Basic Thermodynamics of Electrified Interfaces

Abstract

In this study some general aspects of the thermodynamics of systems with interfaces are discussed, and a brief treatment of interfaces within the framework of classical thermodynamics is presented. Special attention is paid to the theory of electrified interfaces. The intensive parameter conjugate to surface area (“surface tension” or “interfacial tension”) is an important parameter also in the thermodynamic theory of electrodes, because the interactions between the adjacent bulk phases take place via interfaces, for example, via the interface between a metal and an electrolyte solution. As a consequence, the thermodynamic properties of the interface region (i.e., the electronic conductor/ionic conductor interface) directly influence the electrochemical processes. First, to introduce the reader to the topic, basic concepts (such as “surface,” “interface,” “interphase,” “interfacial or interface region,” “dividing surface,” “adsorption”) are reviewed, a reasonably simple thermodynamic treatment of interfaces, together with a brief description of the models widely used in the literature, are presented, and the characteristics of the Gibbs “dividing plane” model and the Guggenheim “interphase” model are outlined. The derivation of the electrocapillary equation, the Gibbs adsorption equation, and the Lippmann equation for an ideally polarizable electrode is given. A simple illustrative example for the application of the electrocapillary equation is presented. Some important mathematical concepts (e.g., theory of homogeneous functions and partly homogeneous functions, Euler's theorem, and Legendre transformation) and various functional relationships of the thermodynamics of surfaces and interfaces are summarized.