First-order wetting transition at a liquid–vapor interface

Abstract

In certain binary solutions the lower of the two liquid phases forms a layer which intrudes between the upper liquid phase and the vapor. We find that such intruding layers form above binary solutions of a fluorocarbon (C7F14) and an alcohol (i-C3H7OH). As the temperature of C7F14–i-C3H7OH solutions is increased, the intruding layer abruptly appears at a characteristic wetting temperature TW=311 K. This temperature is well below the consolute temperature (363 K). At temperatures slightly above Tw the intruding layer’s thickness (measured by ellipsometry) is several hundred angstroms and its variation with temperature is extremely weak. Below Tw, the layer’s thickness may be zero and is no greater than 20 Å when a naive slab model is used to interpret the data. Below Tw, three-phase contact can occur between the vapor and both the upper and the lower liquid phases. Our measurements show that one of the angles (θ) which characterizes this three-phase contact has a very simple temperature dependence: cos θ=1−0.8 (Tw−T)/Tw. Thus, both the temperature dependence of the layer’s thickness and the temperature dependence of the three-phase contact are consistent with a first-order wetting transition at Tw. The thickness of the intruding layer has been monitored as the solutions approach equilibrium. Below Tw the behavior is complex; however, a feature does appear which suggests a nucleation-like event is required for the layer to vanish. Phenomenological theories of the first-order wetting transition predict that a discontinuity in the temperature derivative of σ occurs at Tw where σ is the macroscopic surface tension between the upper liquid phase and the vapor phase. From our data we estimate that this discontinuity is very small: −0.009 dyn cm−1 K−1. A first-order wetting transition is predicted by approximate theories which use long-ranged interatomic potentials (e.g., inverse sixth power attractions), provided that the range of the potential between unlike species is longer than the range of the potentials between like species. We find that the thickness of the intruding layer d depends quite weakly on L in the vicinity of a first-order wetting transition. (L is the height spanned by the upper liquid phase.) This feature is in contrast with earlier ellipsometric measurements of the thickness of intruding layers which showed that d∝L−1/3 in different binary mixtures away from wetting transitions.