were preserved in a desiccator or distilled water. The surface modification ratios of silica particles were measured by thermogravimetric/differential thermal analysis (TG/DTA; TG/DTA300; Seiko Instrument, Inc.). The surface modification ratios were calculated by dividing the number of (12) Mulvaney, P.; Ciersig, M. J. Chem. Soc., Farady Trans. 1996, 92, 3137. (13) Kardassi, D.; Tsiourvas, D.; Paleos, C. M. J. Colloid Interface Sci. 1997, 186, 203. (14) Junno, T.; Anad, S.; Deppert, K.; Montelius, L.; Samuelson, L. Appl. Phys. Lett. 1995, 66(24), 3295. (15) Butt, H.-J.; Kuropka, R.; Christensen, B. Colloid Polym. Sci. 1994, 272, 1218. (16) Utsugi, H. Hyomen 1973, 11, 593.

10.1021/la991134x CCC: $19.00 © 2000 American Chemical Society Published on Web 02/10/2000

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2 SN G

)

σN210-18NA

∫

P′

P)0

Γ ln P

22 400

(1)

(mol-1), Γ is the adsorbed amount (mL STP‚g-1), and P′ represents the relative pressure at which the monolayer is formed on the 2 2 -1 unmodified sample. Here, SN G (m ‚g ) indicates the bare silica surface excluding the area occupied by the modified group at adsorption temperature of -196 °C. Therefore, the excluding area EN2 (nm2/chain) per modifier can be calculated immediately 2 2 -1 as where S′N G (m ‚g ) is the specific surface area for the

EN2 )

Figure 1. TEM image of Aerosil 200. modified groups by the number of silanol groups (2.8 nm-2) on unmodified particles.17 The running temperature of TG/DTA was over the range 25-400 °C at the rate of 2 °C/min, and the combustion of the modified group was confirmed by the exotherm peaks of DTA at 200-300 °C. The surface modification ratios of the modified glass were estimated as equal to those of the modified particles. Preferential Dispersion Test. Hydrophilicity and hydrophobicity were tested by the preferential dispersion test to evaluate the wettability. The preferential dispersion test was performed by dispersing silica particles into distilled water and hexane solution under ultrasonic wave for 5 min, and the dispersion tendency was judged after standing overnight. In advance of the tests, silica particles were outgassed at 160 °C for 4 h under 10-5 Torr to ensure that the surface was free from physisorbed water. Gas Adsorption. Adsorption isotherms of neopentane at 0 °C and nitrogen at -196 °C were determined by the volumetric method with a homemade multigas adsorption device after outgassing in the same manner as in the dispersion test. The saturation vapor pressure of neopentane was taken as 532.28 Torr. The usual value of σN2 ) 0.162 nm2 was taken as the crosssectional area of a nitrogen molecule, and the cross-sectional area of the neopentane molecule was calculated as σneo ) 0.614 nm2 from the monolayer capacity on unmodified samples.18,19 The specific surface areas obtained from nitrogen adsorption were calculated by two methods. One method is the specific N2 (m2‚g-1) calculated by the Brunauer-Emsurface area SBET mett-Teller (BET) equation, and the other method is the surface 2 2 -1 area SN G (m ‚g ) calculated where the work of adhesion is equal to that for the unmodified surface based on the Gibbs adsorption 2 2 -1 equation. The equation to estimate the surface area SN G (m ‚g ) according to the latter method is where NA is Avogadro’s constant (17) Fuji, M.; Iwata, H.; Takei, T.; Watanabe, T.; Chikazawa, M. Adv. Powder Technol. 1997, 8, 210. (18) Kiselev, A. V.; Kuznetsov, B. V.; Lanin, S. N. J. Colloid Interface Sci. 1979, 69, 148. (19) Snyder, L. R.; Ward, J. W. J. Phys. Chem. 1966, 70, 3941.

N2 2 S′N G - SG NR

(2)

unmodified samples and NR is the number of modifier chains on the samples. 2 -1 In the same way, the specific surface area Sneo G (m ‚g ) was calculated from neopentane adsorption isotherm by eqs 1 and 2, because the neopentane molecule tend to adsorb on the bare surface.20-22 The exclusion area Eneo (nm2/chain) from neopentane adsorption is equal to the area occupied by the modified group at the neopentane adsorption temperature of 0 °C. The values of the exclusion area obtained from nitrogen and neopentane adsorptions reflect the geometric structure of modifier chains at -196 and 0 °C, respectively. Surface Imaging and Force Measurement with AFM. All images of glass plates were taken both in water and air with an AFM SPI300 microscope (Seiko Instruments, Inc.) in the contact mode. We used the silicon nitride cantilevers (Olympus Opt., Ltd.) of 100- or 200-µm length with pyramidal sharpened tips and set the loading force on the cantilevers at 0.13 nN in the repulsive mode. The spring constants used were 0.09 and 0.02 N‚m-1, and the radius of curvature of the tip was estimated as 30 nm.23 Before the observation in air, modified samples were rinsed with distilled water and kept at room temperature for ∼1 h. On the contrary, no special treatments were made for the sample imaged in water. The adhesion force between the tip and modified glass plates was measured in water in the “force curve” mode. In this operating mode, the substrate displacement was controlled by the applied piezovoltage, and the interaction force was recorded as the voltage from the split photodiode detector. The photodiode voltage and piezovoltage were converted via calibration standards to the normalized force-separation distance curve by the SPA3700 analyzing system. In advance of the measurements, the cleanness of the surface of the tip was guaranteed by gas etching for 5 s.

Results and Discussion Wettability. The wettability of the samples was first evaluated by the preferential dispersion test. The results of the tests for modified samples, listed in Table 1, showed that the samples exhibited hydrophilicity over the range of surface modification ratio of 0 to 20% and changed completely to hydrophobic at a surface modification ratio of >20%. We can assume from these results that the surface state changed from a surface modification ratio of 20% because of the influence of the modified group. Gas Adsorption. Figure 2 shows the specific surface 2 areas SN BET calculated by the BET method. The specific areas gradually decreased from a surface modification 2 ratio of 20%. Ustugi et al.24 reported that SN BET decreased with the increase in surface modification ratios because the modified group partially closed the pores. However, (20) Carrott, P. J. M.; Roberts, R. A.; Sing, K. S. W. Langmuir 1994, 10, 984. (21) Fuji, M.; Iwata, H.; Takei, T.; Watanabe, T.; Chikazawa, M. Adv. Powder Technol. 1999, 10, 187. (22) Fuji, M.; Iwata, H.; Takei, T.; Watanabe, T.; Chikazawa, M. J. Soc. Powder Technol., Jpn. 1996, 33, 740. (23) Markiewicz, P.; Goh, C. M. Langmuir 1994, 10, 5. (24) Utsugi, H.; Nishimura, S.; Kano, T. Zairyo 1972, 21(225), 528.

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Table 1. Preferential Dispersion Tests for Silica Particles Modified by HMDS for Water and Hexanea surface modification ratio %

wettability

0 3.7 11.5 13.7 19.5 21.2 29.1 45.3

hydrophilic hydrophilic hydrophilic hydrophilic hydrophilic hydrophobic hydrophobic hydrophobic

water D D D D P N N N

hexane

water/ hexaneb

hexane/ waterb

D D D D D D D D

D/N D/N D/P D/P P/P N/D N/D N/D

N/D N/D P/D P/D P/P D/N D/N D/N

a D, dispersion; P, partially dispersion; N, nondispersion. b Water/ hexane means dispersed in water first and hexane next and hexane/ water is the opposite order.

Figure 3. Exclusion areas evaluated by nitrogen and neopentane adsorptions. The areas were calculated where the work of adhesion is equal to that for unmodified particles (eq 1). Scheme 1. Illustration of the Structure of Modifier Chains and the Exclusion Areasa

Figure 2. Specific surface areas determined by nitrogen adsorption as a function of the surface modification ratio. The BET method was used to calculate the specific areas.

this assumption is not valid for the nonporous smooth particles used in this study. A more appropriate explanation has been proposed.25-29 Chikazawa et al.29 reported that nitrogen molecules adsorbed preferentially to silanols, and weak adsorption was observed on organic surfaces compared with silica bare surfaces. Accordingly, the decrease in specific areas by nitrogen adsorption indicates that the silica bare surfaces are covered with modification groups and varied to organic-like surfaces from a surface modification ratio of 20%. This theory is suitable for the results of wettability that changed to hydrophilic also at a surface modification ratio of 20%. Although the change in the surface state has been clarified, the mechanism of how the geometric structure of modifier chains reflects the surface wettability remained unsolved. To discuss this mechanism, the calculated exclusion areas per modifier were plotted in Figure 3. As already described, the exclusion area means the occupied area of modifier chains at each adsorption temperature and reflects the geometric structure of the chains. Therefore, a larger exclusion area means that the modifier chains lie horizontally to the silica surface with extending the chains (Scheme 1a), and the decrease in area indicates that the chains are likely to stand perpendicular to the (25) Kiseleve, V. A.; Korolev, Y. A.; petrova, S. R.; Shcherbakova, D. K. Kolloid. Zh. 1960, 22, 671. (26) Gobet, J.; Kovats, E. Adsorpt. Sci. Technol. 1984, 1, 285. (27) Koberstein, E.; Voll, M. Z. Phys. Chem., Neue Folge 1970, 71, 275. (28) Barthel, H. In Chemically Modified Surfaces, Proc. 4th Symp. Chem. Modif. Surf.; Mottola, H. A., Steinmetz, J. R., Eds.; Elsevier: Amsterdam, 1992; pp 243-250. (29) Chikazawa, M.; Kanazawa, T. Shikizai 1984, 57, 456.

a (a) At low surface modification ratio, modifier chains lay over the silica surface, and the exclusion area was quite large. (b) With the increase in surface modification ratio, modifier chains came to stand because of being interfered with themselves and arranged on the surface. (c) As the packing of the modifier chains had been getting closer and closer, the overestimation of exclusion area generated because of adsorptive could not access between modifier chains.

silica surface (Scheme 1b). It must be remembered that the area would be overestimated after the packing of the modifier chains has been established because the space between the chains would also be added as inaccessible sites (Scheme 1c). Additionally, the modifier chains assume a frozen conformation at nitrogen adsorption temperature.30 On the other hand, interpenetrating of the modifier’s layer among particles might occur in the case of neopentane adsorption for a high surface modification (30) Kessaissia, Z.; Papirer, E.; Donnet, J.-B J. Colloid Interface Sci. 1981, 79, 257.

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ratio, as Barthel proposed.4 However, neopentane adsorption reflected the structure of the modifier chains more practically because the properties of the particles are usually evaluated at room temperature. Figure 3 showed that EN2 slightly decreased throughout the surface modification ratio. This result means that the chains of modifiers assumed a similar frozen conformation irrespective of the surface modification ratios at -196 °C. Takahashi31 reported that long alkoxy chains conformed to the Loop-Train-Structure, with anchoring of the terminal methyl groups by the nitrogen adsorption method. Therefore, we can assume that it is not the modification but the temperature that determines the chain structure by the nitrogen adsorption method. On the other hand, Eneo steeply decreased until reaching a surface modification ratio of 20% and slightly increased from a surface modification ratio of 20%. Because the modifier chains can arrange instead of freeze at neopentane adsorption temperature, the gradual decrease means that the modifier chains came to stand from lying over the silica surface with the increase in surface modification ratios. This change occurred because the modifier chains interfered with themselves as the surface modification ratio increased, and the minimum value at a surface modification ratio of 20% indicates that the modifier chains began to arrange packing over the silica surface. This assumption is supported by the value Eneo ) 0.20 nm2 at surface modification ratio of 20%, which is close to the area of 0.19 nm2 occupied by a hydrocarbon chain in an alkane single-crystal structure. The continuing slight decrease from a surface modification ratio of 20% contributed to the overestimation as the packing of modifier chains had been getting closer and closer. The change in the geometric structure from lying to standing at 20% coincided with the wettability, which also varied to hydrophobic at 20%. Therefore, we can conclude that the geometric structure of the modifier chains has an influence on the wettability from the results of gas adsorption tests. AFM Observation. The geometric structure of modifier chains was also evaluated by AFM imaging in both air and water. In advance of nanoscaled scanning, the unmodified glass plate was observed in water after removing the contamination by H2O2 and thoroughly rinsing with distilled water. Figure 4 shows the topography image of the unmodified glass plate in water. One can recognize that the surface consists of hillocks with an average width of ∼100 nm, and the surface roughness is 22.5 nm. However, the surface image in water changed drastically after modification, especially at high surface modification ratios. Figure 5 shows a typical topography image of a modified glass plate as a comparison. Numerous small grains could be observed spreading on the entire surface area, with a similar surface roughness of 21.5 nm. To observe these small grains more precisely, the scanning area was narrowed and the result is shown in Figure 6A, with the cross-sectional images in Figure 6B. From Figures 6A and 6B, the mean diameter was calculated as 18.6 nm. This value was corrected to 15.1 nm by considering the convolution effect between tip and sample (Scheme 2). Here, the estimated diameter is almost equal to that of silica particles, which were modified in the same batch as the glass plates. Furthermore, it is impossible that the morphology of the glass plate itself had changed with the modification process. Therefore, we conclude that silica particles diffused and attached, spreading uniformly on (31) Takahashi, A. Hyomen 1985, 23, 158.

Fuji et al.

Figure 4. AFM topography image of unmodified slide glass scanned in water. The image area is 1000 × 1000 nm. The surface consists of hillocks, with an average width of ∼100 nm.

Figure 5. AFM topography image of modified slide glass scanned in water. The surface modification ratio is 50.1%, and the image area is 1000 × 1000 nm. Numerous small grains could be observed.

the glass plate, in the modification process using the autoclave method. The slight change in surface roughness from 22.5 to 21.5 nm also agrees with this concept. Of course, it is quite surprising that nanosized particles could be observed without any special fixation; however, it is not necessarily impossible if the attractive force between the modified particle and glass plate is greater than the interaction between the particles and scanning tip. For this question, Islaerachvili et al.32-34 reported the long-range strong attractive force between the hydrophobic surfaces in water media. Accordingly, the strong interac(32) Israelachvili, J. N.; Parshley, R. M. Nature 1982, 300, 341. (33) Parshley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088. (34) Christenson, H. K.; Fang, J.; Ninham, B. W.; Parker, J. L. J. Phys. Chem. 1990, 94, 8004.

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Figure 6. (A) AFM topography image of 50.1% modified slide glass imaged in water. The scanning area is 250 nm × 250 nm. (B) The cross-sectional image on the line in Figure 6A. Scheme 2. Illustration of the Convolution Effect between Tip and Objectsa

a The measured size of an individual particle is 2R + 2W, where 2W is the contribution from the convolution. The real particle radius can be determined as R ) L2(W + R)/L1.

tion should work between particles and scanning tip in this study. As written in the Experimental Section, the interaction between particles and tip was set as the repulsive force mode. Consequently, it is assumed that surface imaging of silica particles was enabled by the right choice of the media for observation and the setting of the interaction force loaded on the tip. According to this sample preparation method, surface images at various surface modification ratios were taken by AFM in water from the large scan area to a smaller area. No significant change in the image could be observed

Figure 7. (A) AFM topography image of 50.1% modified slide glass imaged in water. The scanning area is 5 × 5 nm. (B) Fourier filtered image of Figure 7A. (C) Autocorrelation pattern transformed from Figure 7A.

in the large scanning area of 500 × 500 nm, but regularity appeared at high surface modification ratios in the small area of 5 × 5 nm. Figure 7 A shows the topography image at 50.1% in the 5 × 5 nm area. The regularity that emerged in the center of the image can be recognized in Figure 7A. For the analysis of this regularity, Fourier filtered (FT) image and a transformed image into the auto-correlation (AC) pattern are shown in Figures 7B and 7C, respectively.

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Figure 9. Adhesion force between cantilever and modified slide glass surface as a function of surface modification ratio. The top of the cantilever was made of silicon 〈100〉.

Figure 8. (A) AFM topography image of 50.1% modified slide glass imaged in air. The scanning area is 8 × 8 nm. (B) Fourier filtered image of Figure 8A. (C) Autocorrelation pattern transformed from Figure 8A.

Although obscure, six dots could be recognized in the FT image and a corresponding single brightest spot surrounded by six nearest spots appeared in the AC image. The FT image showed that spots are arranged in loose hexagonal packing, and the area occupied by one spot was calculated35 as 0.31 nm2 from the AC image. Taking the results of the exclusion area into consideration, the modifier chains are closely packing at 50.1%,

and 0.31 nm2 is similar to the exclusion area of ∼0.20 nm2 over a surface modification ratio of 20%. From these considerations, we are able to determine that the spot corresponds to the terminal methyl group of the modifiers packing on the silica particles. Fujii et al.10 gained similar results in observing the terminal methyl group of octadecyltrymethyl chains occupying the area of 0.43 ( 0.07 nm2/chain on the silicon surface. After the observation in water, the same sample was scanned in air for ∼1 h. In the large scanning area, a clear image could not be obtained and the scanning noise appeared quite strongly. This result may be caused by a probe-induced effect;12-15 in other words, the tip swept the attached particles over the glass surface in scanning. This sweeping of particles occurred because only van der Waals force work in air, instead of the hydrophobic attraction force in water, and the van der Waals force is generally too weak to fix the particles. However, in the 8 × 8 nm area, we successfully obtained a clear topography image with high regularity. The topography image and the converted image into an AC pattern and FT image are shown in Figure 8. The regularity consisting of six sites appeared more clearly compared with the image in water, and one spot was calculated as 0.32 nm2 from the AC image. This value is almost equal to the occupied area of the spot in the water and supports the theory that these spots correspond to the terminal methyl group of the modifiers. However, the reason for the high regularity in air had not been discovered. Two possibilities can now be proposed. The first is that the difference in the surface structure between glass slide and silica particles affected the packing arrangement of the modifier chains. The second possibility follows the mechanism suggested by O’Shea et al.,7 who investigated copolymer imaging either in xylene solution or in air and observed the rigid island image consisting of aggregated polymer in air after drying of the xylene solution. They thought the polymer was dragged on evaporation of the solution and compressed within the island shape. In the same way, if the obtained image is the top of the modification group islands formed on evaporation, it would be quite valid that the modifier chains arranged themselves with high regularity on these rigid islands. (35) Fujii, M.; Sugisawa, S.; Fukuda, K.; Kato, T.; Seimiya, T. Langmuir 1995, 11, 405.

Surface Geometry of Chemically Modified Silica

Adhesion Force between Tip and Modified Surface. Finally, the adhesion forces between the substrate and tip were measured in water media by AFM and are plotted in Figure 9. A critical increase was seen from a surface modification ratio of ∼40%. As previously described, this critical increase is supposed to be the result of strong hydrophobic interactions. Rabinovich et al.36 investigated the same tendency for the adhesion force between silylated silica plate and glass beads, and Tsao et al.8 also reported the attraction between a hydrophobic surface and a polar surface. Accordingly, the critical increase in adhesion force was supposed to be generated by the establishment of a complete hydrophobic surface and confirmed that particles fixed on a glass surface could be observed by AFM at a surface modification ratio of 50%. Conclusions We investigated the relationship between geometric structure and wettability on the alkoxylated silica particles and glass plate and concluded that: (36) Rabinovich, Y. I.; Yoon, R.-H. Langmuir 1994, 10, 1903.

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(i) Wettability was determined by the geometric structure of modifier chains, and both of them showed a change at a surface modification ratio of 20%. (ii) Nanosized silica particles attached over the glass plate were observed by AFM in water with the repulsive force loaded on the scanning tip. (iii) The image of hexagonal-packed modifier chains was obtained both in water and in air. The occupied area was estimated as 0.31-0.32 nm2 at a surface modification ratio of 50.1%. (iv) The stabilization of nanosized particles could be explained by a strong hydrophobic attractive force according to the results of adhesion force measurements. Acknowledgment. We express our gratitude to Nippon Aerosil, Ltd., for the supply of Aerosil 200. This work was partly suppoted by the Grant-in-Aid for Scientific Research (No. 11750656) from the Ministry of Education, Science, Culture, and Sports of Japan, and Saneyoshi scholarship foundation. LA991134X