Different types of phase transitions at charged interfaces. A thermodynamic analysis

Abstract

A thermodynamic treatment of surface phase transitions due to orientational and structural effects is presented. It is shown that thermodynamics predict only two types of surface phase transitions: separation of the interface into two new phases and a kind of two-dimensional condensation. The first type takes place when both adsorbate and solvent molecules coexist at the interface. The new phases may be concentrated surface solutions of adsorbate in solvent and vice versa or pure adsorbate and a concentrated surface solution of adsorbate in solvent. In the last case, the adsorbate may be expelled in the form of a surface precipitate or micelles. The second type of surface phase transition occurs only at saturated interfaces with adsorbate molecules. Orientational and structural effects do not lead to independent transitions, as in bulk phases, but coexist in the two types of surface phase transitions. Rigorous relationships, which describe the transition region, are also developed and discussed. © 1991.