Electrostatic potential of phase boundary in Coulomb systems

Any interface boundary in an equilibrium system of Coulomb particles is acc ompanied by the existence of a finite difference in the average electrostat ic potential through this boundary. This interface potential drop is a ther modynamic quantity. It depends on temperature only and does not depend on s urface properties. The zero-temperature limit of this drop (along coexisten ce 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 discuss ed potential drop is directly calculated by numerical simulation of phase t ransitions in Coulomb systems. Properties of the interface potential drop are discussed for several simpli fied Coulomb models (melting and evaporation in the One Component Plasma (O CP)). Some examples of phase transition in real situations are also discuss ed.