Effect of Wettability on Adhesion Force between Silica Particles

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Effect of Wettability on Adhesion Force between Silica Particles Evaluated by Atomic Force Microscopy Measurement as a Function of Relative Humidity 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 October 29, 1998. In Final Form: March 22, 1999 Wettability and the adhesion force between two silica particles were measured as a function of relative humidity. The wettability of silica particles was controlled by rehydroxylation and modification by hexamethyldisilazane and evaluated by a preferential dispersion test, heat of immersion, and water adsorption. The adhesion force between two particles was measured by atomic force microscopy using a “colloidal technique”. The wettability varied from hydrophilic to hydrophobic at trimethylsilyl density ) 1.0 nm-2 as modified groups shielded the residual silanol groups and hindered the formation of a capillary bridge between the two particles. This “hindrance effect” is thought to result in the absence of a critical increase in adhesion force at high relative humidity and also the reduction of surface energy at low humidity where hydrogen bonding dominated the adhesion force.

Introduction The behavior of particles such as dispersions, aggregation, and lubrication is largely dominated by the adhesion force on individual particles; thus, the clarification of the adhesion force mechanism is of great importance in the field of powder technology. Especially the role of relative humidity (rh) related to the adhesion force has attracted attention in the practical handling of powders, because the critical increase in the adhesion force has been pointed out over the range of 60-80% rh. For example, Fridrun et al. reported the deterioration of pharmaceutical particles over 75% rh,1 and McFarlane and Tabor found that the adhesion force between sphere beads rose from a negligible value below 75% rh to a maximum value at 88% rh.2 This critical and tremendous increase at high relative pressure is known as the contribution of capillary condensation generated from the formation of a liquid bridge around the contacting surface. With respect to the theoretical analysis for the liquid bridge, Fisher3 and Islaerachivili4 introduced the capillary force equation, and the validity of this principle has been confirmed by numerous experimental results.5-7 On the other hand, during the last 20 years, important advances of force measurement have been made, owing to the development of direct measurement apparatuses. The atomic force microscope (AFM) is one of these instruments, and it has enabled direct measurement of colloidal surfaces. Ducker et al.8 first utilized AFM for * To whom correspondence should be addressed. E-mail: [email protected]. Tel & Fax: +81-426-77-2850. (1) Fridrun, P.; Michael, N. J.; Michael, B. J. Int. J. Pharm. 1996, 145, 221. (2) Harnby, N.; Hawkins, A. E.; Opalinski, I. Trans Inst. Chem. Eng. 1996, 74, 605. (3) Fisher, R. A. J. Agric. Sci. 1926, 16, 492. (4) Fisher, L. R.; Israerachvili, J. J. Colloid Interface Sci. 1981, 80, 528. (5) Nishino, M.; Arakawa, M. Zairyou 1973, 22, 663. (6) Binggeli, M.; Mate, C. M. Appl. Phys. Lett. 1994, 65, 415. (7) Corn, M. J. Air Pollut. Control Assoc. 1961, 11, 566. (8) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239.

colloidal force measurement by attaching a particle to the top of the scanning probe, and this technique has been rapidly spreading in the applications of force measurement in liquid media.9-12 However, few reports are available on gas-phase applications, and the adhesion force mechanism between single particles still remains unsolved despite the AFM development. Consequently, in this study, we attempted to prove the relationship between the adhesion force and relative humidity using AFM colloidal techniques. We prepared various silicas by either silylation or rehydroxylation and considered how the wettability of silica powders influences the adhesion force. Experimental Section Materials. The nonporous amorphous silica particles Adma Fine SO-C5 (Tatsumori Co., Ltd.) of average diameter 1.7 µm were used. Figure 1 shows the scanning electron microscopy image of SO-C5 particles. Reagent-grade hexamethyldisilazane (HMDS) was purchased from Kanto Chemical Co., Inc., for the silylation. Other chemicals used, aqueous ammonia (29%), ethanol, and hexane, were also purchased from Kanto Chemical Co., Inc.; the Grignard reagent for the measurement of the surface silanol (Si-OH) density was synthesized from metal magnesium and methyl iodide in dibutyl ether.13 Surface Treatment. The wettability of silica particles was controlled by silylation and rehydroxylation. Silylation was performed through the autoclave method by a HMDS reagent at 235 °C under 30 atm for 1.0 h, and the amount of trimethylsilyl (TMS) groups introduced on the surface was controlled by the HMDS volume poured into the reactions. The TMS density was calculated according to eq 1 where D, S, and NA represent the

D)

MNA × 10-18 S×3

(1)

TMS density (TMS/nm2), the surface area of the samples (9) Biggs, S. Langmuir 1995, 11, 156. (10) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Am. Chem. Soc. 1993, 115, 11885. (11) Butt, H.-J. J. Colloid Interface Sci. 1994, 166, 109. (12) Snyder, B. A.; Aston, D. E.; Berg, J. C. Langmuir 1997, 13, 590. (13) Fripiat, J. J.; Uytterhoeven, J. J. Phys. Chem. 1962, 66, 800.

10.1021/la981533c CCC: $18.00 © 1999 American Chemical Society Published on Web 05/27/1999

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Figure 1. SEM image of SO-C5 particles. (m2), and Avogadro’s number (mol-1), respectively. M is the carbon molar quantity (mol) converted from the CO2 volume generated from the combustion of modified samples at 600 °C. Here, the CO2 volume was recorded with gas chromatograph GC-7A (Shimazu Co., Ltd.), which had been corrected for CO2 sensitivity beforehand. On the other hand, rehydroxylation was carried out by hydrothermal treatment, which was performed by exposing silica powders to water steam in an autoclave at 160 °C under 3.5 atm for 2.0 h. The surface silanol density for unmodified and rehydroxylated samples was measured by the Grignard method,13 and that for modified samples was estimated by subtracting the TMS density from the silanol density of the unmodified samples. The surface treatment, TMS density, and silanol density are summarized in Table 1. Wettability. Hydrophilicity and hydrophobicity were examined by a preferential dispersion test at the beginning. A preferential dispersion test was performed by dispersing silica particles into a distilled water and hexane solution under ultrasonic waves for 5 min, and the dispersion tendency was judged after the solution was allowed to stand overnight. In advance of the tests, silica particles were outgassed at 200 °C for 2 h under 10-5 Torr to ensure freedom from physisorbed water. Consecutively, the wettability of the surface was characterized by both heat of immersion and water adsorption. Heat of immersion with distilled water was measured at 25 °C by a multimicrocalorimeter MMC-5111 (Tokyo Riko Co., Ltd.). Silica particles were placed in a glass ampule and measured by nitrogen adsorption using the BET method after outguessing in the same manner as in the dispersion test. The glass ampule was then placed in a calorimeter until it reached thermal equilibrium at 25 °C. After this preparation, the ampule was broken and the silica powders inside were immersed in water. The liberated heat was recorded and calibrated with blank experiments. Water adsorption isotherms were determined at 0 °C by a volumetric method with a multigas adsorption device. Water used as the adsorptive was charged in a vessel attached to a vacuum line, after bubbling nitrogen gas, and degassed to remove the dissolved gas by several freeze-melting cycles. The crosssectional area of a water molecule was estimated as 0.105 nm2 considering that the water molecules were arranged in closest packing over the sample surface. The thickness of one layer was calculated as being out 0.284 nm from eq 2 where MW and d

d)

(MW × NA × d) 1.091

1/3

× 107

(2)

denote the molecular weight and means of the density of the adsorbate (g‚mL-1). Force Measurements. AFM SPI300 (Seiko Instruments Co., Inc.) was used for the force measurement, and force-distance

information was obtained in the “force curve” mode.14 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 a normalized force-separation distance curve by the SPA3700 analyzing system. Fujii et al.15 reported that the pull-off force depends on both a frequency of a cycle toward-away and a loading force. Therefore, the frequency was set constant at 3 s, and the loading force was kept constant by setting the distance of the piezomovement after contact as 200 nm. The value of the loading force guarantees enough contact between particles before separation. The colloidal probe was prepared by mounting one of the silica particles on a cantilever tip using epoxy resin by means of a micromanipulator stage.8 The microfabricated cantilevers (Nanosensors Co., Ltd.) have spring constants ) 1.6 N‚m-1, and the resonance frequency was roughly 25 kHz in air. The other particle in a pair of force measurements was fixed on a flat silicon plate using epoxy resin.16 Prior to the force measurements, the top of a particle fixed on the substrate was confirmed from the scanning surface image by a cantilever with a particle, and the force was measured between these two particles. To control the relative humidity, dry nitrogen gas or humid gas was introduced into the AFM head unit at a controlled rate. The range of measurements was limited to 40-90% rh because the extreme electrostatic influence was reported in the range of 15-40% rh17 and the upper limit of the humidity measurements was 90% rh.

Results and Discussion Wettability. First, the wettability of various SO-C5 samples was evaluated by a preferential dispersion test. The results are shown in Table 2. All samples were highly dispersed in a hexane solution because of the presence of hydrophobic siloxane sites or TMS groups. On the other hand, the dispersion tendency to water changed from dispersion to flotation between SO-C5/0.77TMS and SOC5/1.01TMS. This tendency indicates that the wettability of silica particles varies from hydrophilic to hydrophobic approximately at 0.8-1.0 TMS/nm2, which is converted into a silanol density of 2.7-2.8 OH/nm2. This critical change is a quite remarkable result in that although nearly 70% of hydrophilic silica sites are still exposed to water, these samples exhibited strong hydrophobicity. However, this change point also corresponds to the result of another nonporous silica Aerosil20018 (Nippon Aerosil Co., Ltd.). Consequently, the factor to decide the surface wettability is supposed to be related not to the number of the silanol sites themselves but to the amount of those sites accessible to water molecules. The details of this mechanism will be discussed later in water adsorption isotherms. Second, the heat of immersion was measured as another method to evaluate the change in wettability. The results are plotted as a function of the TMS density in Figure 2. Unmodified SO-C5 showed 190 mJ‚m-2 and gradually decreased in proportion to the increase in the TMS density, reducing the value to nearly equal to zero at 1.0 TMS/ nm2. In general, the hydrophilicity of silica is mainly due to the hydrogen bonding of water to silanol groups, and both the electrostatic force and van der Waals interaction (14) Mizes, H. A.; Loh, K.-G.; Miller, R. J. D.; Ahuja, S. K.; Grabowski, E. F. Appl. Phys. Lett. 1991, 59, 2901. (15) Kanno, T.; Fujii, M.; Fukada, K.; Katou, T.; Seimiya, T. 48th Symp. Colloid Interface Chem. 1995, 466. (16) Shakesheff, K. M.; Davis, M. C.; Jackson, D. E.; Roberts, C. J.; Tendler, S. J. B.; Brown, V. A.; Watson, R. C.; Barrett, D. A.; Shaw, P. N. Surf. Sci. Lett. 1994, 304, 393. (17) Nomura, T.; Yamada, Y.; Masuda, H. Kagaku Kogaku Ronbunshu 1998, 24, 585. (18) Fuji, M.; Iwata, H.; Takei, T.; Watanabe, T.; Chikzawa, T. Adv. Powder Technol. 1997, 8, 210.

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Table 1. Surface Treatment, Silanol Density, and TMS Densitya sample name

surface treatment

silanol density (OH/nm2)

TMS density (TMS/nm2)

SO-C5/rehydrated SO-C5/original SO-C5/0.42TMS SO-C5/0.46TMS SO-C5/0.77TMS SO-C5/1.01TMS SO-C5/1.16TMS

rehydration by a hydrothermal process untreated modified with HMDS (0.03 mL/g) modified with HMDS (0.09 mL/g) modified with HMDS (0.20 mL/g) modified with HMDS (0.37 mL/g) modified with HMDS (0.97 mL/g)

4.6 3.6 3.2 3.1 2.8 2.6 2.4

0 0 0.42 0.46 0.77 1.01 1.16

a Both hydrothermal treatment and surface modification were carried out with an autoclave method. Silanol densities for modified samples were calculated by subtracting the TMS density from the silanol density of an unmodified sample.

Table 2. Preferential Dispersion Tests for Water and Hexanea sample name

wettability

water

SO-C5/rehydrated SO-C5/original SO-C5/0.42TMS SO-C5/0.46TMS SO-C5/0.77TMS SO-C5/1.01TMS SO-C5/1.16TMS

hydrophilic hydrophilic hydrophilic hydrophilic hydrophilic hydrophobic hydrophobic

D D P P P F F

water/ hexane/ hexane hexaneb waterb D D D D D D D

D/D D/D D/D D/D P/D F/D F/D

D/D D/D D/P D/P D/P D/P D/F

a Hydrophilic samples dispersed into water and hydrophobic samples floated over water instead of dispersion. D: dispersion. P: partial dispersion. F: nondispersion. b Water/hexane means dispersed in water first and hexane next; hexane/water is the opposite order.

Figure 2. Heat of immersion for 25 mL of water at 25 °C plotted as a function of TMS density. The data were calibrated by blank tests.

forces are assigned as making only a slight contribution.19 Especially in the case of the silica surface, the electrostatic force is small enough to be neglected. Therefore, small values of immersional heat over 1.0 TMS/nm2 mean that only van der Waals forces exist on these samples and the residual surface silanol groups are hindered from water access by TMS groups. Moreover, this critical density coincides with the concentration where the wettability changed to hydrophobic in the preferential dispersion test. These results indicate that the TMS density ) 1.0 TMS/ nm2 is the critical point for either wettability or heat of immersion and the “hindrance effect of TMS groups” is the key to determination of the wettability. On the contrary, the effect of an increase in silanol groups to the immersional heat had also been considered between rehydroxylated silica (SO-C5/rehydroxylated) and unmodified silica (SO-C5/original). The heat of immersion for SO-C5/rehydroxylated was 274 mJ‚m-2, 84 mJ‚m-2 greater than the value for the SO-C5/original. As previously mentioned, the main contribution for (19) Tsutsumi, K.; Takahashi, H. Colloid Polym. Sci. 1985, 263, 506.

Figure 3. Water adsorption isotherms at 0 °C. All of the isotherms were categorized in IUPAC III type.

immersional heat is the hydrogen bonding; therefore, the increase in immersional heat is a result of the increase in the silanol density. In a word, the rehydroxylation process led to the increase in silanol groups from 3.6 to 4.6 OH/nm2 and resulted in the increase in hydrophilicity as clarified in the heat of immersion. Water Adsorption. Three hydrophilic and two hydrophobic samples were chosen for the water adsorption experiment, including three modified SO-C5s. Among these modified samples, SO-C5/0.42TMS is hydrophilic silica, and both SO-C5/1.01TMS and SO-C5/1.16TMS are hydrophobic samples. Water adsorption isotherms are shown in Figure 3. All isotherms were classified into type III isotherms, which are characterized by convexity toward the relative pressure axis, commencing at the origin, whereas the amounts of adsorbed water were on the order of the silanol density throughout their courses. This trend of the adsorbed amount agrees with the water adsorption mechanism advocated by Iler et al.20 They summarized that the adsorption of water is especially sensitive to the silanol density for the silica surface. However, the gap of the adsorbed amount between hydrophilic and hydrophobic samples at high relative pressure cannot be explained only by the silanol density. This discrepancy can be solved by the occurrence of capillary condensation among individual silica particles and the formation of a continuous two-dimensional liquid layer. Many experiments4-6 concerning the Kelvin equation21 verified that capillary condensation by water occurred from P/P0 ) 0.7, and the occurrence of capillary condensation appeared as steep increases in water isotherms. Therefore, the drastic increases in the adsorbed amount of water for hydrophilic samples are attributed to capillary condensation, and the monotonic increase until reaching saturated pressure for hydrophobic samples can be explained by the absence of capillary condensation. (20) Iler, K. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979. (21) Defay, R.; Prigogine, I.; Bellemans, A.; Everett, D. H. Surface Tension and Adsorption; Longmans: London, 1966.

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Figure 4. Amount of adsorbed water divided by the number of silanol groups on each sample and plotted as a function of the relative pressure.

This difference is suspected by the hindrance effect of TMS groups, which had already been pointed out in the results of immersional heat. To clarify this effect, the scale of adsorption is converted to molecules of water adsorbed per atom of silanol groups in Figure 4. Three hydrophilic samples showed quite similar curves compared to the hydrophobic samples. In particular, SO-C5/1.16TMS which has strong hydrophobicity exhibited a much less adsorbed amount in all ranges of relative pressure. If the hindrance effect appears, the adsorbed water molecules per silanol would be reduced because the residual silanol sites are shielded by the TMS groups. Therefore, the decrease in adsorbed water molecules on hydrophobic samples is evidence of the hindrance effect of TMS groups. With respect to the mechanism of the hindrance effect in water, our previous work18 proved that water molecules are adsorbed, forming a two-dimensional water layer by localized and cooperative adsorption and finally growing the water multilayers. However, during the formation stage of the continuous two-dimensional water layer, the TMS groups structurally hindered such water adsorption. Consequently, water multilayers cannot form, and this hindrance effect leads to the absence of capillary condensation. This effect also results in the flotation of powder particles and then induced the decrease in the immersional heat on hydrophobic samples. Adhesion Force Measurement. The adhesion force was measured as a function of the relative humidity, and the samples were chosen to be the same as those used in water adsorption tests. The values of the measured adhesion force were normalized using eq 322 where f

F ) f/

(

)

1 1 + R1 R2

(3)

represents the adhesion force measured in practical experiments, R1 is the curvature radius of a silica particle attached to the cantilever, and R2 is that of a fixed particle on the silicon plate. R1 was calculated by an optical microscope, and R2 was estimated from an AFM crosssectional image by morphological restoration.23-25 Additionally, the calculated adhesion force can be converted (22) Junno, T.; Anad, S.; Deppert, K.; Montelius, L.; Samuelson, L. Appl. Phys. Lett. 1995, 66, 3627. (23) Junno, T.; Anad, S.; Deppert, K.; Montelius, L.; Samuelson, L. Appl. Phys. Lett. 1995, 66, 3295. (24) Mulvaney, P.; Ciersig, M. J. Chem. Soc., Faraday Trans. 1996, 92, 3137. (25) Markiewicz, P.; Goh, C. M. Langmuir 1994, 10, 5.

Figure 5. Adhesion force and surface energy between particles measured by using an AFM colloidal technique. The adhesion force was normalized using eq 3, and the surface energy was calculated by eq 4.

to surface energy γSV via the Derjaguin approximation,26 with the elastic surface deformations using the JKR theory.27 The approximate equation can be described as

F ) 3π

(

)

1 1 + R1 R2

-1

γSV

(4)

and theoretically γSV increases up to water surface tension 73 mN‚m-1. The results of the adhesion force and surface energy are plotted in Figure 5 as a function of relative humidity. The adhesion force for hydrophilic samples maintained a rather constant value until high relative humidity. The rehydroxylated and unmodified SO-C5 then displayed a gradual increase from approximately 70% rh, and the surface energy approached the maximum value 73 mN‚m-1. These increases in adhesion force over 70% rh are closely related to the drastic increase in the adsorbed amount of water (Figure 2) in the range of 70-80% rh. Consequently, the remarkable increase in the adhesion force can be considered as the occurrence of capillary condensation. Binggeli6 reported a similar tendency between a tungsten tip and a hydrophilic silicon oxide and found that the steep increase of more than 75% rh was generated by capillary condensation, whereas the hydrophilic modified sample SO-C5/0.42TMS did not exhibit the gradual increase until up to 90% rh. This discrepancy is explained by the difference in the contact angle in the Kelvin equation. The change in the contact angle would result in the shift of the Kelvin radius, and the gradual increase in the adhesion force did not occur until 90% rh. However, the disappearance of the gradual increase even at saturated pressure for the hydrophobic samples cannot be explained only by the change in the contact angles. This difference indicates that the capillary force does not exist for the hydrophobic samples, and the hindrance effect of TMS groups worked on the adhesion force as well as wettability. That is to say, capillary condensation was prevented on the hydrophobic samples because of the presence of TMS groups. Consequently, we can conclude that both wettability and the behavior of the (26) Israelachvili, J. N. Intermelecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991. (27) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Ser. A 1971, 345, 301.

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Figure 6. Surface energy replotted as a function of the silanol density of each sample.

adhesion force at high relative humidity depend on the formation of a capillary bridge between particles. On the other hand, we can also discuss the change in surface energy with the increase in relative humidity and in the silanol density from Figure 5. The adhesion force was calculated as a function of the silanol density and plotted at each relative humidity in Figure 6. The values of the surface energy for all relative humidity conditions increased depending on the silanol density. This result is in accordance with the study of the relationship between the surface energy and silanol density by Barthel.28 He did not refer to the relative humidity but proved that the silica surface exhibits higher surface energy than silylated hydrophobic silica because of surface silanol groups. It can also be interpreted from Figure 6 that the surface energy increased with relative humidity. This implies that the surface energy varies with the amount of adsorbed water. To explain this assumption more precisely, the surface energy was rewritten as a function of the amount of adsorbed water per silanol in Figure 7a. In Figure 7a, a steep increase appeared over 1.0 molecular/-OH on the hydrophilic samples. This area is in accordance with the relative humidity over approximately 70% rh; therefore, the steep increase should be attributed to the addition of capillary force. However, the generation mechanism of the surface energy under 1.0 molecular/-OH still remained unsolved. Accordingly, when the range over 1.0 molecular/-OH was neglected, the data were extrapolated into two straight lines (Figure 7b). One is a high-slope line belonging to hydrophobic silicas, and the other one belonged to the hydrophilic silicas having only a gradual increase. These differences indicate that the surface energy of the hydrophobic samples is more sensitive to humidity than that of hydrophilic samples, and it is assumed that the hindrance effect of TMS groups also has an influence on the surface energy. This hypothesis would be quite reasonable if one thought that the surface energy is generated by hydrogen bonding between surfaces.29 The height of the TMS groups shields the surface silanol groups from forming hydrogen bonds, and the adhesion force due to the hydrogen bond increases with the number of adsorbed water molecules which can (28) Barthel, H. Colloids Surf. A 1995, 110, 217. (29) Chikazawa, M.; Kanazawa, T.; Yamaguchi, T. Kona 1984, 2, 54.

Figure 7. Surface energy depending on the adsorbed amount of water per silanol. (a) All of the data were plotted, and the dashed line at 1.0 molecular/-OH corresponds approximately over 70% rh. (b) The data under 1.0 molecular/-OH was enlarged. The steep line belongs to the hydrophilic samples, and the sloped line belongs to the hydrophobic samples. Chart 1. Formation of Hydrogen Bond between Surfaces in the Case of Hydrophilic (a) and Hydrophobic (b) Samples

bond together over the TMS groups. On the contrary, unmodified silica can form hydrogen bonds among the silanol groups without water adsorption, and the total number of hydrogen bonds does not change in proportion to water adsorption. In addition, the hydrogen bonds

Effect of Wettability on Adhesion Force

formed by the silanol groups might be stronger than those formed by water molecules because of the mobility of the -OH groups, and the increase in adsorbed water molecules does not have a significant influence on the adhesion force. This hypothesis is illustrated in Chart 1. Consequently, the adhesion force under 1.0 molecular/-OH, where the relative humidity is approximately lower than 70% rh, can be detected as generated from hydrogen bonding. Conclusions Using AFM, we studied the relationship between wettability and the adhesion force between silica particles as a function of relative humidity and concluded the following. (i) TMS groups shielded the residual surface silanol groups and led to the change in the wettability to

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hydrophobic and the reduction of the heat from 1.0 TMS/ nm2. This hindrance effect of TMS groups resulted in the absence of capillary condensation as a water multilayer cannot grow on the surface. (ii) The absence of capillary condensation resulted in a diminishing of the critical increase in the adhesion force in high relative humidity between modified silica particles. (iii) From the aspect of surface energy, a higher silanol density exhibited high surface energy, and hydrogen bonding was interpreted as the dominant component of the adhesion force in low relative humidity. Acknowledgment. The author is grateful to Tatsumori Co., Ltd., for providing Adma Fine SO-C5. LA981533C