Thermodynamic Analysis of Liquid–Vapor Phase Equilibrium in Binary Systems with One Associating Component

In modeling liquid–vapor phase equilibrium, the actual structure of the solution is often disregarded, and a monomolecular structure of the substances is assumed. This study proposes an experimental combined method for determining the molecular weight of associated components and their degree of association. The method consists of several steps. In the first step, the molar heat of vaporization of the substance is determined experimentally based on the dependence of vapor pressure on temperature. In the second step, the specific (weight-based) heat of vaporization of the substance is measured experimentally. In the third step, the molecular weight of the substance is calculated from the ratio of the specific heat of vaporization to the molar heat of vaporization. Using the water–acetic acid system as an example, the formation of molecular complexes of acetic acid in both the vapor and liquid phases and their effect on phase equilibrium are demonstrated.