(213e) Local Equilibrium of the Gibbs Interface in Two-Phase Systems

The classical approach to equilibrium interfacial thermodynamics in phase coexistence describes the interface as a separate two-dimensional thermodynamic system. Excess densities then characterize the autonomous interface. Gibbs' original formulation relies however on the uniformity of intensive variables throughout the system. Therefore, it cannot be directly applied to nonequilibrium situations such as evaporation/condensation processes or heterogeneous catalysis, where jumps in temperature and chemical potential across the interface can occur. We present a conceptually clear formulation of local equilibrium for interfaces. In particular, we gain new insights by expressing, in terms of gauge transformations, the ambiguity in locating the interface. The gauge invariance of thermodynamic properties appears to be equivalent to conditions for jumps of bulk densities across the interface, and we are notably able to rigorously generalize the Clapeyron relationships to nonequilibrium situations. We support our theoretical predictions by performing stringent tests with nonequilibrium molecular dynamics simulations of a coexisting liquid/vapor Lennard-Jones fluid.