Analisys of Methods for Calculating the Equilibrium Surface Tension of Vapor–Liquid Systems in the Lattice Gas Model

Definitions of the equilibrium surface tension (ST) existing in thermodynamics are analyzed along with reasons for their ambiguity. A molecular statistical analysis of the concept of equilibrium ST at the vapor–liquid interface is given and a procedure for calculating it within the simplest microscopic model of statistical physics (the lattice gas model) is formulated. Equilibrium ST is shown to be a mechanical characteristic calculated under the condition of a rigorous phase equilibrium over three partial equilibria (mechanical, energy, and chemical). Violation of the chemical equilibrium results in non-equilibrium ST. The emergence of metastable STs is due to the artificial introduction of a foreign film boundary into the model through the Laplace equation, which distorts the real properties of the system. Means of statistical physics for obtaining existing definitions of equilibrium ST in the theory of integral equations for fluids and molecular dynamics are discussed along with a criterion for distinguishing between equilibrium and non-equilibrium STs, metastable and otherwise. Analysis shows that none of the current means of statistical physics ensures correct calculations of equilibrium STs with regard to all molecular features of the considered systems.