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Dewetting Modes of Surfactant Solution as a Function of the Spreading Coefficient J. T. Woodward and D. K. Schwartz* Department of Chemistry, Tulane University, New Orleans, Louisiana 70118 Received July 17, 1997. In Final Form: October 28, 1997X The surface coverage of an adsorbed monolayer of octadecylphosphonic acid on mica was varied via solution concentration and immersion time, thereby controlling the surface free energy. For low surface coverage, a stable wetting layer of deposition solution was formed. For slightly greater coverage, the substrate was initially wetted by a thin liquid layer which quickly ruptured, leaving the sample dry. For large coverage, the liquid layer receded from the sample edge. Atomic force microscopy and video microscopy showed that the intermediate regime corresponds to dewetting via nucleation and growth of holes (0.25-5 µm in diameter) in the liquid layer while the dewetting at large coverage proceeds from the sample edge. The dependence of the wetting mode upon surface coverage and solution concentration was explained in terms of a competition between two rates: the nucleation rate of holes in the liquid layer and the speed at which the three-phase line advances.
There is a great interest in wetting and dewetting because of their importance to processes such as painting, coating, and lubrication.1 The equilibrium wetting of a solid surface by a liquid is controlled by the spreading coefficient, S ) γSV - γLV - γSL,2 where γ is an interfacial free energy and the subscripts specify the particular interface (S, solid; L, liquid; and V, vapor). When S > 0, the liquid layer “wets” the solid surface, and when S < 0, droplets of liquid with a well-defined contact angle are formed. If a metastable liquid layer initially covers the surface in a system where S < 0, it will eventually “dewet” and form droplets. This dewetting is generally thought to occur in one of two ways: (1) spontaneous nucleation and growth of holes in the wetting layer or (2) dewetting from the sample edge.3-6 In this Letter we show a systematic dependence of the observed mechanism, for the dewetting of a surfactant solution, upon solution concentration and surface free energy in the regime where S is very small. We take advantage of the well-known fact that complete self-assembled monolayers (SAMs) are generally “autophobic”sthey are not wet by their own deposition solution. However, the bare substrate is often wet by that solution. One would expect, therefore, that at some point during monolayer formation the spreading coefficient must pass through zero. SAMs of octadecylphosphonic acid (OPA) were deposited by immersing freshly-cleaved mica disks into solutions with concentration in the range 0.01-2 mM (using tetrahydrofuran as a solvent) for various immersion times. The details of OPA synthesis, contact angle goniometry, and atomic force microscopy (AFM) imaging were reported previously.7-9 The contact angle measurements were improved over those in our previous publications7 in that * To whom correspondence should be addressed: Fax, 504-8655596; Phone, 504-865-5573; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) deGennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (2) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990. (3) Redon, C.; Brochard-Wyart, F.; Rondelez, F. Phys. Rev. Lett. 1991, 66, 715. (4) Brochard-Wyart, F.; deGennes, P. G. Adv. Colloid Interface Sci. 1992, 39, 1. (5) Safran, S. A.; Klein, J. J. Phys. II 1993, 3, 749. (6) Elender, G.; Sackmann, E. J. Phys. II 1994, 4, 455. (7) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626. (8) Woodward, J. T.; Schwartz, D. K. J. Am. Chem. Soc. 1996, 118, 7861. (9) Woodward, J. T.; Doudevski, I.; Sikes, H. D.; Schwartz, D. K. J. Phys. Chem. B 1997, 101, 7535.
S0743-7463(97)00799-3 CCC: $14.00
Figure 1. The cosine of the static contact angle of water on various incomplete SAMs plotted versus the surface coverage measured directly using AFM demonstrating the excellent correlation between these quantities. A perfectly linear dependence would be in exact agreement with the Cassie equation.
they were performed under saturated vapor pressure conditions. AFM images and IR spectra showed that these incomplete SAMs were partially covered by densely packed submonolayer “islands” of OPA. Therefore, the fractional surface coverage of these islands could be quantitatively determined using straightforward image analysis. In previous work, we established that the molecules were deposited in these islands via a combination of two mechanisms. While the substrate is in solution, some islands nucleate and grow due to a combination of adsorption from solution and surface diffusion. Also, during removal from solution, additional molecules are deposited on the substrate via a quasi-Langmuir-Blodgett (LB) transfer of the Gibbs monolayer at the solution-air interface. Some of these molecules attach to pre-existing islands and others form new islands. There was excellent correlation between surface coverage extracted from AFM images and the measured contact angle of water, θw, on the film. Figure 1 shows cos θw plotted versus the measured surface coverage. The dependence is monotonic, nearly linear as expected from the mean-field Cassie equation.10 (10) Cassie, A. B. D. Discuss. Faraday Soc. 1952, 75, 5041.
© 1997 American Chemical Society
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Figure 2. Behavior of the wetting layer of SAM deposition solution as a function of solution concentration and the cosine of the contact angle of water. At low coverage (high cos θw), the substrate remained wetted by solution after removal (filled triangles, W region). At intermediate surface coverage, the substrate emerged wet but the wetting layer broke rapidly (open circles, B region). At high coverage, the substrate emerged dry, therefore the wetting layer dewetted from the sample edge (filled circles, D region). The dotted lines are provided as guides to the eye to distinguish the three regimes. Although it would be more physically meaningful to specify surface coverage instead of cos θw for the vertical axis of this graph, an accurate measurement of surface coverage involves extensive AFM imaging (to achieve good statistics) and image analysis. Given the excellent qualitative correlation between surface coverage and cos θw demonstrated by Figure 1, it was more convenient to measure the static contact angle as a parameter to demonstrate the trend in dewetting behavior.
During removal from deposition solution, a given sample had one of three possible appearances. Samples immersed in low concentration solution for short times emerged wet and remained wetted by solution until rinsed. At the other extreme, samples immersed for fairly long times emerged dry, displaying the autophobicity typical for SAMs. Careful observation uncovered a middle regime where the sample emerged wet originally but the thin liquid film broke almost immediately forming a dry area in the central part of the sample that spread rapidly toward the edges. We, therefore, classified samples according to their appearance during removal as “wet”, “breaks”, or “dry” (W, B, or D). There was a clear dependence of the removal behavior on solution concentration and surface coverage (or, equivalently, water contact angle). As shown in Figure 2, for a given solution concentration, the films progressed through the sequence W to B to D with increasing coverage (or decreasing cos θw). However, for greater solution concentrations, the transition from W to B and the transition from B to D each occurred at lower surface coverage. For solution concentrations greater than 0.2 mM, the samples emerged dry after the shortest possible immersion time (about 1 s); therefore no W or B samples were observed. AFM images of W or D films showed a random, but macroscopically uniform, coverage of submonolayer islands with no other features. However, B films displayed distinctive circular features as shown in Figure 3. These features ranged in diameter from 250 nm to about 5 µm; we noted no correlation between the size of the circles and immersion time, concentration, or average coverage. Areas inside circles had slightly less coverage than the areas between circles. The rim of the circles was marked by increased island coverage. The correlation between liquid film breakage and the appearance of these circles
Figure 3. AFM images of partially formed SAMs where the thin liquid film broke after removal from solution (B samples). Features are observed which have a circular region of reduced coverage surrounded by a ring of greater coverage. (a) A 15 µm image of a film immersed for 1 s in 0.2 mM OPA solution. Many rings several micrometers in diameter are observed. Rings are sometimes enclosed by larger oblong rings. Several rings only a few hundred nanometers in diameter are also apparent. (b) Higher resolution image of a single ring. The interior region has a lower average coverage than the exterior. The ring itself is composed of large islands that are densely packed but do not form a single continuous island.
led us to hypothesize that the circles were remnants of holes that served as nucleation sites for the dewetting process. The increased coverage outside the circle may be due to quasi-LB deposition that does not occur inside the circle during hole nucleation. A particularly large amount of material might be deposited at the rim during the hole nucleation event. To further test our hypothesis regarding the nucleation of holes, we deposited a drop of adsorption solution on a clean mica substrate and videotaped the results (Figure 4). The entire sample was held in a closed container saturated with THF vapor. After an initial waiting time,
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Figure 4. A sequence of images digitized from videotape showing the dewetting process of a wetting layer of OPA solution (in THF) from a clean mica surface under saturated THF vapor: (a) 30 s after depositing a drop of solution on the substrate, a uniform wetting layer of 0.02 mM solution covers the surface. The various features are defects on either surface of, or internal to, the mica substrate. (b) 57 s after deposition, many small dots appear scattered around the surface. (c) 72 s after deposition, a dewetting front can be seen passing across the surface. (d) 102 s after depostion, the solution has completely receded, leaving only isolated droplets. We believe that these dots correspond to the circular features shown in Figure 3.
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small “dots” appeared on the substrate. Shortly afterward, the liquid layer was observed to dewet and bead up on the surface. A drop of pure THF on clean mica, under the same conditions, formed a stable wetting layer on the surface. However, if a drop of THF was placed on a surface from which the solution layer had just dewetted (after removal of the droplets of solution), dots formed in the exact same locations. If a droplet of solution was placed on mica under conditions where the container was not saturated with THF vapor, the dots still formed; however, the dewetting “front” moved much more quickly across the surface after the appearance of the dots. A pure THF layer, under unsaturated conditions, formed dry areas at obvious defect sites (e.g., steps), which grew quickly as the THF evaporated. We believe that the dots were due to light scattered by holes in the liquid layer that were too small to resolve with our video camera/lens assembly. This implied that the first dewetting process that occurred was due to nucleation of holes through the wetting layersnot dewetting from the sample edge. This was consistent with the sequence of W to B to D observed during removal of samples from solution after increasing immersion times. As mentioned above, there was a slight trend for solution dewetting to occur at lower surface coverage for increasing solution concentration. This is demonstrated in Figure 2 by the small positive slope of the upper dotted line (drawn as a guide to the eye) which divides the W region from the B region. This trend is counterintuitive since the decrease in γSL and γLV due to increased concentration would tend to stabilize the wetting layer by increasing the spreading coefficient. In actual fact, however, the measured change in γLV over this concentration range is only about 0.3% (Henry’s law regime), so one would expect the effect to be negligible. In addition, a wetting layer of solution is known to increase in thickness with increasing concentration due to various effects including the difference in the chemical potential of the solvent between the vapor and the thin layer.6 We did not measure the wetting layer thickness exactly. However, it was clear to the naked eye that more concentrated solution resulted in thicker wetting layers. The theory of dewetting via spontaneous hole formation predicts that surface fluctuations with wavevectors smaller than a critical wavevector grow, while fluctuations with larger wavevectors shrink.5 Translating into real space, a hole must be greater than a critical size in order to form a stable nucleus for dewetting (thus the analogy with the kinetics of phase transitions). The critical dimension for the hole is proportional to γLVd4, where d is the wetting layer thickness. Therefore, the rate of hole formation decreases dramatically with increasing wetting layer thickness. Thus, both equilibrium and kinetic considerations would tend to argue that dewetting should occur at greater coverage for high solution concentrations, contradicting our observations. We offer the following hypothesis as an explanation. We imagine that surface fluctuations are constantly present; however, when the spreading coefficient is positive any holes that form are unstable and are immediately filled. However, our earlier work9 showed that the amount of OPA deposited via quasi-LB deposition increased significantly with solution concentration (presumably because the two-dimensional concentration of the Gibbs monolayer was greater). One must remember that this deposition occurs whenever the liquid-vapor interface comes into contact with the solid surface, such as during the formation of a hole. The features observed with AFM on the B films are examples of this deposition; the high density of OPA islands around the rim would stabilize the contact angle around the hole. If the surface coverage were such that the spreading coefficient was close
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to zero (but still slightly positive), then the additional amount of OPA deposited during a spontaneous hole formation event could serve to stabilize that hole and it would not disappear. These holes would then serve as preformed nuclei for the dewetting process. In support of this hypothesis is our observation that pure solvent layer spread on a surface which has previously dewetted in this manner forms holes in the same locations as the original solution wetting layer. Once formed, the circular features on the surface serve as sites for stable hole formation. We now address the transition from dewetting via hole formation to dewetting from the sample edge (B to D). Figure 2 shows that for a given surface coverage, low concentration solution dewets via hole formation (B sample) while high concentration solution dewets from the edge (D sample). We believe that this is due to competition between two kinetic processesshole formation versus the spreading of dry areas. If the rate of hole formation is very fast compared to the velocity of the threephase line, then many holes will form and the wetting film will break from within. However, the edge of the sample always represents a three-phase line, and if the velocity of the three-phase line dominates the formation rate of holes, then the sample will dewet from the edge before holes have a chance to form. As noted above, wetting layers are thicker for higher concentration solution, resulting in lower rates of hole formation. The velocity of the three-phase line is given by the expression υ ∝ (γLV/η)θ3,4 where η is the solution viscosity and θ is the equilibrium contact angle of the solution on the solid surface (in the small angle approximation). As noted above, we can neglect the effect of concentration on γLV; the same is true for the viscosity. Of course, the contact angle increases with surface coverage but depends only weakly on concentration of the solution. Therefore, for a given concentration of solution, the velocity of the “dry” front will be slow at lower coverage and dewetting by hole nucleation will dominate, whereas at higher surface coverage the three-phase line will move quickly in from the edge, overwhelming the nucleation of holes. This also explains the trend with increasing concentration at a given coverage. Since the front velocity is approximately constant as a function of concentration, the dewetting mode depends on the hole nucleation rate. Thin films (low concentration) will nucleate holes quickly and form B samples while thick films (high concentrations) will dewet from the edge to form D samples. Although these experiments involve the dewetting of a
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solution containing OPA from a surface partially covered by the same molecules, we believe that they address a different issue from studies of so-called “selfdewetting”sdewetting of a pure substance from underlayers of the same substance. Such experiments have been performed on Langmuir-Blodgett multilayers11,12 and thin films of polymers which form layered structures.13 The major difference is that the dewetting liquid, in our case, is predominantly solvent (the maximum OPA concentration is 2 × 10-3 M). Although the submonolayer patches of OPA on the surface play a crucial role in varying the surface free energy, we believe that the OPA in solution plays only a minor role in determining the dewetting mechanism. For example, if one were able to vary the surface free energy by an alternative method, the same evolution from dewetting via hole formation to dewetting from the sample edge should be observed as a function of surface energy, regardless of the solute used. In conclusion, we have observed the evolution from wetting, to dewetting by spontaneous hole nucleation, to dewetting from the sample edge, as a function of increasing surface coverage of incomplete self-assembled monolayers by surfactant solution. The transition from wetting to dewetting by hole nucleation had an anomalous dependence on solution concentration (higher concentration solution dewetted at lower surface coverage) that may be due to the stabilization of otherwise unstable holes in the wetting layer due to quasi-LB deposition at the threephase line. The transition from dewetting via hole formation to dewetting from the sample edge is explained by the competition between the hole nucleation rate and the velocity of the spreading of dry areas. At high surface coverage the three-phase line moves from the sample edge quickly, dominating the hole nucleation process. Acknowledgment. We thank Abraham Ulman for preparation of the OPA. This work was supported by the National Science Foundation (Grant No. CHE-9614200) and the Center for Photoinduced Processes (funded by the National Science Foundation and the Louisiana Board of Regents). LA9707991 (11) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 10444. (12) Riegler, H.; Asmussen, A.; Merkyl, C.; Schabert, F.; et al. In XVth Moriond Workshop, “Short and Long Chains at Interfaces”; 1995; pp 307-312. (13) Sheiko, S.; Lermann, E.; Mo¨ller, M. Langmuir 1996, 12, 4015.
