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Onset of Hydrophobic Attraction at Low Surfactant Concentrations V. Yaminsky,* C. Jones, F. Yaminsky, and B. W. Ninham Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Institute of Advanced Studies, The Australian National University, Canberra, ACT 0200, Australia Received January 29, 1996. In Final Form: April 29, 1996X We report here on the interaction between silica surfaces in a range of CTA+ (cetyltrimethylammonium cation) concentrations up to the point of zero charge (5 × 10-5 M) in a background electrolyte (3 × 10-3 M sodium acetate). In the absence of CTA+ the static interaction is repulsive at all distances. The addition of CTAB up to 10-5 M has no effect on the interaction. Between 2 × 10-5 and 3 × 10-5 M the surface charge as deduced from interaction at large separations remains unchanged, but the interaction becomes attractive at distances less than a Debye length. The strength of the attraction increases several times as surfactant concentration increases to 4 × 10-5 M with simultaneous reduction in the electrostatic repulsion at larger distances. At 5 × 10-5 M the interaction is attractive at all distances. The results confirm that CTA+ adsorption is cooperative and nonlinear in concentration and is enhanced by surface proximity. Both effects lead to a long range hydrophobic force.
Introduction Adsorption of cationic surfactants induces a long range “hydrophobic” attraction between mica1-3 and silica4 surfaces. The origin of this attraction has been explained recently.5 By making use of the Gibbs equation it was shown that the surfactant-induced attraction corresponds to an increase of adsorption with decreasing separation. Neutralization of the surface charge is enhanced due to cooperative tail association within hydrophobic monolayers. This charge regulation reduces the electric potential below the constant limit and shifts the point of zero charge (p.z.c.) to lower surfactant concentrations at smaller separations. While the general conclusions are straightforward, experimental data are limited to a few concentrations. With this it is not even clear how the transition from repulsion in pure water to attraction around the p.z.c. actually occurs. To allow quantitative thermodynamic analysis, data have to be collected in fine surfactant ion concentration intervals at constant concentrations of other components. Measurements under controlled dynamic conditions are needed to show an involvement of kinetic and equilibrium effects. The surface force apparatus (SFA technique) based on contact interferometry6,7 is elaborate and not well suited to sufficient data collection. Force measurements using commercial atomic force microscopy (AFM)8 are of a limited value. They typically involve ill-defined geometry and purity of contacting microparticles, presence of a shearing * Author to whom all correspondence should be addressed: telephone, (06) 249 4693; fax, (06) 249 0732; e-mail vvy110@ rsphysse.anu.edu.au. X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) Israelachvili, J. N.; Pashley, R. M. Nature 1982, 300, 341. (2) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088. (3) Kekicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989, 40, 31. (4) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706. (5) Yaminsky, V. V.; Ninham, B. W.; Christenson, H. K.; Pashley, R. M. Langmuir 1996, 12, 1936. (6) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (7) Parker, J. L.; Christenson, H. K.; Ninham, B. W. J. Phys. Chem. 1988, 92, 4155. (8) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239.
S0743-7463(96)00091-1 CCC: $12.00
component during measurement of normal force, etc. We report here on an experimental development that enables rapid collection of extensive data under widely changed and easily controlled conditions. New results for silica surfaces at low concentrations of CTAB are presented that illustrate the new techniques and confirm the mechanism of surfactant-induced hydrophobic interactions. Materials and Techniques Surface force measurements using solid state sensors9-12 are ideally suited to our purposes. The basic setup is described in ref 13 and will be termed an interfacial gauge. Spheres several millimeters in diameter are melted at an end of silica rods and installed in the gauge while hot. With this any “cleaning” procedure like cold plasma treatment8 is unnecessary. Unlike for SFA interferometer, the scaling radii coincide with the macroscopic radii of curvature which can be measured with any practically desired accuracy. For extended bibliography of using these surfaces in force measurements, see ref 13. Measurements are done by applying an external magnetic load and monitoring the electric response of the piezoelectric sensor. This gives interaction forces between the two surfaces vs their mutual displacement and speed. The speed and the load can be varied by changing the period and the amplitude of the loading ramps. The new data acquisition system enables collection of points at a frequency up to 50 kHz. The load and the displacement are at the normal to the surfaces at contact so that shearing movement is avoided. No parts of the gauge except the silica samples themselves are immersed in a liquid contained in a silica beaker. Concentration is changed by adding stock solution and stirring by rotating the beaker while surfaces are under the liquid. Solutions were prepared from Millipore water, CTAB (Aldrich), NaBr, and NaAc (reagent grades). Reagent purity is controlled through measurement of surface tension, wetting tension, and forces between nonpolar surfaces (methylated glass). Wetting tension tests for absence of a hysteresis with a flame-polished silica glass and freshly cleaved mica plates assure absence of surface active contaminants in water with affinity to hydrophilic negatively charged solid surfaces. (9) Parker, J. L. Langmuir 1992, 8, 176. (10) Parker, J. L.; Stewart, A. M. Prog. Colloid Polym. Sci. 1992, 88, 162. (11) Stewart, A. M.; Parker, J. L. Rev. Sci. Instrum. 1992, 63, 5626. (12) Stewart, A. M. Meas. Sci. Technol. 1995, 6, 114. (13) Yaminsky, V. V.; Ninham, B. W.; Stewart, A. M. Langmuir 1996, 12, 836.
© 1996 American Chemical Society
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Figure 2. Interaction of fused silica surfaces (R ) 1.62 mm) in 0.3 M NaBr. The initial speed was 2.5 nm/s. The exponential form holds down to contact. The pre-exponential factor is of the same order as in pure water. The Debye length is reduced 50 times. Data are on linear (a) and logarithmic (b) scales. Figure 1. Interaction of fused silica surfaces in Millipore water. The force (F) is scaled with the mean radius (R ) 1.42 mm) according to the Derjaguin approximation F ) πRE to give the free energy (E) of interaction per unit area. The scaling has this physical meaning only for static interaction of hard core bodies. The loading speed is 1.14 dyn/s, the initial approach at 30 nm/ s. Data are on linear (a) and logarithmic (b) scales.
Results Measurements at any nonzero speed involve viscous effects. Even though the radii of the samples in the present study are an order of magnitude smaller than in an ordinary SFA experiment (the latter are typically 1-2 cm) while the viscous force is proportional to radius squared, we still find a number of kinetic effects. The regime of a constant rate of change of external load used in this study has been considered earlier.14 On approach the viscous interaction is repulsive and noticeable at sufficiently small distances. On separation the viscous interaction gives rise to an attraction. Only when the samples are moved sufficiently slowly in pure water is the interaction free of viscous hysteresis. It then is repulsive over the whole range of distances. When the initial speed of approach is 30 nm/s or less, the repulsion down to at least the last 3 nm is speed independent and decays exponentially with a characteristic length of about 30 nm (Figure 1). The repulsion at shorter ranges noticeably increases at higher speeds but (14) Steblin, V. N.; Shchukin, E. D.; Yaminsky, V. V.; Yaminsky, I. V. Hydrodynamic Surface Interactions in an Electrolyte Solution. New Method of Investigating Surface Forces Using Capacitance Ultradynamometer. Kolloid. Zh. 1991, 53, 684 (in Russian; English translation in: Colloid J. USSR 1991, 53, 577).
also shows some variation with time elapsed since the samples were melted and then first immersed in water. The latter effect is indicative of a surface rearrangement (e.g., hydroxylation) which apparently occurs under water. The range of the double layer repulsion decreases by addition of an electrolyte through reduction of the Debye length. In decimolar solutions the screening length is less than 1 nm. When the loading is sufficiently slow the repulsion on approach is independent of the speed. An exponential form with the short decay length holds up to the last several angstroms (Figure 2). Below this distance the samples come into a hard wall contact. For 10-1 M strong inorganic 1:1 electrolyte the apparent surface charge is almost as in water. Kinetic adhesion on separation depends on rates of loading and unloading and on the maximum force in contact. Depending on the particular set of the dynamic parameters and electrolyte concentration which influences the static repulsion, the samples can proceed from the hard wall interaction regime into the exponential double layer regime at either positive or negative loads. When the repulsion is suppressed by adding electrolyte, the dynamic effect on separation is especially pronounced with the interaction being purely attractive. The standard viscous drag formula for continuum media diverges at zero distance and is thus of limited significance in analyzing the separation data.14 In practice the situation is further involved. We have observed that when the speed is changed instantaneously by changing the period of loading cycles, the kinetic interaction does not respond immediately. Adhesion changes from the original to the new value gradually over several periods of a
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Figure 3. Interaction of fused silica spheres (R ) 1.42 mm) in 0.003 M NaAc. The free sample moves with a speed (25 nm/s) determined by the loading rate (10 mN/s) and the spring constant (400 N/m). Both ways, in (a) and out (b), are shown. The time interval between each two subsequent points is 10 ms. Interaction in pure water for these two spheres is shown in Figure 1.
changed frequency. Apparently the effect has some memory of the history of the previous cycles and cannot be accounted for by simple hydrodynamic models. Even in surfactant-free electrolyte solutions the kinetic behavior is complicated. Underlying mechanisms demand thorough dynamic investigation. Here we consider effects of CTAB for a single set of dynamic parameters. Frequency and amplitude of triangle loading ramps are fixed. The contact (point of zero force) is approximately in the middle of the ramp. The speed outside contact is 25 nm/s, the rate of loading is 10 µN/s, and the maximum contact force is about 100 µN. All measurements are in a background of 3 × 10-3 M sodium acetate. This salt was chosen because it does not suppress the solubility of CTAB. At such a concentration the double layer is still well developed, while the ionic strength is maintained essentially constant over the range of CTAB concentrations of interest (below 5 × 10-5 M). Only the chemical potential of the CTA+ ion is varied in this range. Figure 3 shows the interaction in the reference sodium acetate solution before addition of CTAB. The Debye length, about 7 nm, is as expected for such a concentration. The pre-exponential factor corresponds to a relatively low
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surface potential, about 25 mV. This is slightly higher than for pure water because pH is higher as a result of the buffering action of the hydrolyzable salt. The dissociation of silanol groups increases rapidly with increasing pH, from zero at pH below 3 to almost 100% at pH above 9. For the chosen dynamic conditions adhesion is in the positive (repulsive) range. The exact value of the load at which the samples go out of the contact shows some variation which can depend on the exact contact duration which is only roughly constant in repeated measurements. The time spent in water by the silica surfaces is a possible contributing factor. Apart from this difference, results a few minutes after immersion, and a day later, are similar. When CTAB is added to a concentration of 2 × 10-6 and further to 8 × 10-6 M the interaction pattern remains unchanged. At concentrations of 1.5 × 10-5 M the prefactor extrapolated from long distances is still the same as before addition of CTAB. But at separations below a Debye length the interaction undergoes a remarkable transformation. The repulsion reduces and goes through a minimum at around 5 nm from the hard wall position. This minimum occurs in the positive energy range, which means that the force reduces but still remains repulsive. The interaction pressure, which is the gradient of the static force, can be attractive at these distances, but the overall force between spheres includes the repulsion from more distant areas of the surfaces. Repulsion dominates again at distances below 2 nm. This subsequent short range repulsion is apparently even more steep than prior to addition of CTAB. This could be taken for an indication of a molecular rearrangement and an enhanced effective viscosity of molecular layers. The effect arises immediately after the concentration of CTAB is increased to this value and persists for all 12 h of observation. At the next concentration studied, 2.5 × 10-5 M, the minimum becomes deeper, but no qualitatively new features are introduced. The results are shown in Figure 4. By 3 × 10-5 M this minimum in the force curve shifts downward a long way and is now in the negative range. At this concentration the extrapolated charge as seen from large distances is reduced substantially (Figure 5). At 5 × 10-5 M there is no repulsion at all even at large distances (Figure 6). The monotonic attraction is noticeable at distances less than 15 nm. Below 10 nm the mechanical stability of the spring is lost and the surfaces jump into a hard wall contact. The latter type of behavior is typical of a hydrophobic attraction as observed by many authors. Its range is variable from system to system. In the latter respect we refer to one our own observations. We notice that when a strong stream of bubbles was passed occasionally through the small volume (about 5 mL) cell with a syringe, the range of the attraction was substantially increased (Figure 7). The range then reduced slowly and, after several hours, was back again to the original value observed before the bubbling. At higher concentration recharging occurs. The interaction is again repulsive.4 We do not consider this range of surfactant concentrations here. Discussion At CTAB concentrations several times lower than the p.z.c. (about 5 × 10-5 M, the precise value being both salt and distance dependent) adsorption and its variation with separation are negligible according to the experimental results. Only at concentrations above 10-5 M, or more than a quarter of the p.z.c., is there a substantial
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Figure 6. Continued from Figures 3 to 5. CTAB concentration was 5.8 × 10-5 M.
Figure 4. Experiment continued from Figure 3. CTAB is added to a concentration 2.6 × 10-5 M. Data are on linear (a) and logarithmic (b) scales.
Figure 5. Continued from Figures 3 and 4. CTAB concentration was 4 × 10-5 M.
enhancement of the surfactant adsorption from a separation below 10 nm. Recent considerations of such hydrophobic interactions5 included several assumptions: (1) adsorption is insignificant at concentrations several times lower than the p.z.c.; (2) it rises nonlinearly with concentration; (3) its increase with decreasing separation is faster than for a
Figure 7. Continued from Figure 6. Measurement was after a high-speed jet of microbubbles was passed through the cell.
simple charge neutralization model in the range of distances where the constant potential and constant charge DLVO results diverge. Our observations confirm the notion of cooperative adsorption enhanced by the proximity of the surfaces.5 It has been shown earlier15 that adsorbed layers of CTAB at isolated silica surfaces are rarefied. Even at the p.z.c., the area per molecule is 700 A2 in a correspondence with a similarly low surface charge of silica in water. Adsorption at the silica-water interface is further much lower at lower concentrations. But that adsorption increases up to several times for two surfaces in adhesive contact. At this equilibrium distance the surfaces are separated by a condensed surfactant layer. The present results confirm the recent consideration5 which shows that condensation is enhanced at quite large separations long before contact is reached, from distances below a Debye length. The change in adsorption, Γ - Γ∞ on going from infinity to a given separation is the derivative -dE/dµ of the free energy of interaction at this separation with respect to the chemical potential. Here E ) πRF, F is the measured force, R ) R1R2/(R1 + R2) for two hard spheres of radii R1 and R2, dµ ) kTd ln a. The hard sphere (Derjaguin) approximation holds out of contact, and activity a can be substituted for concentration C in our case. In the concentration range from 3 (15) Yaminsky, V. V. Langmuir 1994, 10, 2710.
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× 10-5 to 5 × 10-5 M adsorption increases by 1013 molecules/cm2 when the separation is decreased to 3-4 nm. An additional CTAB ion adsorbs per each 1000 Å2. This is indeed comparable in order of magnitude with the initial value of p.z.c. adsorption at isolated surfaces. The surfaces can still retain a large charge at infinite separation, but when the distance becomes less than the Debye lengths, all the charge is neutralized and p.z.c. is reached. The adsorption further increases many times in the range of shorter distances down to equilibrium contact. At very large separations the small increase of adsorption is due to charge regulation. But after a critical density of the order of 103 Å2 per molecule is reached and hydrophobic tails begin to associate, a spontaneous condensation occurs driven by additional free energy gain which results from this association. The process goes with acceleration until formation of a fully condensed layer at equilibrium separation. Simple estimates based on Gibbs formalism extended by Hall16 and Radke and Everett17 in direct application to experimental results lead to a unified account of various short and long range surface forces.5,15,18,19 More detailed results can be based on careful consideration of various kinetic conditions related to equilibrium adsorption as a function of separation. A short range kinetic repulsion at smaller distances before the hard wall contact as observed at intermediate concentrations could be attributed to surfactant condensation in a narrow gap of (surfactant) molecular thickness.5 A high viscosity typical of concentrated gel-like surfactant systems shows itself as a repulsion on approach and as an attraction (“hydrodynamic” adhesion14) on separation. And indeed, adhesion, while substantially enhanced, also remains time and speed dependent at these transition concentrations below the p.z.c. Static attraction, which results from surfactant adsorption, increases with concentration and dominates the interaction at the p.z.c. also at short distances. A time dependent adhesion at CTAB concentrations many times lower than the p.z.c. had earlier been attributed to enhanced adsorption in narrow slits.20 However, we show here that at such concentrations, the effects of CTAB on static forces in 3 × 10-3 M NaAc are quite negligible. Even in the absence of CTAB, when only simple electrolyte is present, there is a kinetic attraction which has essentially the same characteristic times. A reduction of the double layer repulsion by electrolyte favors hydrodynamic adhesion.14 A similar effect can be caused by CTAB, which is a potential determining ion, at lower concentrations in the absence of a foreign salt. In fact what we observe is that at higher concentrations the adhesion becomes less frequency dependent, taking on more of a static nature at the p.z.c. At the p.z.c. there is no such short range “non-DLVO” repulsion any more. The attraction is strong and the surfaces jump rapidly into a hard wall contact. In the absence of CTAB the ordinary exponential double layer (16) Hall, D. G. J. Chem. Soc., Faraday Trans. 2 1972, 68, 2169. (17) Everett, D. H.; Radke, C. J. In Adsorption at Interfaces; ACS Symposium Series 8; Mittal, K. L., Ed.; American Chemical Society: Washington, DC, 1975; p 1. (18) Yaminsky, V. V.; Christenson, H. K. J. Phys. Chem. 1995, 99, 5176. (19) Pethica, B. A. Colloids Surf., in press. (20) Parker, J. L.; Rutland, M. W. Langmuir 1993, 9, 1965.
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form also holds down to at least the last few angstroms provided the speed is sufficiently low. An involvement of silica swelling responsible for an enhanced effective viscosity cannot be dismissed as a contributing factor. A systematic study of kinetic aspects of the interaction will shed more light on these mechanisms. It has been shown earlier21 that like for the two surfaces in adhesive contact, equilibrium CTAB adsorption at the silica-vapor interface is many times larger than at the silica-water interface. While at the latter interface the layer remains rarefied up to the p.z.c., a condensed hydrophobic monolayer is deposited when the plate is retracted and solution recedes from the surface. This deposition apparently occurred during the bubbling experiment. Even though water is saturated with air, the microbubbles are under excess Laplace pressure and dissolve rapidly. For deposited CTAB monolayers the desorption is much slower. Adsorption exceeds the equilibrium value over a substantial period of time.15,21 The supersaturated monolayers more easily form condensates in contact. Attraction of a similarly increased range is indeed observed if the samples are retracted from CTAB solution and reimmersed in the solution or even in pure water. An even longer range attraction is observed for mica surfaces with deposited Langmuir-Blodgett monolayers.22 The latter transferred at high pressures to the solid-air interface are further away from equilibrium when under water. Being laterally mobile they escape the thermodynamically unfavorable contact with water at the three phase line and onto the water-air interface in wetting experiments.23 They also fill contact slits and form bulky swollen condensates which induce a capillary type attraction (cf. ref 5). A similar attraction of a further longer range is induced by vapor adsorption on freshly molten commercial glasses.24 Soda and other hygroscopic components segregate at the surface during melting and then dissolve in adsorbed aqueous film with respect to which water vapor remains supersaturated (the effect known as “polywater”25). Adhesion according to high surface tension of electrolyte solutions is particularly high, but after the jump out of contact under external load, the capillary bridge (which in the case of swollen surfactant phases in water hardly can be detected interferometrically) is preserved and induces long range attraction noticeable in the polywater case from distances greater than a micrometer. Acknowledgment. We wish to thank Tim Sawkins for his contribution to the development of user friendly hard- and software for the interfacial gauge. The developments in technique used here rest substantially on a long term project initiated by John Parker in this department. LA9600917 (21) Yaminsky, V. V.; Yaminskaya, K. B. Langmuir 1995, 11, 936. (22) Christenson, H. K. In Modern Approaches to Wettability: Theory and Applications; Schrader, M. E., Loeb, G., Eds.; Plenum Press: New York, 1992; p 29. (23) Yaminsky, V. V.; Nylander, T.; Christenson, H. K.; Ninham, B. W. Submitted for publication in Langmuir. (24) Yaminsky, V. V. In preparation. (25) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990; pp 282-283.
