  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.
