Langmuir 1997, 13, 5979-5990
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Dewetting of Mica Induced by Simple Organic Ions. Kinetic and Thermodynamic Study Vassili Yaminsky,*,†,‡ Barry Ninham,‡ and Marilyn Karaman§ Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Institute of Advanced Studies, Australian National University, Canberra, ACT 0200, Australia, and Department of Physical Chemistry 1, Center for Chemistry and Chemical Engineering, University of Lund, Box 124, S-221 00 Lund, Sweden Received July 18, 1996. In Final Form: July 7, 1997X Aqueous solutions of organic electrolytes for which either the cation or anion contain hydrocarbon moieties form finite contact angles on freshly cleaved mica. After electrolytes are spread on the surface, a solution then recedes spontaneously to take up an equilibrium contact angle value. Among electrolytes studied which show the effect are tetraethylammonium chloride, tetrabutylammonium bromide, and sodium acetate. Concentrated solutions of such low molecular weight dissociating organic compounds form ideal lenses that exhibit no contact angle hysteresis and slide on mica without friction if the plate is tilted. From equilibrium contact angle and the surface tension measurements, wetting tension isotherms were obtained. Interpreted as surface pressure difference isotherms, these were converted into adsorption difference isotherms. The dewetting can then be shown to occur because the organic ion adsorbs preferentially at the mica-air interface. A monolayer of close packed tetrabutylammonium cations is deposited behind the receding liquid front at solution concentrations as low as 10-4 M and at higher concentrations for smaller ions. The ions desorb when the solution advances onto the surface. The layer readsorbs again on the areas from which the liquid has receded. The rate of adsorption is controlled by the rate of diffusion of the solute to the three-phase line. Equilibration times are less than a second at high concentrations. At smaller concentrations this adsorption becomes the limiting factor that reduces the rate at which the receding angle increases to equilibrium. But even here, equilibration times do not exceed several minutes. Desorption from areas onto which the solution has advanced is fast. The advancing angle equals the equilibrium angle at all concentrations unless the spreading occurs on to areas of the mica which were never in contact with the solution. The initial spreading onto clean areas proceeds at a faster rate than the rate of adsorption. This ultimate rate, controlled by inertia/viscosity, is the same for water and solutions. A characteristic reoscillation occurs, with a rapid decrease of the contact angle and subsequent increase to the equilibrium value, at a rate controlled by adsorption. The dewetting effects of simple organic ions are similar to those of long chain surfactants. But equilibrium angles are smaller, and the equilibration is orders of magnitude faster than in the case of surfactants.
1. Introduction The Problem of Surfactants. Curious dewetting transitions are caused by adsorption of ionic surfactants on hydrophilic surfaces of opposite charge.1 A well-known example is hydrophobization of silica by cationic soaps.2 “Hydrophobic forces” between mica surfaces in solutions of single and double chain cationic surfactants have attracted a good deal of attention recently.3,4 Water containing trace amounts of CTAB (cetyltrimethylammonium bromide) forms large contact angles at these hydrophilic substrates. The surfaces then attract each other from large distances. Cationic head groups of surfactant molecules bind to anionic surface sites. Through this binding the hydrophilic moieties are removed from contact with water. Water * Author to whom all correspondence should be addressed: telephone, (616) 249 4693; fax, (616) 249 0732; e-mail, vvy110@ rsphysse.anu.edu.au. † On leave from the Institute of Physical Chemistry, the Russian Academy of Sciences, Moscow. ‡ Current address: Physical Chemistry 1, Chemical Centre, Lund University, Box 124, 221 00 Lund, Sweden. § Department of Chemistry, Australian National University. X Abstract published in Advance ACS Abstracts, September 1, 1997. (1) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989. (2) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (3) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (4) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088.
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makes contact with hydrophobic tails of the surfactant on the outer side of the adsorbed layer. Such is the usual picture of a hydrophobic monolayer. It is a simple picture. But it is not correct. Indeed, were it so, the implicit assumption is that the adsorption is irreversibile. This makes for self-consistent consideration only in the case of chemisorbed surfactants. Here the layer is “frozen” and adsorption-desorption equilibrium is hindered. In applications to wetting phenomena, this would mean that a displacement of the three-phase line cannot affect the hydrophobic monolayer. The amount adsorbed would here not change when the liquid advances on the solid substrate or recedes from it. But if adsorption is reversible, the situation is very different. Equilibrium values of adsorption at the solidliquid, and at the solid-vapor interfaces, are generally not equal to each other. An adsorption discontinuity must occur at the three-phase line. A displacement of the line forces the surfactant to adsorb on one side of it and to desorb on the other. At low concentrations of CTAB in aqueous solution, equilibrium adsorption at the mica-air interface is larger than at the mica-water interface. This is because of the additional free energy gain which results from the transfer of hydrophobic tails from water into air. The process can be analyzed quantitatively by combining the Young equation, which accounts for contact angle equilibrium, with the Gibbs equation which accounts for adsorption equilibrium. This explains the mechanism behind the apparent hydrophobicity of the surface. Equilibrium contact angles © 1997 American Chemical Society
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are large not because the hydrophobic tails are exposed to water. Rather, surfactant molecules desorb from the parts of the solid which are under water and adsorb on dry areas. The molecules migrate across the three-phase line and settle on the vapor side of it. The liquid is pushed away from the solid by this adsorption, and contact angle increases. On surface regions from which water has receded a high density surface deposit forms. It is washed away if the liquid is forced to advance onto these regions again. This description of what happens with surfactants can be rephrased quantitatively in terms of adsorption and surface pressure. The two-dimensional adsorption pressure that acts on the wetting line from the vapor side of the solid exceeds the opposing surface pressure from the other side which is under the liquid. This is what drives the contact angle increase through surfactant adsorption. Adsorption equilibrium does not necessarily extend over the entire macroscopic area of the solid. This condition is satisfied for surfaces of liquids. But for solids the attainment of global equilibrium can be hindered. This is because solid surfaces lack the tangential mobility of surfaces of liquids. Also, for adsorbed components, molecular mobility is restricted. Especially is that so on dry areas where, for nonvolatile adsorbates, lateral migration only by mechanisms of solid state diffusion can occur. However, for an equilibrium contact angle to be formed, it is sufficient that adsorption equilibrium is maintained locally, in the vicinity of the three-phase line. These considerations show that the conditions for chemical and mechanical equilibrium in dewetting transitions induced by surfactant adsorption are linked. The principle is trivial in consideration of contact angles formed between two liquids. But rare attempts have been made to apply those notions to solids (e.g., ref 5). We have already used this kind of analysis to explain the dewetting transitions caused by adsorption of CTAB on to silica6 and on to mica.7 The fact that the wetting behavior depends here on CTAB concentration in the solution has long been known. That observation provides a clear indication that adsorption is reversible. However, nonequilibrium effects are involved too: a large contact angle hysteresis occurs. This is the usual experimental obstacle to quantitative wetting studies. It may explain why thermodynamic conclusions could not be reached earlier. Long duration capillarography6-9 is the new relevant technique. What is new is that it solves the problem of equilibrium contact angles. Further it enables one to monitor the evolution to equilibrium of liquids on solid substrates that ordinarily occurs over hours, days, and even much greater times. The method rests on straightforward but rigorous principles of capillarity. The thermodynamic condition for mechanical equilibrium based on the Young equation is exact. So is the condition for chemical equilibrium. The method follows from consideration of concentration dependencies of the equilibrium wetting tension in terms of Gibbs adsorption excesses. This approach provides a firm foundation for the Wilhelmy plate/meniscus height techniques. These are here redevized as a means of thermodynamic characterization of solid substrates. With the primary experimental quantity taken as the difference of the values of the surface (5) Vogler, E. A. Langmuir 1992, 8, 2005, 2013. (6) Yaminsky, V. V.; Yaminskaya, K. B. Langmuir 1995, 11, 936. (7) Eriksson, L. G. T. ; Claesson, P. M.; Eriksson, J. C.; Yaminsky, V. V. J. Colloid Interface Sci. 1996, 181, 476. (8) Yaminsky, V. V.; Nylander, T.; Ninham, B. W. Langmuir 1997, 13, 1746. (9) Yaminsky, V. V.; Claesson, P. M.; Eriksson, J. C. J. Colloid Interface Sci. 1993, 161, 91.
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tension at the coexisting solid-liquid and solid-vapor interfaces, the Gibbs analysis leads to the corresponding difference of the equilibrium values of adsorption. The thermodynamic analysis itself applies to equilibrium data. The molecular picture which here emerges in turn gives insights into kinetic mechanisms that lie behind dynamic wetting tension measurements. Contact angle hysteresis occurs here on ideally smooth, and compositionally uniform, surfaces of crystalline and vitreous planes. The hysteresis occurs and is explained not by the usual invocation of surface roughness and heterogeneity but by low rates of adsorption equilibration. Restrictions of molecular mobility on dry areas of a solid are significant, particularly for long chain surfactants, and especially for water insoluble lipids. However, even in the latter case the thermodynamic principles continue to apply. They show a self-consistent picture of adsorption phenomena that lies behind Langmuir-Blodgett transfer. This gives an entirely new physical insight into the molecular mechanisms of deposition, hydrophobicity, and instability of lipid monolayers.8 The Problem of Inorganic and Organic Ions. With this as background, we consider now what happens if we progress successively from insoluble lipids through soluble surfactants down to simpler organic electrolytes. These are composed of smaller and less anisotropic/anisometric ions for which molecular mobility can be high, and equilibration times indeed become much shorter. We remark parenthetically that dewetting does not occur for solutions of inorganic electrolytes. Here, as for pure water, contact angles are zero. This is so even for concentrated solutions for which the surface tension is markedly increased. By virtue of the Young equation, this is a factor which in itself would favor an increase in contact angle. This, however, does not occur. The solutions, as for water itself, spread as a film, and no contact angle forms. However, simple organic ions behave differently. Thus, for example a 1 M solution of sodium acetate forms a contact angle of 10° on a mica surface. The wetting perimeter of a droplet forms an ideal circle, with no irregularities along the three-phase line. The slightest tilt causes the monoconvex liquid lens to slide smoothly along the surface under the effect of gravity. Both the ideal geometry and the absence of static friction show that the wetting is free of hysteresis. The equilibrium contact angle decreases to below 1° at concentrations less than 0.1 M. Similar behavior is found for other ions which contain small hydrocarbon groups. Thus the contact angle behavior for tetraethylammonium cations at similar concentrations is close to that of acetate anion. Here also contact angles are small as compared to those observed with solutions of long chain cationic surfactants, and there is no hysteresis. For simple organic salts equilibrium is attained in milliseconds and seconds rather than hours and days as for CTAB or the much longer periods required for LB film equilibration. Droplets of aqueous solutions of simple organic electrolytes on mica are reminiscent of biconvex lenses of a liquid that float freely on the surface of another immiscible liquid. 2. Theory The mechanical equilibrium of a liquid on an ideal surface of a solid satisfies the condition
τ ) γLV cos θ
(1)
Here θ is the contact angle for which the surface tension (γ) at the liquid (L)-vapor (V) interface and the wetting tension (τ) which acts on the three-phase line in the normal
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direction in the plane of the solid (S) are always balanced. At thermodynamic equilibrium (a unique value of the contact angle for a given liquid on a given solid), the wetting tension is the difference of the surface free energies at the solid-vapor and solid-liquid interfaces
τ ) γSV - γSL
(2)
Equations 1 and 2 together constitute the Young equation.10 It combines the conditions of mechanical and thermodynamic equilibrium. Changes of interfacial tension with concentration (C) are related to solute adsorption (Γ) by the Gibbs equation
-dγ )
∑ Γi dµi
(3)
where for the chemical potential (µ)
dµ ) kT d ln a
(4)
where a ) C for ideal solutions and µ ) µi and C ) Ci for ions of symmetric electrolyte. At an uncharged, or at an air-water interface, adsorption of cations and anions proceeds to an extent that is dominated by the need to maintain electroneutrality. On mica, cations alone can be adsorbed. The original counterions are then released. For pure water these could be protons, for example. As long as the chemical potential of water does not change much (up to moderate electrolyte concentrations), and the pH stays constant, the proton contributions need not be counted in the summation over components in eq 3. The same is true for any other counterion that might be due to the occurrence of an ionic impurity in distilled water. For the main component, a symmetric electrolyte, C+ ) C- and a+ ≈ a- up to moderate concentrations. The Debye-Huckel activity coefficients could always be introduced as a correction. Let the cation adsorb at both the mica-water and micaair interfaces by the mechanism of ion exchange. From the application of the Gibbs equation (3) to the thermodynamic definition of the wetting tension (2), the Lucassen-Reynders equation follows:
-dτ/dµ ) ΓSV - ΓSL
(5)
This adds the condition of chemical equilibrium to the Young equation. All the conditions of mechanical, thermodynamic, and chemical equilibrium are here satisfied. Equation 5 shows that a measurement of the wetting tension as a function of concentration is essentially a measurement of the difference of the values of solute adsorption at the solid-vapor and solid-liquid interfaces. The way it appears here applies equally to exchange of an ion and to adsorption of a nonionic solute. Hydrophobic effects of soluble cationic surfactants6,7 and insoluble zwitterionic lipids8 have been shown to conform with, and are explained by, this equation. In the present paper we extend this thermodynamic analysis to study dewetting effects of simple organic electrolytes.
cm3) with the vapor from a cup of water or of the solution under investigation. A droplet was then formed with a microsyringe introduced through a Teflon seal in the top plate. The syringe could be moved up and down in the seal on friction to adjust the height of the circular orifice of a thin needle above the surface. The droplet profile was recorded with the help of a long working distance (about 5 cm) videomicroscope (magnification up to 50×) in transmitted light on a white background. The process was monitored from the instant of placement of the droplet until its shape stopped changing with time. Hysteresis tests were done by tilting the plate for a free droplet or by expanding and contracting the liquid meniscus that connects the surface and the needle of the syringe. The image was digitized, and the angles were read with a commercial software (NIH Image for Macintosh). The accuracy of a single measurement is (1°. Surface tension was measured via the maximum bubble pressure method. Details of the setup have been reported earlier.11 The accuracy of the measurement is (0.05 dyn/cm (mN/m). Surface tension does not depend on the bubble period for solutions studied here. This shows that adsorption equilibrium is maintained for these measurements. Dynamic and equilibrium conditions for contact angle measurements are the essence of our study and explained in detail in the results section. From the contact angle and the surface tension data the wetting tension was calculated by eq 1. In addition to droplet goniometry two other methods of capillarography were used, the Wilhelmy plate and the meniscus height techniques. By measurement of the capillary force acting on a partly immersed plate, the wetting tension was determined. With this setup, an accuracy of (0.1 dyn/cm was obtained, and contact angles larger than 5° can be resolved. This method is very well suited for larger contact angles, but the resolution is poor in the range where contact angles are small. Here it is better to use the other technique. The height of the meniscus on a flat wall was measured with the videomicroscope. In this situation the plate was 3 cm in width and immersed in a Petri dish of about 10 cm in diameter to avoid edge effects. The level of the flat area of the liquid was determined from the reflection of the plate in the liquid before it touches it. The height accuracy, (10 µm, corresponds to an angular resolution of about 1°. The height is related to the contact angle via the surface tension and the density by an exact (for ideally smooth wetting line) analytical expression. In both techniques the partly immersed vertical plate was driven by a microtranslation stage to study static and dynamic hysteresis. Solutions were prepared from deionized and distilled Millipore water and salts of reagent grade. Experiments were also done with solutions equilibrated with charcoal. Adsorption capacities were sufficient to remove any surfactant contaminants out of the salts. In no case had the charcoal treatment any effect on the results except that the surface tension could be changed by an insignificant amount due to the adsorption of the solute itself. Most of the measurements reported were carried out with brown muscovite mica, but green mica gives similar results.
4. Results
In most of the experiments contact angles were measured directly. A mica sheet, after cleavage, was placed in a holder in a custom built optical glass/Teflon/stainless steel cell. The procedure typically takes less than a minute. This time is not critical. A longer exposure to ambient air between mica cleavage and closing the cell does not influence the results. A few minutes more were allowed to saturate the internal volume (about 50
At the instant when a droplet touches the surface, it begins to spread. The spreading occurs under the effect of adhesion forces. The apparent contact angle decreases rapidly. For pure water it starts at a value of between 15 and 30°. This is observed about 10 ms after the droplet comes in contact with the mica and is ripped off the needle (Figure 1). It reduces to values of less than 1-2° half a second later (Figure 2). Then the droplet spreads into an almost flat spot. For 3 M sodium acetate the droplet spreads initially at a similar rate. But in less than a half a second a contact angle of about 12° is reached (Figure 3). Thereafter it does not change with time. At lower concentrations, the final angle is smaller (Figure 4), and below about 0.1 M the wetting behavior of the solution is indistinguishable
(10) Spelt, Li.; Neumann, A. W. In Modern Approaches to Wettability; Schrader and Loeb, Eds.; Plenum Press: New York, 1992.
(11) Yaminsky, V. V.; Ninham, B. W.; Stewart, A. M. Langmuir 1996, 12, 836.
3. Experiment
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Figure 1. Images of a droplet of 1 M solution of sodium acetate before and after it has been detached from the needle of the syringe and adheres to a mica surface. These are captured in one frame. The time interval between the two images is about 10 ms. The droplet on the videorecord appears symmetrical with respect to the horizontal plate due to reflection in the mica sheet. The diameter of the droplet before it has been detached is about 0.5 mm.
Figure 3. Contact angle vs time of spreading for 3 M solution of sodium acetate. Data on linear (a) and logarithmic (b) time scales.
Figure 4. Equilibrium contact angle vs concentration of sodium acetate. Figure 2. Apparent contact angle of water vs time of spreading measured from the moment when the droplet touches the mica surface. The two symbols show the values measured at the right side and at the left side of the image. Data on linear (a) and logarithmic (b) time scales.
from that of pure water. Many other acetates and acetic acid show a similar effect.
For 0.5 M tetraethylammonium chloride the equilibrium contact angle is 7°. A similar contact angle was observed for sodium acetate at similar concentrations. However, unlike sodium acetate, for tetraethylammonium chloride the contact angle decreases again at concentrations of 1 M and higher. For tetraethylammonium chloride the contact angle reduces to values below 1-2° at more than ten times lower concentrations (i.e., 6 × 10-3 M) than for sodium acetate (cf. Figures 5 and 4).
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Figure 5. Equilibrium contact angle vs concentration of tetraethylammonium chloride.
Figure 6. Equilibrium contact angle vs concentration of tetrabutylammonium bromide.
The contact angle isotherm for tetrabutylammonium bromide also has a maximum (Figure 6). The maximum value of the equilibrium contact angle is about 30°, three or four times higher than for tetraethylammonium chloride. The maximum is reached at concentrations a hundred times lower, between 10-3 and 10-2 M (compare Figures 5 and 6). At a concentration of 1 M the contact angle is about 5°, somewhat smaller than for sodium acetate, and close to the value for tetraethylammonium chloride at similar concentrations. For 1 M tetrabutylammonium bromide, the contact angle decreases monotonically with time after placing the droplet, and the equilibrium plateau value is reached within half a second (Figure 7). This is typical of the behavior of the other two salts at such high concentrations. At concentrations an order of magnitude lower, the contact angle for tetraethylammonium chloride becomes smaller. But for tetrabutylammonium bromide the contact angle increases and interesting observations can be made. At such small concentrations the kinetic pattern becomes qualitatively different. The droplet spreads initially to give a contact angle of approximately 8°. It then shrinks rapidly again until it attains a higher contact angle of around 18° (Figure 8). At lower concentrations down to 3 × 10-3, the equilibrium contact angle continues to increase, while the transient minimum angle remains constant at the same value of about 8°. The whole effect is then more pronounced (Figure 9) than before.
Figure 7. Contact angle vs time of spreading for 1 M solution of tetrabutylammonium bromide. Data on linear (a) and logarithmic (b) time scales.
Figure 8. Contact angle vs time of spreading for 0.171 M solution of tetrabutylammonium bromide.
Below 10-3 M (Figures 10 and 11) the minimum value of the contact angle to which the droplet spreads in half a second, constant at a level of 8° at a higher concentrations, becomes smaller. It decreases below 1-2° at concentrations less than 10-4 M. In all these cases, after the minimum transient contact angle is reached, the liquid begins to recede back again. The contact angle increases. Equilibration times over
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Figure 9. Contact angle vs time of spreading for 2.87 × 10-3 M solution of tetrabutylammonium bromide.
Figure 10. Contact angle vs time of spreading for 3.76 × 10-4 M solution of tetrabutylammonium bromide.
Figure 11. Contact angle vs time of spreading for 8.66 × 10-5 M solution of tetrabutylammonium bromide.
which the droplet collects increase by orders of magnitude when the concentration decreases by orders of magnitude (Figure 12); an apparent plateau occurs in a range between 10-2 and 10-3 M. Accidentally rather than coincidentally, as we shall see later, it is in this range that the values of the equilibrium contact angle go through a maximum. We remark that this is the same range of concentrations in which the minimum transient angle remains constant.
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Figure 12. Time of attainment of equilibrium contact angle vs concentration of tetrabutylammonium bromide.
At 8.7 × 10-5 M the droplet spreads in a second into an irregularly shaped spot. For a period of time there are no noticeable changes in its appearance. One is left with an impression that the process is over and nothing else is going to happen. This is not so. After a minute or so, the spot begins to bulge at the edges and to collect back into a droplet! The process continues for 10 min (Figure 11). Here a quite high equilibrium value of 14° is reached. At this stage the droplet becomes perfectly circular again. At low concentrations where the attainment of equilibrium is slow, dynamic contact angle hysteresis occurs. We observed that when the liquid is sucked rapidly out of the droplet with the syringe, the contact angle decreases to allow for the decrease of the volume. However, the contact diameter of the annulus then decreases spontaneously at about the same rate as that at which the droplet collects after the initial spreading. Through this process the equilibrium angle is re-established. If more liquid is pushed into the droplet from the syringe the three-phase line responds instantaneously and slides outward. The increase of the volume occurs at a constant value of the contact angle, which, for these (advancing) conditions, coincides with the equilibrium angle. However, if the advancing wetting perimeter extends beyond the area reached during the initial expansion which occurred when the droplet was first placed, the liquid suddenly spreads onto areas which have not been wetted previously. A similar process of subsequent increase of the contact angle that occurred on the initial placement is then repeated. The characteristic reoscillation recurs. The contact angle rapidly goes down, and then goes back up again, until the equilibrium angle is re-established. We remark again that for solutions of tetraethylammonium chloride, dewetting effects arise at much higher concentrations than for tetrabutylammonium bromide. At such concentrations diffusion is fast and equilibration times are short. Apparently, the attainment of equilibrium as observed in these systems at lower concentrations is slower because it becomes diffusion controlled. Also for tetraethylammonium chloride at smaller concentrations there is a tendency for the reoscillation in which the droplet first expands to a smaller angle and then contracts to conform with a larger equilibrium contact angle. As for the case with tetrabutylammonium bromide this effect arises at decimolar and lower concentrations. However, for tetraethylammonium chloride, at such concentrations the equilibrium angle itself is quite small. This circumstance makes the kinetic effect not as pronounced as for tetrabutylammonium bromide for which
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the equilibrium contact angle continues to increase with decreasing concentration in this range. For sodium acetate the dewetting occurs only at high concentrations of the order of 1 M. At these high concentrations, equilibration is always fast and no reoscillation to a smaller angle occurs. In the whole range of concentrations contact angle decreases monotonically with time, reaching the equilibrium value within half a second. It follows from the above results that the rates of initial spreading for all salts and at all concentrations studied are not very different. They are similar to those observed for pure water. The process, driven by the adhesion between mica and water, is apparently controlled by viscosity and/or inertia. The subsequent reoscillation, when it occurs, is in no way due to the inertia of the droplet as a whole. The origin of the effect is quite different. It also is not related to the equilibrium value of the contact angle itself. One can make a comparison for one and the same value of this angle at a higher and at a lower concentration on either side of the maximum, or for two different salts at two concentrations for which contact angles are equal. Whatever the value of this angle is, the reoscillation occurs only at concentrations about and below 0.1 M. At higher concentrations the contact angle decreases monotonically with time to its equilibrium value. After this value is reached, it stops changing. No reoscillation occurs when concentrations are in the molar range. The initial rate of spreading over fresh surface areas which have not been in contact with the solution previously is fast. It does not depend on the type and concentration of the solute. It typically takes a fraction of a second for a solution to reach the minimum angle, whether transient or equilibrium. This rate is not different from that for spreading of pure water. Before discussing this situation further, we point to some other experimental observations which may be relevant. We have observed that for sodium acetate solutions, the contact angle increases monotonically with concentration up to a highest concentration limited only by the solubility of the salt. Acetic acid shows a similar wetting behavior at similar concentrations. However, acetic acid is completely miscible with water, and the range of concentrations can be easily extended up to 98%. The contact angle here goes through a maximum at molar concentrations and falls to zero again on the transition to pure acetic acid. A similar maximum at several times smaller concentrations occurs for tetraethylammonium chloride and for tetrabutylammonium bromide at concentrations further hundred times lower. The contact angle measurements to which we refer here were performed in a sealed cell saturated with vapor of the solution. In some cases similar results were obtained when measurements were done in ambient atmosphere of air which was not presaturated with water. In some other cases, however, results in saturated vapor and in drier air were quite different. For example, for 1 M tetrabutylammonium bromide the equilibrium angle is about 5° at saturation. It increases to 10° if the same measurement is repeated in air. In both cases the droplet spreads to a stationary value in half a second, but these are two different values. Similarly the angle rises if the seal is broken and dry air is admitted into the cell. By contrast, for tetrabutylammonium bromide at concentrations below 0.2 M, essentially the same results were obtained in air and in vapor. An increase of the contact angle on demoisturizing also occurs for sodium acetate. Note that for this salt dewetting occurs only at high concentrations. Only at such concentrations is a marked sensitivity to humidity observed.
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For an independent control of the results we repeated several measurements with the Wilhelmy balance and the meniscus height. The values of contact angles obtained by these two alternative techniques were the same as for the droplet goniometry. 5. Discussion It can be readily noticed that there is no straightforward correlation between the surface tension of the solutions and their wetting behavior. Sodium acetate has negative surface activity. The surface tension here increases above that of water with increasing salt concentration. As for most inorganic salts, and also for sodium acetate, this increase is roughly linear in concentration and the value of dγ/dC is of the order of 1 mN/m per 1 mol. For tetraethylammonium chloride, surface activity is positive, but it is equally small in absolute magnitude. Some inorganic electrolytes, many mineral acids in particular, also have small positive values of dγ/dC. However salts or acids or bases, whether with negative surface activity or positive, including those with ions of organic origin which however do not contain hydrocarbon groups, carboxyl or ammonium, for example, do not induce dewetting. Compared to other salts with lower surface activity, tetrabutylammonium bromide reduces the surface tension of water quite significantly. For concentrated solutions of this salt the surface tension is almost as low as for micellar surfactant solutions. The main difference is that critical micelle concentration (cmc) values of long chain surfactants are typically orders of magnitude lower concentrations as compared to those which we consider here. For acetic acid the surface activity is as high as for tetrabutylammonium bromide, but the dewetting effect is much smaller. Similar values of surface tensions at similar concentrations can be obtained for nonionic solutes, while dewetting here does not occur. For example, aqueous solutions of ethyl alcohol form zero contact angles with mica over the whole range of concentrations from pure water to pure ethanol. However this is true only if equilibrium conditions are carefully maintained. Under ordinary nonequilibrium conditions in ambient air, water-ethanol mixtures are known to form droplets on the hydrophilic walls of a glass container. The effectsknown as tears in winesis a famous example of a wide group of nonequilibrium phenomena which arise from surface tension gradients. They are linked under the common name of Marangoni effects.12 The tears can be readily seen on a glass with a strong wine and with a mica sheet brought into contact with a solution of ethanol in water. If the same experiment is done in saturated vapor the solution forms an ordinary wetting meniscus on the sheet. However, when the seal is broken, ethanol, which is more volatile and has a lower surface tension, evaporates from the wetting film. The surface tension of the film then becomes larger. The subsequent effect is fascinating at a microscopic level: The film rapidly climbs high up the vertical wall, bulges, splits into streams in a firework fashion. The rising droplets then slow down and slide back under the effect of the gravity. When such a droplet reaches the meniscus it begins to coalesce. Because the more concentrated parent solution which fills the container has a lower surface tension, there is a surface tension gradient along the coalescing neck which makes it unstable. The neck breaks almost at the instant of its formation. The droplet, instead of being sucked into the liquid, makes a swallowing (12) Scriven, L. E.; Sterling, C. V. Nature 1960, 187, 186.
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motion and is released again. These characteristic drinking impulses continue at diminishing intensity and frequency. Each time when the droplet takes in some more of the liquor, concentrations tend more to equalize. Finally the droplet loses sharp boundaries and dissolves. With a droplet placed on a horizontal surface, the same phenomenon results in characteristic ejection of satellite droplets and is equally amusing. We do not observe such distinctly pronounced effects for the aqueous solutions of organic electrolytes studied here. In our case solutes show a different sign and magnitude of surface activity and are generally involatile, although in the case of sodium acetate the presence of acetic acid due to hydrolysis can easily be recognized by the smell of the solution. The involvement of Marangoni phenomena is a factor which contributes to an increase of contact angles for concentrated solutions on demoisturizing. Otherwise, dewetting effects of organic salts which are observed in saturated vapor are of thermodynamic, rather than of nonequilibrium, origin. In many respects they in fact resemble hydrophobic effects of typical surfactants. Contact angles for simple organic electrolytes are several times smaller and equilibration times are smaller by orders of magnitude as compared to long chain surfactant cations. Dewetting transitions occur at much higher concentrations. But the basic principles are the same. Both the presence of nonpolar (e.g., hydrocarbon or fluorocarbon) groups and the ionic nature of a solute prove here to be important for the occurrence of the effect. As we have already noticed nonionic organic solutes like ethanol do not show this effect. Solutions of water in ethanol, or of ethanol in water, wet mica under equilibrium conditions when the system is maintained at saturation. The equilibrium dewetting is a property of organic electrolytes. More precisely, it is a property of those organic electrolytes that contain a small hydrophobic residue. Whether these hydrophobic groups are included in the cation or in the anion appears to be a matter of a secondary importance in the wetting behavior for such salts. Dewetting occurs both with acetates and quaternary ammonium salts. Nevertheless, for cations the effects are generally stronger. This trend becomes further obvious by consideration of longer chain surfactants. Cationic soaps like CTAB are strong dewetters of mica, while ordinary soaps or anionic SDS13 never form large contact angles on the hydrophilic substrate which adopts negative charge in water. The presence of a single methyl group as in the acetate ion is sufficient to induce a dewetting. We have observed that carbonates, formates, and many other more sophisticated organic ions (such as citrate etc.) which do not contain hydrocarbon elements in their structure do not show the effect. This is also true for inorganic cations and anions independent of their charge, valence, and other chemical properties. Among simple organic salts that we studied here tetrabutylammonium bromide shows the strongest effect and is covered by the most extensive data. The effect arises at small concentrations when corrections for nonideality in the thermodynamic equations are small and straightforward. The contact angle for tetrabutylammonium bromide goes through a pronounced maximum with concentration. The same is true for CTAB, although the resemblance, as we shall see, is superficial. Indeed, for CTAB, apart from the fact that the angles are larger and concentrations at which the contact angle (13) Birch, W. R.; Knewtson, M. A.; Garoff, S.; Suter, R. M.; Satija, S. Langmuir 1995, 11, 48.
Yaminsky et al.
Figure 13. Difference of surface pressures at mica-air and mica-water interfaces (squares) and the surface pressure at water-air interface (diamonds) vs concentration of tetrabutylammonium bromide. The straight line fitted to the pressure difference data corresponds to a constant difference of 167 Å2 per tetrabutylammonium ion at the mica-air and mica-water interfaces.
effect begins to show up are much smaller than for the tetrabutylammonium salt, both the contact angle and the wetting tension have an extremum. For CTAB the contact angle goes over a maximum at concentrations above the point of zero charge and decreases again below the cmc. Here a monolayer adsorbs at the water-air interface and the surface tension decreases. The wetting tension goes through a minimum when in the same range of concentrations a hydrophilic outer layer builds up at the micawater interface. This results in the characteristic bilayer structure. For tetrabutylammonium bromide the wetting tension changes monotonically with concentration (Figure 13). The maximum in the contact angle occurs simply because the surface tension at the water-air interface at high concentrations decreases more rapidly then does the wetting tension. The wetting tension for tetrabutylammonium bromide changes almost linearly with logarithm of concentration over the entire concentration range. According to the basic thermodynamic relation, eq 5, this corresponds to a constant value of the adsorption difference for the coexisting solid-vapor and solid-liquid interfaces. We know a priori, and as a matter of general validity, that the tetrabutylammonium ion can in principle be adsorbed at both of these interfaces by a mechanism of ion exchange. From surface tension measurements we know also that this salt has a quite significant surface activity at the water-air interface. The latter circumstance is apparently due to the large size of the tetrabutylammonium cation which consequently is hydrophobic. The hydrophobic (cavity) self-energy increases as the square of the radius while the hydrophilic (electrostatic charge interaction) self-energy diminishes accordingly. The dispersion self-energy still can be large, but this is not sufficient to counteract the tendency for the hydrophobic ion to be expelled out of water. At the water-air interface the counterion inevitably has to be coadsorbed to maintain electroneutrality. The interface charges up due to this adsorption and a double layer forms. The factors which work in favor of preferential adsorption at the solid-vapor interface are in their essence similar to those established earlier for typical surfactants like CTAB. While the hydrophobic energy gain for the
Dewetting of Mica
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adsorption of an organic cation at the solid-air interface can be as large as for the water-air interface, the hydrophilic counterion need not be coadsorbed in the former case. The cation alone is adsorbed on mica by the charge neutralizing ion exchange mechanism. This adsorption occurs by maintaining electroneutrality rather than charging the interface up. The free energy of ion exchange can be positive or negative depending on the position of the exchanging ions in the lyotropic series. The electrostatic interaction is more favorable for the adsorption process in this case than for the water-air interface when counterion coadsorption always makes a repulsive contribution to the free energy. While the change in the electrostatic energy associated with ion exchange may alone prove insufficient to maintain a substantial adsorption at the solid-liquid interface, the energy gain at the solid-vapor interface resulting from this sort of ion binding is supplemented by the hydrophobic energy gain. This is equivalent to that which occurs when the ion is transferred from the bulk of the solution to its interface with air. However, for ion exchange adsorption at the initially charged solid-liquid interface the hydrophobic free energy is not diminished by the opposing counterion contribution. The latter occurs only at the water-air interface. It is this effect associated with charging of the interface and formation of the double layer that makes the free energy of adsorption at the waterair interface smaller. Since the free energy of adsorption occurs in a Boltzmann exponent, the magnitude of adsorption at the solid-vapor interface can exceed the sum of the values of adsorption at the other two interfaces. Because for the mechanism of ion exchange the counterion does not coadsorb in the process, it does not contribute to the surface pressure. The corresponding term is absent from the Gibbs equation. We can assume that the counterion adsorption is zero. Let the exchanging ion be a cation (+), for illustration. We can write
dµ+ ) kT d ln C In this case then we have
-dτ/(kT d ln C) ) ΓSV - ΓSL
(6)
where Γ ) Γ+ refers to the excess adsorption of the exchanging ion, here say tetrabutylammonium cation. The slope of the straight line in Figure 13 corresponds to an adsorption difference of 6 × 10-13 molecules/cm2, or in other words to an area of 160-170 Å2 per molecule (ion). Molecular area in an adsorbed monolayer is generally restricted by the hard core size of the adsorbate. The tetrabutylammonium cation is several times larger than most common ions. The large experimentally determined value for the molecular area is consistent with estimates based on the geometrical cross sectional area for a tetrabutylammonium cation. The effective cross section can be larger than the van der Waals cross section due to the ionic repulsion. What we see experimentally is that the state of an incompressible monolayer is reached. The adsorption difference is maintained constant over the entire range of concentrations studied. The monolayer that consists of close packed ions is formed at concentrations as low as 10-4 M and cannot be compressed much further. Indeed, going from 10-4 to 1 M in concentration corresponds to a chemical potential effect that represents an almost 10-fold (by ln(104)) increase of the lateral pressure applied to a condensed monolayersand the measured adsorption difference does not change over this range within experimental accuracy.
The limiting molecular area could in principle be larger (adsorption density smaller) than what follows from such packing considerations if matching/electroneutrality conditions were further involved. This factor is of minor importance for the present system. Indeed, the area per each chargeable adsorption site, 48 Å2 per aluminate ion of the basal plane of mica, is three or four times smaller than the limiting monolayer value dictated by the size of tetrabutylammonium. The density of the adsorption sites may well be the limiting factor for adsorption of much smaller ions such as protons. To obtain the absolute value of adsorption at the solidvapor interface, the value of the adsorption at the solidliquid interface has to be subtracted from the measured adsorption difference. For the present system, however, the value of the adsorption at the solid-liquid interface apparently can be neglected. It is small as compared with the much larger adsorption at the solid-vapor interface. If this adsorption on mica under water were to be significant, then to account for the experimental value of the adsorption difference we would be compelled to assume that the ion at the solid-vapor interface occupies an area smaller than its own size. This might be so in the case of a BET isotherm for multilayer adsorption of the vapor of a liquid near the saturation pressure. But a bilayer of similarly charged spheres seems to be an inappropriate and absurd notion. It follows from consideration of the data that while the adsorption at the solid-liquid interface remains small up to molar concentrations, the solid-vapor interface is filled with the large hydrophobic cations. The ions in the form of a condensed monolayer are here present already beginning at concentrations of the coexisting solution as low as 10-4 M. This is not really so surprising if one recalls that a monolayer of CTA+ ions at the mica-air interface is formed at solution concentrations which are lower still by several orders of magnitude. While the overall hydrocarbon content for the two alkylammonium ions is almost the same, the diameter of the trimethylammonium head group of CTAB is much smaller than of tetrabutylammonium ion. The limiting area per CTA+ ion is accordingly several times smaller. The surfactant ion can be packed at a high molecular surface density to conform with the area per charge site of mica surface. The measured adsorption for a condensed CTAB monolayer at the mica-air interface is indeed several times higher than for tetrabutylammonium bromide. The adsorption result which follows from the corresponding contact angle studies, is in accordance with molecular size considerations for the two alkylammonium ions. The anionic adsorption sites of mica and the cationic head groups and alkyl chain cross sections for CTAB all match up favorably. The free energy of adsorption is particularly high within such a surfactant monolayer. This follows from the small concentrations at which this monolayer forms and is explainable given structural considerations of packing and interactions. Because of the smaller trimethylammonium head group size, its attractive Coulombic interaction with the matching solid phase anions is stronger. The effective area over which the hydrocarbon chains of the aligned cetyl radicals are in contact with each other is also much larger. Only the terminal methyl group has to be left outside exposed to the environment. The quasispherical symmetric quaternary ammonium cation, while of a similar gross composition, matches all these conditions less favorably. These factors taken together explain why the free energy of adsorption is much larger for CTAB than for the symmetric tetrabutylammonium bromide. Limiting ad-
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sorption densities are larger and the monolayer condensation occurs at lower concentrations. This leads to higher surface pressures and corresponingly to higher equilibrium contact angles. Larger free energies of adsorption and higher packing densities all relate to much lower molecular mobilities for the surfactant monolayer. This explains larger hysteresis and longer equilibration times. Unlike the long chain surfactant the more weakly adsorbed tetrabutylammonium ions are mobile. The spherical ions with a repulsive lateral interaction roll freely on the nonmatching substrate and desorb easily. Only at low concentrations is there noticeable dynamic hysteresis. It is explained by diffusion controlled adsorption. Relaxation times that occur here are orders of magnitude smaller than for CTAB. In the following discussion we emphasize equilibrium aspects of the interaction and then consider kinetic aspects of contact angle and adsorption equilibration. A change in equilibrium wetting tension equals the difference of the surface pressures (γ0 - γ) at the solidvapor and solid-liquid interfaces; viz.
τ0 - τ ) (γSV0 - γSV) - (γSL0 - γSL)
(7)
In our case τ0 equals the surface tension of pure water (γLV0) for which contact angle is (close to) zero. For CTAB solutions the minimum for the wetting tension occurs when the surface pressure at the solid-liquid interface begins to increase rapidly with concentration above the point of zero charge (pzc). It here increases faster with concentration than the surface pressure of the monolayer adsorbed at the solid-vapor interface. This shows that in this concentration range ΓSL becomes larger than ΓSV. This result can be interpreted as bilayer adsorption that occurs prior to the cmc. For symmetric quaternary ammonium ions, bilayers cannot be formed. What we observe is that at all concentrations adsorption of tetrabutylammonium bromide at the solid-liquid interface is insignificant. The hydrophobic energy gain that results from lateral contacts of hydrocarbon chains creates the driving force for condensation of CTA+ monolayer at this interface, but this gain is much smaller in the tetrabutylammonium case. The electrostatic contribution alone can rather be unfavorable, because one has to exchange a small proton with a much larger tetrabutylammonium ion. By neglecting ΓSL the second bracket in eq 7 equals zero. The change of the wetting tension equals the surface pressure at the solid-vapor interface up to the highest concentrations. Surface pressure here as for any interface increases with concentration monotonically. No minimum in the wetting tension occurs under such conditions. According to the Gibbs equation, surface pressure for a condensed monolayer when adsorption remains constant changes with concentration logarithmically. For the wetting tension effect of tetrabutylammonium bromide this logarithmic dependence holds from 10-4 to 1 M. Over most of the same concentration range the surface pressure at the liquid-air interface is nonlinear on a logarithmic concentration scale. At concentrations below 10-2 M this surface pressure remains small. At higher concentrations it rises steeply, with a limiting slope which is almost twice as large as that for the wetting tension isotherm. For the liquid-air interface where each cation adsorbs with its counterion, the Gibbs equation takes the form
-dγ ) Γ+ dµ+ + Γ- dµBy taking into account the circumstance that Γ+ ) Γ- ) Γ (to satisfy electroneutrality, for a symmetric electrolyte)
and that dµ+ ) dµ- ) kT d ln C (assuming ideality), we arrive at
-dτ/(2kT d ln C) ) ΓSV - ΓSL
(8)
From comparison of eqs 6 and 8, which differ by a factor of 2 in their left hand sides, it follows that the twice larger limiting slope for the surface tension isotherm compared to the wetting tension isotherm corresponds to one and the same limiting value for the corresponding values of the adsorption (ΓLV) and the adsorption difference (ΓSV ΓSL). At concentrations below 10-2 M adsorption at the airwater interface remains small. Adsorption at the solidliquid interface is also small. The only adsorption which is large starting from low concentrations is that at the mica-air interface. However, while adsorption at the mica-water interface remains low over the entire concentration range, for the water-air interface adsorption builds up in a range of higher concentrations. At concentrations above 10-2 M the values of adsorption at the mica-air and water-air interfaces become equal. In spite of the fact that the mechanisms of adsorption at the two interfaces are different, being due to ion pair adsorption at the water-air interface and to ion exchange adsorption at the mica-air interface, the limiting factor which is the size of the ion is the same in both cases. At the water-air interface this limiting adsorption for tetrabutylammonium bromide is reached above a concentration, C2, of about 10-2 M. This is 2 orders of magnitude higher then the corresponding extrapolated concentration C1 of about 10-4 M for the mica-air interface. The last result shows that the free energy of adsorption within the condensed monolayer is smaller for the waterair interface than for the mica-air interface, by approximately kT ln C2/C1. This corresponds to about 5 units of kT. Adsorption at the air-water interface is less favorable. This is because at this interface, in order to maintain electroneutrality, counterions should be coadsorbed. In this process the counterions are transferred from larger mutual separations in the bulk to smaller distances within the adsorbed double layer. In the case of the solid-vapor interface the charge of the adsorbing organic cations is compensated by the aluminate ions present in the mica lattice. No repulsive electrostatic/ osmotic double layer free energy associated with concentrating counterions near the interface is involved here. If we turn now to the reasons for differences in behavior of the two different quaternary ammonium ions, we observe that the much smaller tetraethylammonium cation is less hydrophobic than the tetrabutylammonium cation. Because of the smaller hydrophobic contribution to the free energy of adsorption, the surface activity at the water-air interface is much smaller than for the tetrabutylammonium ion. Dewetting effects for tetraethylammonium chloride are qualitatively similar to those observed with tetrabutylammonium. But they are smaller and occur at higher concentrations. With tetraethylammonium ions, a full scale monolayer is not developed at the mica-air interface up to concentrations of 1 M (Figure 14). For such a 1 M solution the area per tetraethylammonium ion at the mica-air interface is 100-110 Å2. This is smaller than for the saturated monolayer of tetrabutylammonium ions. The result is consistent with the smaller size of the tetraethylammonium cation. As for tetrabutylammonium bromide, so for tetraethylammonium chloride the steepness of the surface tension isotherm is almost twice that of the wetting tension isotherm in the
Dewetting of Mica
Figure 14. Difference of surface pressures at mica-air and mica-water interfaces (squares) and the surface pressure at water-air interface (diamonds) vs concentration of tetraethylammonium chloride.
limit of high concentrations. The limiting values of adsorption are similar for the mica-vapor and water-air interfaces also in the tetraethylammonium chloride case. For a long chain cation like CTA+ the hydrophobic free energy gain within the monolayer, similar for water-air and mica-air interfaces, is much larger than for the quasispherical tetrabutylammonium cation. A smaller size of the trimethylammonium head group, which is electrostatically more strongly attracted to mica, is another contributing factor that favors the adsorption. Taken together they lead to a very large free energy of adsorption for CTAB onto mica in air. A condensed monolayer of this surfactant at the mica-air interface forms at concentrations almost as low as 10-7 M. For the water-air interface the adsorption energy is smaller due to electrostatic effects associated with formation of the double layer. This contribution is even more unfavorable for CTAB than for tetrabutylammonium bromide because the limiting adsorption/surface charge for CTAB is higher. Condensation of a CTAB monolayer at the air-water interface is completed at concentrations above 10-4 M, prior to the cmc which is at about 10-3 M. The value of the difference of free energies of adsorption at these two interfaces, kT ln C2/C1, which is of the same order of 5 kT, is still somewhat higher for CTAB than for tetrabutylammonium bromide. Similarly, one can compare free energies of adsorption for the two salts. These are about 5 kT units higher for CTAB than for tetrabutylammonium bromide, at either of the two interfaces. Ion exchange energies involved in adsorption at the mica-air interface are more favorable for CTAB. Double layer energies that contribute to adsorption at the water-air interface are more in favor of tetrabutylammonium bromide. This electrostatic/ osmotic contribution is higher for CTAB because of a higher packing density and accordingly a higher surface charge at the water-air interface that here occur. But this makes only a second-order consideration for the observed differences. The main reason is the difference in the hydrophobic energy. This is 5 kT higher for CTAB than for tetrabutylammonium bromide. This result is among many other that follow from the analysis of our experimental data. The fact that the dewetting effect occurs also for acetate salts is of further interest. An anion cannot be adsorbed onto (anionic) mica by an ion exchange process. For acetates the adsorption at the mica-air interface must
Langmuir, Vol. 13, No. 22, 1997 5989
occur as ion pairs where the cation can be further exchanged for a proton. This would mean that the adsorbed entity is essentially neutral acetic acid. Implications of this mechanism are that one might probably need to include more detailed considerations of counterion specificity, due, e.g., to dispersion forces acting on ions. High molecular mobility for small and/or symmetric organic ions results in fast adsorption re-equilibration at the three-phase line. At high concentrations when the rates of diffusion are high, a hysteresis-free behavior is observed. At lower concentrations the attainment of adsorption equilibrium becomes slower, and this influences contact angle equilibration. This is clearly shown by our kinetic observations. At such concentrations the contact angle goes through a minimum with time. The initial spreading during placement of the droplet occurs over a bare mica surface. Suppose that the rate of supply of the adsorbing organic ions is slower than the rate at which the droplets spread under the capillary effect of the wetting tension of water on mica. Then the equilibrium adsorption behind the three-phase line cannot be maintained. A solution droplet spreads at the same rate as a droplet of pure water. By doing so it can pass the equilibrium position and expand to a larger diameter than that which corresponds to the equilibrium contact angle at this concentration. As the contact angle decreases the restoring force of the surface tension increases and the spreading slows down. A complete theory of this fast spreading follows by consideration for effects of inertia and viscosity the way they arise during the motion of the liquid. As the rate of the spreading decreases, the solute diffuses and adsorbs behind the three-phase line. The droplet shrinks again being pushed off the solid by adsorbing ions. The contact angle increases to the equilibrium value. The reoscillation does not occur at high concentrations when the rate of diffusion is fast on the time scale of this fast spreading. It also does not occur if a droplet of the solution is placed on preadsorbed area of the mica sheet over which ions have already been exchanged by previous contact with the solution. If after the reoscillation the droplet is forced to expand, the three-phase line slides over the pre-adsorbed area over which the surface potassium cations or protons have been exchanged for tetraalkylammonium cations. This is the area from which the solution spontaneously receded after the initial spreading. The dynamic contact angle stays at the equilibrium static value. This shows in particular that as water advances desorption from the wetted areas occurs almost instantaneously. Desorption is a kinetic reaction of zero order, like that of dissolution or evaporation, for which the rate does not depend on the bulk concentration. The result shows that at all concentrations the process of desorption is fast enough to maintain the equilibrium adsorption difference across the three-phase line. The conclusion that the organic cation is desorbed from areas of mica surfaces which are under water is further supported by the fact that if the solution is sucked out, the three-phase line does not respond synchronously as it does for advancing conditions. Initially it does not retreat. In order to allow for a smaller volume of the meniscus, the contact angle decreases below the equilibrium value. Then spontaneous retreat of the liquid front occurs. It continues until the equilibrium contact angle if re-established. The rate of this contact angle relaxation is determined by the diffusion and re-adsorption kinetics. This rate depends on concentration and indeed is the same as for the retreat observed in reoscillation that occurs after the initial spreading.
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Consider the three-phase line advancing with a constant equilibrium angle over a pre-adsorbed area of mica surface at which exchange for the organic ion has earlier taken place. When it reaches the boundary of this area, the liquid suddenly spreads again onto fresh areas of the mica surface. Then, following the kinetics of adsorption, it recedes back. A reoscillation is repeated. It is during this retreat that ion exchange always occurs. The result shows, in particular, that even though reequilibration for tetrabutylammonium ions is much faster than for CTA+ the adsorbed monolayer does not diffuse spontaneously far beyond the original area over which it has been deposited by the retreat of the wetting line. In the case of CTAB a characteristic stick-slip behavior is observed when a freshly cleaved sheet advances into the solution. Sharp peaks in the force occur due to sudden jumps of the meniscus. The liquid is stopped at the boundary of the hydrophobic layer that spontaneously forms in front of it and cannot be desorbed as easily as for organic ions studied here. Under such conditions, when the liquid is forced to advance but cannot move, the contact angle increases above the equilibrium value. To counterbalance the changed surface tension component, the monolayer adsorbed at the mica-air interface is effectively compressed to a high surface pressure which exceeds the equilibrium value. After a critical stress is reached, which makes the supercompressed monolayer collapse or water to climb on top of it, the liquid begins to move. The threephase line jumps over the hydrophobic band that there forms and creates a force barrier and spreads onto the hitherto clean areas of the surface. The kinetics of this spontaneous receding by which monolayer deposition occurs is similar for long chain surfactants and simpler solutes, even for monolayers of insoluble lipids. There is no activation needed for readsorption. Deposition of a lipid, a surfactant, or a simple ion occurs spontaneously at a rate controlled by diffusion of the amphiphilic component to the three-phase line. This rate at which the relaxation of the receding contact angle occurs ranges from many hours to milliseconds depending on the bulk concentration and related to the area per molecule at the water-air interface for insoluble lipids. By slow movement of a mica sheet across the surface of a solution of tetrabutylammonium bromide the wetting tension does not change. At higher speeds a hysteresis becomes noticeable. This occurs when the rates of the adsorption re-equilibration become comparable to the rate of protrusion of the plate. The reoscillation mechanism the way we observed it by placing a droplet of the solution on a freshly cleaved mica surface takes here the form of a slight periodic variation of the wetting tension when the plate moves at a constant speed down into the liquid. This damped oscillatory mode of displacement of the advancing wetting line occurs only at small concentrations and sufficiently high speeds. For all systems studied the kinetics of the attainment of the contact angle equilibrium reflects the kinetics of attainment of adsorption equilibrium around the threephase contact line. The time scales are greatly different depending on concentration and type of the solute, but otherwise hysteresis effects of simpler organic electrolytes
Yaminsky et al.
resemble those of cationic surfactants6,7 and zwitterions of insoluble lipid amphiphiles.8 Dewetting transitions induced by a variety of ionic organic compounds on chargeable hydrophilic substrates have been unnoticed or relegated to the status of idle curiosities. This is in spite of the fact that the notion of wetting transitions has been central to many problems for years. Phenomena that involve soils and bone tissues, amino acids, and polyelectrolytes are common materials that depend intimately on this type of interaction. Dynamic molecular self-assembly occurs here through effects imposed by the existence of the three-phase line. It acts as a molecular pump, thermodynamically driven by the transfer itself. In turn, the arising reaction makes the line move in the opposite direction. This completes the transfer. Deposition occurs spontaneously. The role of the barrier on Langmuir trough that raises the chemical potential to facilitate the transfer for solutes is played by concentration. This universal phenomenon occurs almost everywhere where water, air, organic matter, and minerals come together. There are numerous applications and implications in nature and technologies. In Conclusion For more than a century the Gibbs equation has constituted the backbone of adsorption studies for surfaces of surfactant solutions. For liquid-fluid interfaces the surface tension is a readily measurable quantity. Given a surface tension isotherm, the corresponding adsorption isotherm can be reconstructed. This paper complements a recent study of equilibrium contact angles of CTAB solution on silica6 and mica.7 It shows how the thermodynamic principle applies to three-phase solid-liquidfluid systems. The primary experimental quantity which substitutes for the surface tension of classical capillarity is the equilibrium wetting tension. This is an experimentally accessible parameter. It equals the difference of surface tensions at the solid-vapor and solid-liquid interfaces. As for surface tension isotherms, there is a corresponding adsorption difference isotherm behind each wetting tension isotherm. The variety of solids extends the problem far beyond its traditional preoccupation with water-oil interfaces. While equilibrium contact angles are thermodynamically related to the equilibrium values of adsorption at the interfaces which join together at the three-phase line, contact angle hysteresis is associated with the kinetics of this adsorption. Kinetic effects are diverse when compared to the limited scope of dynamic surface tension behavior of liquids and are easy to study. We have shown here that adsorption equilibration for simple organic ions that induce dewetting transitions on mica is almost as fast as for liquid-fluid interfaces. By this effect the three-phase line does not experience a frictional resistance. Droplets of aqueous solutions of organic salts slide smoothly over a mica surface. These are like lenses of oil floating on the surface of water. Acknowledgment. We are grateful to Anthony Hyde for making the sealed optical cell used in these experiments. LA960713O
