Study of phase equilibria and thermodynamic properties of liquid mixtures using the integral equation theory: Application to water and alcohol mixtures

A theoretical method for calculating the thermodynamic properties and phase equilibria of liquid-liquid mixtures using the integral equation theory is proposed. The solvation chemical potentials of the two components are evaluated by the integral equation theory and the isothermal-isobaric variation of the total density with composition is determined to satisfy the Gibbs-Duhem relation. Given the density of a pure component, the method can calculate the densities of the mixture at any composition. Furthermore, it can treat the phase equilibrium without thermodynamic inconsistency with respect to the Gibbs-Duhem relation. This method was combined with the reference interaction-site model integral equation theory and applied to mixtures of water + 1-alcohol by changing the alcohol from methanol to 1-butanol. The destabilization of the mixing Gibbs energy by increasing the hydrophobicity of the alcohol and demixing of the water-butanol mixture were reproduced. However, quantitative agreement with experiments is not satisfactory, and further improvements of the integral equation theory and the molecular models are required. Published under an exclusive license by AIP Publishing.

Automated summary

A theoretical method for calculating the thermodynamic properties and phase equilibria of liquid-liquid mixtures using integral equation theory is proposed. The method evaluates the solvation chemical potentials and determines the density variation with composition to satisfy the Gibbs-Duhem relation. It can calculate the densities of mixtures at any composition and treat phase equilibrium without thermodynamic inconsistency. However, quantitative agreement with experiments is not satisfactory and further improvements are needed for the theory and molecular models.