Electrostatic Potential of Phase Boundary in Coulomb Systems

Any interface boundary in an equilibrium system of Coulomb particles is accompanied by the existence of a finite difference in the average electrostatic potential through this boundary. This interface potential drop is a thermodynamic quantity. It depends on temperature only and does not depend on surface properties. The zero-temperature limit of this drop (along coexistence curve) is an individual substance coefficient. The drop tends to zero at the critical point of the gas-liquid phase transition. A special critical exponent can be defined to describe this behavior. The value of the discussed potential drop is directly calculated by numerical simulation of phase transitions in Coulomb systems. Properties of the interface potential drop are discussed for several simplified Coulomb models (melting and evaporation in the One Component Plasma (OCP)). Some examples of phase transition in real situations are also discussed.