ABSTRACT
At the First International Symposium on Contact Angle, Wettability, and Adhesion [1], held in honor of Robert J. Good, we addressed the problem of the adsorption isotherm, that describes the formation of sessile drops as the end product of adsorption and condensation, of vapor on a solid surface. The isotherm was originally presented by Derjaguin and Zorin [2], and later quantified and modified by Adamson and co-workers [3-5]. These two groups must be credited with an outstanding contribution to sessile drop science by advancing toward the goal of integrating basic thermodynamics with the empirical and semi-empirical ap-
proaches used by experimentalists in the field. The approach adopted by Derjaguin nevertheless had a puzzling aspect, with which he and others struggled valiantly. The adsorption isotherm "crossed the/?° line", i.e., the vapor pressure had to exceed the saturation vapor pressure, p°, of the liquid in order to bring about condensation. However, since the totality of sessile drops that had condensed on the surface was regarded as a single mass of liquid, it was expected that its vapor pressure would be p° (saturation vapor pressure of liquid) and that the isotherm would rapidly "return to the p° line" and possibly cross it again [4]. This presented difficulties which were bypassed when the thermodynamics of formation of a single drop was developed [1, 6], as compared to treating the totality of multiple drops as a single quantity of formless condensate. It turned out that, because of the curvature at the liquid-vapor interface, the vapor pressure must exceed p° for drop formation (on thermodynamic grounds, not merely because of activation energy), and should remain more than p° as long as the drop exists. Thus, due to the well-known relationship between the free energy change and the logarithm of vapor pressure, it was shown that sessile drop formation is accompanied by an increase in free energy [1,6], not a decrease as many had thought.
