Interaction Forces between Two Silica Surfaces in an Apolar Solvent

In the apolar system, no electrostatic repulsive forces were measurable between two .... The polished silica substrates for the atomic force microscop...
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Langmuir 2004, 20, 1791-1798

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Interaction Forces between Two Silica Surfaces in an Apolar Solvent Containing an Anionic Surfactant Cathy E. McNamee,*,† Yoshinobu Tsujii,‡ and Mutsuo Matsumoto‡ Organic and Macromolecular Chemistry, OC3, The University of Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany, and Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Received September 16, 2003. In Final Form: December 11, 2003 The forces between two silica surfaces in a surfactant-containing apolar solvent were studied by atomic force microscopy, as a function of the silica surface type, surfactant concentration, compression speed, and effect of water presence. In the apolar system, no electrostatic repulsive forces were measurable between two hydrophobized silica surfaces in dodecane. However, the introduction of the anionic surfactant of sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) into the dodecane caused a measurable surface potential because of the adsorption of the hydrophobic bis(2-ethylhexyl) sulfosuccinate (AOT) anions to silica surfaces. Increasing the NaAOT concentration from the critical micelle concentration of 7.6 to 500 mM caused a maximum in the surface potential at 100 mM because of the subsequent adsorption of the sodium cations onto the AOT anions that have already adsorbed. Oscillations in the force curves were only observed for the NaAOT concentration of 500 mM; a smaller concentration did not give enough reverse micelles to cause measurable structuring effects. The experimental results in this study are well-explained by an extended Derjaguin-Landau-Verway-Overbeek theory, which includes electrostatic, van der Waals, and structural forces.

1. Introduction Nonaqueous dispersions are invaluable in our daily use, as seen by their employment in paints, inks, fillers, and greases. Reverse micelles formed by ionic surfactants in nonaqueous solvents can be used for such applications as hosts for proteins and small molecules1,2 and the production of nanoparticles,3 nanowires,4 cosmetics, pigments, and paints.5 The understanding of intermolecular forces in such systems is necessary to improve their macroscopic properties, such as texture, dispersability, stability, and thixotropy.6 The surface charge, solvent and its molecular size, and presence of surfactants or water are some factors that can affect the intermolecular forces. The presence of a charge in a nonaqueous system is known from the petroleum industry because of the reports of fire resulting from electrostatic charging in petrol.7 In nonaqueous solvents, unlike in aqueous solvents, salts do not dissolve well and are only partly dissociated because of the low dielectric constant of a nonaqueous solvent. The adsorption of dissociated molecules or the proton exchange between a solvent and surface are acceptable charge-determining mechanisms in nonaqueous solvents.6 In apolar solvents, only minute amounts of the dissociated salts are required to give very thick electrical double layers * Author to whom correspondence should be addressed. Tel.: +49 731 502 2896. Fax: +49 731 502 2883. E-mail: cathy. [email protected]. † The University of Ulm. ‡ Kyoto University. (1) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochim. Biophys. Acta 1988, 947, 209. (2) Pitre´, F.; Regnaut, C.; Pileni, M. P. Langmuir 1993, 9, 2855. (3) Arcoleo, V.; Liveri, V. T. Chem. Phys. Lett. 1996, 258, 223. (4) Rees, G. D.; Evans-Gowing, R.; Hammond, S. J.; Robinson, B. H. Langmuir 1999, 15, 1993. (5) Kitahara, A.; Watanabe, A. Electrical Phenomena at Interfaces Fundamentals, Measurements, and Applications; Marcel Dekker: New York, 1984. (6) Lyklema, J. Adv. Colloid Interface Sci. 1968, 2, 65. (7) Klinkenberg, A.; van der Minne, J. L. Electrostatics in the Petroleum Industry; Elsevier: Amsterdam, 1958.

and therefore stable dispersions. Keir and others8 showed by electrophoresis that a hydrophilic silica surface is charged in a sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT)-containing decane system, explained by a sitebinding model of the charging mechanism. Several reviews6,9,10 exist about the stability of colloidal dispersions in nonaqueous media. Force measurements between two smooth surfaces in organic solvents, such as cyclohexane,11 tetrachloromethane,12 benzene,12 2,2,4-trimethylpentane,12 and octamethylcyclotetrasiloxane,13,14 have shown that these larger molecules form structured layers between the surfaces, producing measurable oscillations in the force curves. The presence of such oscillations has made direct measurement of the forces in nonaqueous dispersions difficult. In an aqueous system, the presence of micelles has been shown to cause oscillations in the measured force curves because of their structuring.15-18 An increase in the number of micelles increased the number and magnitude of these oscillations. On the other hand, there appear to be no studies about the forces between two surfaces in the presence of only ionic surfactants in nonaqueous solvents. The presence of water affects the properties of nonaqueous dispersions, even in the ppm concentration range. (8) Keir, R. I.; Suparno; Thomas, J. C. Langmuir 2002, 18, 1463. (9) Kitahara, A. In Electrical Phenomena at Interfaces Fundamentals, Measurements, and Applications; Ohshima, H., Furusawa, K., Eds.; Marcel Dekker: New York, 1988; p 139. (10) Leal-Calderon; Poulin, P. Curr. Opin. Colloid Interface Sci. 1999, 4, 223. (11) Christenson, H. K.; Horn, R. G.; Israelachvili, J. N. J. Colloid Interface Sci. 1982, 88, 79. (12) Christenson, H. K. J. Chem. Phys. 1983, 78, 6906. (13) Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1981, 75, 1400. (14) Gee, M. L.; Israelachvili, J. N. J. Chem. Soc., Faraday Trans. 1990, 86, 4049. (15) Nikolov, A. D.; Wasan, D. T. J. Colloid Interface Sci. 1989, 133, 1. (16) Basheva, E. S.; Danov, K. D.; Kralchevsky, P. A. Langmuir 1997, 13, 4332. (17) Richetti, P.; Kekicheff, P. Phys. Rev. Lett. 1993, 68, 1951. (18) Milling, A. J. J. Phys. Chem. 1996, 100, 8986.

10.1021/la035730+ CCC: $27.50 © 2004 American Chemical Society Published on Web 01/30/2004

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Conductivity has been measured in water-in-oil microemulsions,19-23 with the magnitude affected by the amount of water present. Electrophoretic studies on systems involving an apolar solvent, hydrophilic surface, and trace amount of water have shown that the water tends to adsorb at the hydrophilic surface.6,24 The forces between the two hydrophilic mica surfaces in the presence of NaAOT reverse micelles and water were measured with a piezoelectric bimorph force sensor.25 Oscillations were observed because of the reverse micelles. The effect of water inside a reverse micelle on the interaction forces between the two hydrophobized silica surfaces in a solution of reverse micelles, however, has not been studied before. Here, we aimed to study for the first time the effect of the surface hydrophobicity on the forces in an apolar solvent. We investigated the effect of the presence of NaAOT reverse micelles on the interaction forces between the two hydrophobic silica surfaces in dodecane, solubilization of the NaAOT reverse micelles, and compression speed of the two surfaces. The effect of varying the compression speed has up to now only been minutely studied in aqueous solvents.26 The investigations in this experiment are essential for the understanding of nonaqueous dispersions and possibility of discovering further applications. 2. Experimental Section 2-1. Materials. The following electrolytes and surfactants were used in this study: sodium bis(2-ethylhexyl) sulfosuccinate [Aerosol-OT, NaAOT, critical micelle concentration (cmc) ) 7.6 mM27]. NaAOT was synthesized at the Imano Laboratory from the Chemical Engineering Department of the Tokyo Science University. Dodecane (99% purity) was obtained from Nacalai Tesque. The polished silica substrates for the atomic force microscopy (AFM) force measurements were purchased from Nipponchikagaku, Ltd., Japan. The silica particles (Ube Nittoh Kagaku, Japan) for the probe in the AFM force measurements were prepared by a sol-gel method and had a radius of 5 µm. 2-2. Sample Preparation. 2-2-1. NaAOT Reverse Micelle Solutions. The NaAOT reverse micelle solutions for the NaAOT concentrations of up to 500 mM were prepared by dissolving the NaAOT directly into the dodecane. The water-solubilized NaAOT reverse micelles were prepared by the method of Kawai and others.27 Briefly, 26.4 mL of water was added in two equal parts to a 1 kg solution of NaAOT containing dodecane to enable optimal solubilization. The highest concentration of NaAOT in dodecane that we could solubilize was 100 mM. 2-2-2. Hydrophobization of the Glass and Silica Surfaces. A 1.5 cm2 glass square and probe silica particle were hydrophobized in approximately 30% dimethyldichlorosilane (Wako Chemicals) in hexane (high purity, Nacalai Tesque) for 10 min. They were rinsed with hexane and then with distilled water before use. 2-3. Surface Force Measurement Technique. 2-3-1. Preparation. The following procedure was undertaken to prepare a cantilever with an attached silica particle. The silica particles for the probe were cleaned by dispersing them in a 15% H2O2 (19) Boned, C.; Clausse, C.; Lagourette, B.; Peyrelasse, J.; McClean, V. E. R.; Sheppard, R. J. J. Phys. Chem. 1980, 84, 1520. (20) Eicke, H. F.; Meier, W.; Hammerich, H. Langmuir 1994, 10, 2223. (21) Lague¨s, M.; Sauterey, C. J. Phys. Chem. 1980, 84, 3503. (22) Stilbs, P. J. Colloid Interface Sci. 1984, 99, 290. (23) Schlicht, L.; Spilgies, J. H.; Runge, F.; Lipgens, S.; Boye, S.; Schu¨bel, D.; Igenfritz, G. Biophys. Chem. 1996, 58, 39. (24) Romo, G. D. Discuss. Faraday Soc. 1966, 42, 232. (25) Parker, J. L.; Ritchetti, P.; Ke´kicheff, P. Phys. Rev. Lett. 1992, 68, 1955. (26) McNamee, C. E.; Tsujii, Y.; Ohshima, H.; Matsumoto, M. Langmuir In press. (27) Kawai, T.; Hamada, K.; Shindo, N.; Konno, K. Bull. Chem. Soc. Jpn. 1992, 65, 2715.

McNamee et al. solution for 24 h and ultrafiltered more than 20 times with distilled water. They were then further ultrafiltered with pure ethanol several times and kept in ethanol in a glass vessel. The silica particles in ethanol were subsequently spread onto a cleansed glass-slide plate. The tip end of the micropyramid of a v-shaped cantilever (Olympus Optical Co., Ltd.) was glued using a 5-min-curing epoxy resin (Araldite, Ciba-Geigy Japan, Ltd.), and then a single silica particle from the glass plate was transferred onto the tip end. The spring constant (k) of the cantilever (k ) 0.68 N/m) was determined by the Cleveland method28 and was within (10% of the spring constant supplied by the manufacturer. The silica plates were cleaned prior to a force experiment by soaking them in a 1:1 solution of concentrated H2SO4 (Wako Pure Chemical Industries, guaranteed grade) and concentrated HNO3 (Nacalai Tesque, guaranteed grade) for 24 h. They were then transferred into a 1 part 31% H2O2 (Santoku Chemical Industries, guaranteed grade), 1 part 28% NH3 (Nacalai Tesque, guaranteed grade), and 2 parts distilled water solution for another 24 h and then rinsed with copious amounts of distilled water. They were then dried in air before use. AFM images showed the absence of detectable contamination on the silica surfaces and a surface roughness of 0.35 nm over 25 µm2. The silica probe, attached to the cantilever, was cleaned before setting up and commencing an AFM force experiment in a 1 part 31% H2O2, 1 part 28% NH3, and 2 parts distilled water solution for 5 min and then rinsed with distilled water. The probe was then dried in air before use. 2-3-2. Measurements. The surface forces between a silica substrate and silica particle in a dodecane solution were measured at room temperature (25 ( 1 °C) as a function of their distance using an atomic force microscope (Seiko Instruments, Inc., SPA400). The piezo actuator (Seiko Instruments, Inc., PZT FS20A, z-scan size, 20 µm) was calibrated and its nonlinearity compensated for as described elsewhere.29 The forces between a substrate and silica probe in the surfactant solutions were measured using a circular Teflon liquid cell with a radius of 1 cm, in which the substrate was placed. Approximately 2 mL of the surfactant solution was added to the liquid cell, which was then placed on the piezo actuator. The liquid cell was in contact with the cantilever holder, leaving almost no space for the liquid to evaporate. The force measurements were performed according to the method of Ducker and others.30 Briefly, the cantilever with the silica probe fixed on the cantilever holder was positioned in solution to face the substrate. The change in the deflection of the cantilever (∆x) was measured as a function of the piezo displacement, using the differential intensity of the reflection of the laser beam off of the cantilever onto a split photodiode. The force (F) was calculated from Hooke’s law (F ) k∆x) where k is the spring constant of the cantilever. Because the radius of the silica probe (R) was always much greater than the separation distance, the force could be normalized to the interaction free energy (G) by the Derjaguin approximation:

F/R ) 2πG

(1)

Zero separation was defined from the position of the onset of the linear compliance region in the force profile, which was due to the further compression of both surfaces after contact was made. Surface separation (D) was estimated from the displacement of the substrate relative to this constant compliance region. The zero-force position between the surfaces was defined from the baseline of no deflection of the silica probe cantilever at large probe-substrate separations. 3. Theory. 3-1. Force Theory. There are two main forces in the solutions that contain colloids. These are the van der Waals31 (28) Cleaveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403. (29) Yamamoto, S.; Ejaz, M.; Tsujii, Y.; Matsumoto, M.; Fukuda, T. Macromolecules 2000, 33, 5602. (30) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (31) Verway, E. J. W.; Overbeek, J. Th. G. Theory on the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948.

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and electrostatic forces.31 Additional forces have been reported for solutions that contain particles or macromolecules. The depletion force has been observed as an attractive force. It appears when the separation between the two surfaces is smaller than the size of a particle32 or polymer33-37 that coexists in that system. This attraction was explained by Asakura and Osawa38 as osmotic in origin and a result of the lower number density of particles or polymers between the two surfaces compared to that in the bulk solution. The particles or macromolecules32-40 can arrange into structured layers when the two surfaces are separated by appropriate distances. A change in the separation distance of the two surfaces causes the packing of the particles to also change. This alters the liquid density, resulting in a structural force41 that oscillates with the separation distance. The analytical expression for the interaction free energy of Kralchevsky and Denkov42 takes into account both of the depletion and structural forces for a particles-containing liquid confined between two flat plates (see Appendix 1). The total forces, FTot, in a colloid solution has been explained by the Derjaguin-Landau-Verway-Overbeek (DLVO) theory31,43 to be a summation of the electrostatic force, FEl, and the van der Waals force, FVDW, as shown by eq 2. The extended DLVO theory

FTot ) FEl + FVDW

(2)

suggests that other force types, such as the oscillation force, FOs, can also be added to the summation. This can be expressed by a general equation:

FTot )

∑F

i

(3)

i

where Fi are the force components. In the case of electrical, van der Waals, and oscillatory force components, the total force is given by

FTot ) FEl + FVDW + FOs

(4)

The expressions for FEl, FVDW, and FOs are given in Appendix 1. 3-2. Calculation of the Micelles Volume Fraction. The number of micelles in a solution increases with an increase of the surfactant concentration above its cmc. The number of micelles (NM) in a 1000 mL solution can be calculated by

NM ) (C* - Ccmc)(NA/Nag)

(5)

where C*, Ccmc, NA, and Nag are the total concentration of the surfactant ions, critical micelle concentration, Avogadro’s number, and aggregation number, respectively. Because the shape of the NaAOT aggregates is essentially spherical,44-47 the volume fraction of the NaAOT reverse micelles (φ) is the volume of 1 micelle multiplied by the number of micelles: (32) Sober, D. L.; Walz, J. Y. Langmuir 1995, 11, 2352. (33) Milling, A. J.; Kendall, K. Langmuir 2000, 16, 5106. (34) Milling, A. J. J. Phys. Chem. 1996, 100, 8986. (35) Ruths, M.; Yoshizawa, H.; Fetters, L. J.; Israelachvili, J. N. Macromolecules 1996, 29, 7193. (36) Pagac, E. S.; Tilton, R. D.; Prieve, D. C. Langmuir 1998, 14, 5106. (37) Kuhl, T. L.; Berman, A. D.; Hui, S. W.; Israelachvili, J. N. Macromolecules 1998, 31, 8250. (38) Asakura, S.; Osawa, F. J. Chem. Phys. 1954, 22, 1255. (39) Sharma, A.; Walz, J. Y. J. Chem. Soc., Faraday Trans. 1996, 92, 4997. (40) Sharma, A.; Tan, S. N.; Walz, J. Y. J. Colloid Interface Sci. 1997, 190, 392. (41) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1994. (42) Kralchevsky, P. A.; Denkov, N. D. Chem. Phys. Lett. 1995, 240, 385. (43) Derjaguin, B. V.; Landau, L. Acta Physicochem. URSS 1941, 14, 633. (44) Lossia, S. A.; Flore, S. G.; Nimmala, S.; Li, H.; Schlick, S. J. Phys. Chem. 1992, 96, 6071. (45) Huruguen, J. P.; Authier, M.; Greffe, J. L.; Pileni, M. P. Langmuir 1991, 7, 243. (46) Turro, N. J.; Khudyakov, I. V. J. Phys. Chem. 1995, 99, 7654. (47) Peri, J. B. J. Colloid Interface Sci. 1969, 29, 6.

4 φ ) NM πRh3 3

(6)

where Rh is the radius of the NaAOT reverse micelle. The period of the oscillation in a force curve is determined by the size of the NaAOT reverse micelle. The radius of a reverse micelle is given by48

[H2O] R0h ) 1.5 + 0.175 [NaAOT]

(nm)

(7)

where [H2O] and [NaAOT] are the concentrations of water and NaAOT in the system. Because the charged groups of the NaAOT are in the center of the reverse micelle, there will be no counterion atmosphere extending from the outer surface of the reverse micelle into the dodecane bulk. Therefore, in contrast to an aqueous system described by Nikolov and Wasan,15 the 1/κ term for the counterion atmosphere does not need to be added to the radius to give an effective diameter.

4. Results and Discussion 4-1. Surface Forces between the Two Silica Surfaces in Dodecane. Figure 1 shows the interaction forces between the two hydrophilic silica surfaces in dodecane. In the compression force curve, no force was measured between the two hydrophilic silica surfaces until there was a separation of approximately 7.5 nm. At separations smaller than this, a repulsive force was observed. The decompression force showed a weak repulsion originating from the zero separation until there was a separation of approximately 15 nm. Figure 2 shows the interaction forces between the two hydrophobized silica surfaces in dodecane. No repulsive force was measured for the compression or decompression force curve at all of the separations. A weak attraction, commencing at approximately 2.5 nm, was observed in the compression force curve. The absence of a measurable force in the hydrophobized silica case and presence of a repulsive force in the hydrophilic silica case mean that the observed force for the hydrophilic silica case must originate from the silica surface. The origin of the short-ranged force measured for the hydrophilic silica can be further understood by plotting the log of the force versus the distance (see the inset of Figure 1). The linear log(F/R) versus D plot results indicate that the force may be entropic41 or electrostatic in origin. The slope also gives the concentration of free ions in solution (Cfit).49 Vigil and others50 showed that a silica gel layer of several nanometers can exist on the surface of the hydrophilic silica in water. Our experiments were not performed under dry conditions. It is therefore perceivable that a small amount of water may have adsorbed into the dodecane from the environment and adsorbed onto the hydrophilic surfaces,5 forming a silica gel layer. Additionally, the silica probe and substrate were only dried in air at room temperature. However, silica must be dried at temperatures greater than 100 °C to remove all of the adsorbed water.51 The unfavorable entropy associated with compressing the silica gel layers between the surfaces could cause an osmotic repulsive force.41 An electrostatic force may exist in an apolar solvent when the surface is charged and ions are present in the system. The surface charge in a nonaqueous solvent may (48) Charlton, I. D.; Doherty, A. P. J. Phys. Chem. B 2000, 104, 8061. (49) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (50) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. J. Colloid Interface Sci. 1994, 165, 367. (51) Iler, R. K. The Chemistry of Silica Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; John Wiley & Sons: New York, 1979; p 631.

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Figure 1. Interaction force measured between the two hydrophilic silica surfaces in n-dodecane, with compression (b) and decompression (0) force curves. The dotted line in the inset shows the best fit to the force curves, using the summation of an electrical force component with the constant charge model, assuming the identical surface potentials of 38 mV interacting in a 0.306 mM 1:1 electrolyte solution in n-dodecane and a van der Waals force component with a nonretarded Hamaker constant of 5.73 × 10-21 J.

Figure 2. Interaction force measured between the two hydrophobized silica surfaces in n-dodecane, with compression (b) and decompression (O) force curves. The solid line shows the best fit to the force curve, using a van der Waals force component with a nonretarded Hamaker constant of 3.42 × 10-21 J.

be acquired by a dissociation of the surface groups of the solid. Lyklema6 suggested a dissociation based on the idea of a proton donor and proton acceptor:

SH2+ + B- a SH + BH a S- + BH2+

(8)

where SH and HB are the surface groups of the solid and solvent molecules, respectively. Verwey52 showed that the hydrophlic silica is highly acidic. Additionally, in the case of an aprotic solvent, a small amount of water is usually present, unless an elaborate drying process is carried out.5 The dissociation of the water as a weak electrolyte provides protons and hydroxyl ions. Because the silica surface is more acidic than the aprotic solvent dodecane, an acidbase theory for our system is

S + B + H+ + OH- a SOH- + B + H+

(9)

To test this picture, we fit the experimental force curves for the hydrophilic silica-dodecane system with a summation of the electrostatic force between two identical surfaces using the constant surface charge boundary condition (eq A1 in Appendix 1) and the van der Waals (52) Kitahara, A.; Watanabe, A. Electrical Phenomena at Interfaces Fundamentals, Measurements, and Applications; Marcel Dekker: New York, 1984; p 124.

force (eq A3 in Appendix 1) using the Hamaker constant of 5.73 × 10-21 J. The Hamaker constant for the silicadodecane-silica case was calculated using the microscopic procedure (eq A4 in Appendix 1),53 using the individual Hamaker constant values of 8.96 × 10-20 J for hydrophilic silica54 and 5.00 × 10-20 J for dodecane.41 The Cfit determined from the AFM force curves was 0.306 mM in our hydrophilic silica-dodecane system. This low value can be attributed to the protons and hydroxyl ions with the water present. Because dodecane has a low dielectric constant, a small concentration of ions would be sufficient to cause the determined moderate surface potential of 38 mV. Using the values of Cfit and Ψ0, we can calculate the surface charge density (σ) of the silica surface as 266 µC m-2 (eq A11 in Appendix 2). If we consider that a silica surface in water can dissociate to give a surface charge density of up to 2140 µC m-2 (the value depending on the pH of the solution),55 then the values obtained here are reasonable. Because the electrostatic force theory explains the observed force in our hydrophilic silica-dodecane system well and a strong steric force was not seen for the same silica type in water,26 we conclude that the observed repulsive force is electrostatic in origin. The hydrophobized silica therefore did not show any repulsive force because of its apparent inabilty to dissociate. A hysteresis was seen in the compression and decompression force curves of the hydrophilic silica case. No hysteresis was observed for the hydrophobized silica case. The origin of the hysteresis is therefore probably due to the surface-surface interaction changes during contact. Two possibilities are the decharging of the surface and a structural change of the water molecules at the silica surface. The weak attraction that was observed in the compression force between the two hydrophobic silica surfaces was fit with a van der Waals force. The experimentally determined Hamaker constant of 3.42 × 10-21 J is comparable with the Hamaker constant of 2.53 × 10-21 J that was calculated for the hydrophobized silicadodecane system, using the microscopic procedure53 with the individual Hamaker constant values of 7.5 × 10-20 J for a hydrophobic surface and 5.0 × 10-20 J for dodecane.41 Oscillations in the force curves have been observed by the AFM and surface force apparatus for the nonpolar solvents of octamethylcyclotetrasiloxane,56-58 n-tetradecane,58 and n-hexadecane58 between the two smooth, hard surfaces. This was reported as being due to the structuring of the solvent molecules between the two surfaces. No oscillation was observed for dodecane in our experiment, probably because of its smaller molecular size. 4-2. Effect of NaAOT on the Surface Forces between the Two Silica Surfaces in Dodecane. To determine the effect of the addition of the ionic surfactant of NaAOT on the force between the two silica surfaces in dodecane, it is desirable that we have no forces in the absence of the surfactant. Because the hydrophobized silica-dodecane system gave no detectable force, we decided to use this system. Figure 3 shows the compression force curves for the NaAOT concentrations of 7.6 (the cmc), 100, and 250 mM. (53) Hamaker, H. C. Physica (Utrecht) 1937, 4, 1058. (54) McNamee, C. E.; Matsumoto, M.; Hartley, P. G.; Mulvaney, P.; Tsujii, Y.; Nakahara, M. Langmuir 2002, 17, 6220. (55) McNamee, C. E.; Matsumoto, M.; Hartley, P. G.; Nakahara, M. Colloids Surf., A 2001, 193, 175. (56) Lim, R.; Li, S. F. Y.; O’Shea, S. J. Langmuir 2002, 18, 6116. (57) Christenson, H. K. Chem. Phys. Lett. 1985, 118, 455. (58) Chan, D. Y. C.; Horn, R. G. J. Chem. Phys. 1985, 83, 5311.

Interaction Forces between Two Silica Surfaces

Figure 3. Effect of increasing the number of NaAOT reverse micelles on the interaction forces between the two hydrophobized silica surfaces, when the NaAOT concentrations are too low to cause oscillations in the compression force curves. The concentrations and measuring velocities of the NaAOT solutions are as follows: 4, 7.6 mM and 51.2 nm s-1; O, 100 mM and 50.1 nm s-1; and 0, 250 mM and 51.3 nm s-1. The solid, dashed, dotted, and dash-dotted lines in the inset show the best fits to the force curves for concentrations 7.6, 100, and 250 mM, using the summation of an electrical force component using the constant charge model, assuming the identical surface potentials of 16, 23, and 20 mV interacting in 0.153, 0.166, and 0.166 mM 1:1 electrolyte solutions in n-dodecane, respectively, with a van der Waals force component using a nonretarded Hamaker constant of 3.42 × 10-21 J, an oscillatory structural force component with the volume fractions of 0.1, 0.15, and 0.15, and an effective micellar reverse diameter of 3.7, 3.6, and 3.7 nm, respectively.

The average compression velocity of the AFM for the force measurements was 50.9 ( 0.7 nm s-1 (apparent compression time of 5 s). We found that this rate was slow enough to measure the equilibrium forces between the two surfaces as a function of the distance, because the forces did not vary with slower compression velocities. A repulsive force was observed in the presence of the NaAOT for each of these concentrations. The most repulsive force was observed with 100 mM NaAOT; a lower or higher NaAOT concentration gave a less repulsive force. The inset in Figure 3 shows the log(F/R) versus D plot, from which the Cfit was determined. Cfit increased with an increase in the NaAOT concentration. The fact that no oscillations were observed indicates that not enough reverse micelles existed to cause observable structural effects. The effect on the compression force curves of further increasing the NaAOT concentration to 500 mM is shown in Figure 4. The average compression velocity of the AFM for the force measurements was 40.3 nm s-1 (apparent compression time of 5 s). This time oscillation was observed in the force curve. Additionally, the magnitude of the repulsion is less than that observed for 250 mM NaAOT, and the concentration of ions in the dodecane solution is greater than that determined for 250 mM NaAOT. The inset in Figure 4 shows an enlargement of the oscillations. The surface potential of the hydrophobized silica surfaces (for 7.6, 100, 250, and 500 mM NaAOT), effective diameter of the particles, and volume fraction of the particles (φ) in the dodecane (for 500 mM NaAOT) were determined by fitting the experimental force curves with the resultant force from the electrostatic force between the two identical surfaces using the constant surface charge boundary condition, van der Waals force using the Hamaker constant of 3.43 × 10-20 J, and oscillatory structural force. These results are summarized in Table 1. Calculation of the charge density (σ)59 variation with an increase in NaAOT shows an initial increase from 0 to 100 mM, after which it slightly decreases (see Table 1).

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Figure 4. Observation of oscillations in the compression force curves between the two hydrophobized silica surfaces when the high NaAOT concentration of 500 mM in n-dodecane is used. The measuring velocity is 43.6 nm s-1. The solid line shows the best fit to the force curve by the summation of an electrical force component using a constant charge model, assuming the identical surface potentials of 18 mV interacting in a 0.383 mM 1:1 electrolyte solution in n-dodecane, with a van der Waals force component using a nonretarded Hamaker constant of 3.42 × 10-21 J, an oscillatory structural force component with a volume fraction 0.04, and an effective micellar reverse diameter of 3.5 nm. Table 1. Parameters Obtained from the Forces between the Two Hydrophobized Silica Surfaces in Dodecanea concn of NaAOT (mM)

Ψ0 (mV)

1/κ (nm)

Cfit (mM)

σ (µC m-2)

dfit (nm)

φfit

φcalcd

7.6 100 250 500

16 24 21 19

3.95 3.79 3.79 3.53

0.153 0.166 0.166 0.194

2.33 3.71 3.22 3.13

3.5

0.04

0.095

a

Concn of NaAOT, Ψ0, 1/κ, Cfit, σ, dfit, φfit, and φcalcd are the actual concentration of NaAOT used, surface potential of the hydrophobized silica surface, decay length, actual concentration of ions in solution determined from the force curves, hydrophobized silica surface charge density, effective diameter of the micelles, volume fraction of the micelles determined from the force curves, and calculated volume fraction, respectively.

The surface charge in a nonaqueous solvent may be acquired by the dissociation of the surface groups or adsorption of the surfactant cation or anion. In the former case, the dissociation is the result of a proton charge transfer between the solvent and particle, as was demonstrated above in eq 8. In the latter case, the sign of the surface depends on the relative adsorbability of the surfactant cation and anion.6 The hydrophobicity determines the adsorption; a hydrophilic ion (e.g., Na+) adsorbs onto a hydrophilic surface (e.g., metal oxides), and a hydrophobic ion (e.g., RSO3-) adsorbs onto a hydrophobic surface (e.g., carbon black).9 The anion active surfactant of NaAOT in xylene or heptane has been shown by electrostatic studies to give carbon black a negative charge.60 The hydrophobic surface of silica used in this case means that the adsorption or dissociation of a proton is unlikely. The surface charge of the hydrophobic silica in the presence of NaAOT in dodecane must therefore be negative because of the adsorption of the anion of the active surfactant of NaAOT onto the hydrophobic silica surface. This mechanism is (59) Usui, S. In Electrical Phenomena at Interfaces Fundamentals, Measurements, and Applications; Kitahara, A., Watanabe, A., Eds.; Marcel Dekker: New York, 1984; p 28. (60) Sato, T. J. Appl. Polym. Sci. 1971, 15, 1053.

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Langmuir, Vol. 20, No. 5, 2004

S + AOT-Na+ a SAOT- + Na+

McNamee et al.

(10)

where S depicts the hydrophobic silica surface. As expected, the results of this experiment showed an increase in the surface potential of the hydrophobic silica in dodecane from 0 to 24 mV when NaAOT was added to dodecane to give a concentration of 100 mM. An increase in the NaAOT concentration from 100 mM to 250 and 500 mM caused the surface potential to decrease from 24 mV to 21 and 19 mV, respectively. This decrease can be explained by an electrostatic adsorption of the oppositely charged sodium cation onto the bis(2-ethylhexyl) sulfosuccinate anions that have already adsorbed onto the hydrophobic silica (see eq 11). Kitahara and others61,62

SAOT- + AOT-Na+ a SAOTNa + AOT- (11) measured the ζ potential of the carbon black particles in the presence of NaAOT in cyclohexane and heptane. They also obtained a maximum in the absolute value of their ζ potential values as the concentration of NaAOT in a dodecane solution was increased. The lowest NaAOT concentration used here was 7.6 mM, the value corresponding to the critical concentration for the formation of the NaAOT reverse micelles in dodecane. However, the concentration of ions was seen to increase with the surfactant concentration above the cmc. According to the phase separation model, the concentration of the free surfactant molecules should be constant at and above the cmc, and all of the additional molecules should be incorporated into micelles.63 An increase in the ion concentration for the surfactant concentrations above the cmc has been reported for several aqueous surfactant solutions.26,64,65 This increase was explained by an increase in the counterion concentration because of the finite ion dissociation of the surfactant. Similarly, in our case, the increase in the ion concentration can be attributed to an increase in the counterion concentration. Oscillations in the experimental force curves, which include information about the structuring of the micelles, were observable only for the 500 mM NaAOT case. Here, the oscillations were superimposed onto a repulsive force and occurred with a period of 3.5 nm. The diameter of a NaAOT reverse micelle in dodecane can be calculated from eq 6 as 3 nm. The inclusion of a small fraction of water or excess hydrophilic sodium cations in or near the center of the NaAOT reverse micelle could increase its effective diameter, giving a possible explanation for the slightly greater observed value. The calculated volume fraction (φcalc) of reverse micelles in dodecane for 500 mM NaAOT is 0.095, when we use eq 5, the mean aggregation number of 44 for NaAOT in dodecane,66 and a radius of a NaAOT reverse micelle of 1.5 nm (assuming water is not present). This calculated value is greater than the value determined from the fitting of the AFM force curve (φfit) of 0.04 (see Table 1). This difference may also be a result of the nonnegligible electrostatic force that is also present. The accuracy of φfit decreases when the electrostatic repulsions dominate. (61) Kitahara, A.; Karasawa, S.; Yamada, H. J. Colloid Interface Sci. 1967, 25, 490. (62) Kitahara, A.; Amano, M.; Kawasaki, S.; Kon-no, K. Colloid Polym. Sci. 1977, 255, 1118. (63) Myers, D. Surfaces, Interfaces, and Colloids Principles and Applications; VCH: New York, 1991; p 309. (64) Bossev, D. P.; Matsumoto, M.; Nakahara, M. J. Phys. Chem. B 1999, 103, 8251. (65) Pashley, R. M.; Ninham, B. W. J. Phys. Chem. 1987, 91, 2902. (66) De, T. K.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95.

Figure 5. Decompression force curves measured between the two hydrophobized silica surfaces in the NaAOT solutions in n-dodecane: 4, 7.6 mM and 51.2 nm s-1; O, 100 mM and 50.1 nm s-1; 0, 250 mM and 51.3 nm s-1; and ], 500 mM and 43.6 nm s-1.

The experimental force curve for 500 mM NaAOT is larger than the calculated force curve using eq 3 for small surface separations of less than 4 nm. In particular, the oscillation in the force curve at the smallest separation is underestimated. The inclusion of an additional electrostatic force between the hydrophobic silica surfaces, which are charged by the adsorbed AOT anions, and the NaAOT micelles would give a greater repulsion at smaller separations. The decompression force curves of 7.6, 100, 250, and 500 mM NaAOT are shown in Figure 5. The decompression force curves are the same as the compression ones for 100 and 250 mM NaAOT. Hysteresis is however seen in the force curves for 500 mM NaAOT; the decompression force curves are smaller in magnitude and display a weak attraction. The origin of this difference may be a solvophobic force, depletion force, or surface-surface interaction changes (such as decharging) during contact. If the Na+ ions that have adsorbed onto the S-AOT- sites are situated at the silica-dodecane interface, then an unfavorable interaction of the hydrophilic Na+ ions with the hydrophobic dodecane could result in an energetically driven solvophobic force. Additionally, the large volume fraction of micelles for 500 mM NaAOT would give rise to a significant depletion attraction when the micelles are forced out between the two surfaces. This may be observed as an attraction. A hysteresis is also seen in the compression and decompression curves for 7.6 mM NaAOT. The attraction observed in the decompression force curve for 7.6 mM NaAOT may be explained by a van der Waals attraction. The hysteresis may be due to the surfacesurface interaction changes. To see the effect of the compression time on the force between the two hydrophobized silica surfaces in the presence of 500 mM NaAOT in dodecane, the force was also measured at faster times (see Figure 6). The oscillations are clear and sharp for the slow measuring times from 40.3 nm s-1 (apparent compression time of 5 s). However, when the faster measuring times of 67.1 nm s-1 (apparent compression time of 1.5 s) and 4364.9 nm s-1 (apparent compression time of 0.05 s) were used, the oscillations can no longer be recognized. A similar velocity dependence on the oscillations has been seen for the sodium dodecyl sulfate micelles between the two hydrophilic silica surfaces.26 Overbeek also suggested that the speed of collision of the particles may affect the interactions between the colloid particles.67 A possible explanation for the nonobservance of oscillations when the two surfaces (67) Overbeek, J. Th. G. J. Colloid Interface Sci. 1977, 58, 408.

Interaction Forces between Two Silica Surfaces

Langmuir, Vol. 20, No. 5, 2004 1797

5. Conclusions

Figure 6. Effect of interaction velocity on the compression forces between the hydrophobized silica surfaces in dodecane containing 500 mM NaAOT: (A) 0, 4364.9 nm s-1 (apparent compression time of 0.05 s); (B) O, 67.1 nm s-1 (apparent compression time of 1.5 s); (C) b, 43.6 nm s-1 (apparent compression time of 5 s).

In an apolar solvent, there exist no forces between the hydrophobized silica surfaces. We have revealed first that the addition of NaAOT to the system brings about an electrostatic repulsive force induced by the adsorption of the AOT anions and subsequently of the sodium cations. This is the main mechanism for the stabilization of the colloidal dispersion in an apolar solvent with NaAOT. This is the first time for such an effect to be shown. The solubilization of water in the reverse micelles was shown for the first time to not affect the surface forces between the two hydrophobized silica surfaces in an apolar solvent because of its preference to reside in the reverse micelles rather than at the hydrophobized silica or reverse micelle surfaces. Acknowledgment. This study has been indebted to the Science Promotion Grant 10640558 of the Ministry of Education, Japan, and to the Humboldt Foundation, Germany. Appendix 1: Equations for the Electrostatic (FEl), van der Waals (FVDW), and Oscillation Forces (FOs) 1. The electrostatic interaction free energy (GEl) between the two parallel plates separated by a distance h and assuming a constant charge density is

[ (κh2 ) - 1]

GEl(h) ) r0κΨ02 coth Figure 7. Difference in the interaction force between the two hydrophibized silica surfaces in 100 mM NaAOT, when the reverse micelles are not solubilized (b) or solubilized with 1469 mM kg-1 water (O). The measuring velocities for the nonsolubilized and solubilized cases were 50.1 and 40.3 nm s-1, respectively.

approach each other very fast is the presence of the strong hydrodynamic force, which forces fluid out from between the two surfaces. 4-3. Effect of Solubization of Water in the NaAOT Reverse Micelles on the Surface Forces between the Two Silica Surfaces in Dodecane. The compression forces between the two hydrophobized silica surfaces in dodecane with solubilized and nonsolubilized NaAOT reverse micelles are shown in Figure 7 by the open squares and solid circles, respectively. We note that the forces appear to be the same. Because the silica used in our experiment was hydrophobic, it would be energetically unfavorable for the added water to adsorb at the silicadodecane interface. Rather, it would be more favorable for the water to be involved in the NaAOT reverse micelle as solubilized water. The surface potential of the silica surface would therefore be unchanged by the presence of water, which was observed. This result implies that the inner substance of a reverse micelle does not affect the outer surface properties of a reverse micelle. The radius of the NaAOT reverse micelle can be calculated from eq 7 as an increase from 1.5 to 4.1 nm upon solubilization. Such a radius increase would affect the force curves as an increase in the period of the oscillations. However, because 100 mM NaAOT does not give enough reverse micelles to cause observable structural effects, no increased radius effect was noted in these measured force curves.

(A1)

where r, 0, and Ψ0 are the relative permittivity of the solvent, permittivity of vacuum, and surface potential, respectively. The Debye length (1/κ) is

1 κ

)

[ ] r0kBT

∑i

1/2

(A2)

zi2e2Cei

where kB, T, zi, e, and Cei are the Boltzmann constant, absolute temperature, valency, elementary charge, and concentration of ions in the solution in molecules m-3. 2. The van der Waals interaction free energy (GVDW) between two parallel plates is given by

GVDW(h) ) -

A 12πh2

(A3)

where A is the Hamaker constant. The microscopic procedure of calculating the Hamaker constant (A131) for two identical surfaces with a Hamaker constant of A11 in a medium with a Hamaker constant of A33 uses eq A4.

A131 ) (xA11 - xA33)2

(A4)

3. The oscillation interaction free energy expression (GOs) of Kralchevsky and Denkov42 accounts for the depletion and structural forces through the osmotic pressure and the geometric packing of the particles (see eq A5 and A6).

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Langmuir, Vol. 20, No. 5, 2004

(

McNamee et al.

) ( ) () [

h d3 d12d2 d2 d1 2πh cos 2 d d d1 1 2 2 4π + d2

P0d1 exp GOs(h) )

2π sin

(

( )] 2πh d1

for h g d (A5)

) ( ) () [

d3 d d12d2 d2 d1 2πd cos 2 d d d1 1 2 2 4π + d2

P0d1 exp GOs(h) )

2π sin

( )] 2πd d1

- P0(d - h)

for 0 e h e d (A6)

where h is the separation distance and d is the size of the particle. P0 is the particle osmotic pressure and is given by

P0 )

6φ 1 + φ + φ2 - φ3 kT πd3 (1 - φ)3

(A7)

where φ is the volume fraction. The parameters that take into account the packing of the particles between the two surfaces are d1 and d2. These are defined as

[x

d1 ) d

]

2 + 0.23728∆φ + 0.63300(∆φ)2 3

Figure 8. Kralchevsky and Denkov GOs for a solution with a volume fraction of 0.01 (solid line), 0.1 (dashed line), and 0.4 (dotted line) for particles of radius 1.5 nm.

Figure 8 demonstrates GOs for a solution with a volume fraction of 0.01 (solid line), 0.1 (dashed line), and 0.4 (dotted line) for particles of radius 1.5 nm. 4. The above free energy equations for the parallelplate geometry can be converted to force equations for a sphere-parallel-plate geometry via the Derjaguin approximation, given by eq 1 in the text, provided the separation distance of the sphere and plate is much less than the radius of the sphere. Appendix 2: Equations for the Surface Charge Density (σ)

(A8) The surface charge density is given by

( )

and

0.48663 - 0.42032 d2 ) d ∆φ

(

)

σ ) (8Cer0kBT)1/2 sinh

(A9)

eΨ0 2kBT

(A11)

where Ce is the bulk electrolyte concentration in molecules m-3.

with

∆φ ≡

π -φ 3x2

(A10) LA035730+