  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. The discussed 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 the coexistence curve) is an individual substance coefficient. At high
temperature the drop tends to zero at critical point of gasliquid phase
transition. A special critical exponent can be defined to describe this
behavior. Study of the interface potential drop is illuminative in simplified
Coulomb models: i.e. for melting and evaporation in variants of One Component
Plasma model (OCP), or for model of Charged Hard/Soft Spheres (CHS/CSS) etc. In
all these cases properties of the potential drop can be easily calculated by
the DNS methods (direct numerical simulation) when the two-phase coexistence in
Coulomb system is really simulated. Electrostatics of phase boundaries in real
systems could be elucidated in analytical calculation of two-phase coexistence
via finite-temperature DFT approach (density functional theory).
