Diffuse layer effect and effect of curvature on the mechanism and the properties of phase transitions at electrified interfaces

Abstract

The criteria for an adsorbed layer to undergo a reversible phase transition on an electrode surface are examined by means of classical thermodynamics. It is shown that, although this process takes place always at a constant value of the potential drop across the adsorbed layer, it may occur at either a constant value of the applied potential or over a certain range of applied potentials. The first possibility can be realised when the concentration of ions in the diffuse layer is high enough to produce an approximately equipotential plane just beyond the adsorbed layer or when the diffuse layer is imposed to undergo a kind of phase transition characterised by its separation into two parts with different total electrical charge but the same potential drop or the transition extends throughout the interface. If neither of the above processes can be realised, then the second possibility is very likely due to a diffuse layer effect. For the examination of the diffuse layer effect in more detail, the classical supersaturation theory is extended to two-dimensional phase transitions on electrode surfaces and an attempt is made to explain the existence and extent of metastable states as well as the mechanism and the properties of irreversible transitions. The results obtained are compared with current views on two-dimensional phase transitions.