Analysis of a Thermodynamic Determination of the Surface Tension of a Two-Component Vapor–Liquid System

A numerical analysis is performed of a thermodynamic determination of the surface tension (ST) of a vapor–liquid binary mixture and interfacial tension (IT) between two liquid phases as the excess value of free energy ΔF of a two-phase system with and without for a phase boundary. Calculations are made in the simplest version of the lattice gas model (LGM), allowing for the interaction of nearest neighbors in a quasi-chemical approximation. Each node of a two-component mixture in the LGM system can be occupied by mixture components A + B and vacancy V. The two main ways of calculating ST, expressed in terms of different partial contributions Miq to excess free energy ΔF (where i = A, B, V are vacancies; 1 ≤ q ≤ κ, q is the number of the monolayer inside the boundary; and κ is its width), are compared, as are calculations of ST and IT as excess free energy ΔF for a lattice model with no vacancies. The ambiguity of ST and IT values depending on the type of Miq functions is obtained by calculating the temperature dependence for a flat boundary and the dependence of ST and IT on droplet size at a fixed temperature. The role of vacancies in the LGM as the main mechanical characteristic of the system under the condition of strict phase equilibrium in three equilibria (mechanical, energy, and chemical) is discussed.