Molecular Dynamics Simulations Study of Phase Behavior and Interfacial Structure in the Benzene–Methanol Azeotrope

Abstract

Azeotropes are liquid mixtures that exhibit constant boiling behavior and behave as single substances during phase change, posing significant challenges in separation processes while playing important roles in chemical manufacturing and purification. Understanding the molecular origins of azeotropic behavior is therefore essential for the design of efficient separation strategies. In this study, molecular dynamics (MD) simulations were employed to investigate the microscopic behavior of the benzene-methanol azeotrope and its liquid-vapor interface over a range of temperatures and compositions. The reliability of the employed force field was first assessed by calculating fundamental thermophysical properties, including liquid density and enthalpy of vaporization, for the pure components as well as the azeotropic mixture. The simulated values show excellent agreement with experimental data, demonstrating the suitability of the force field for describing the system. The liquid-vapor interfacial analysis reveals that benzene exhibits preferential evaporation at lower benzene concentrations, whereas methanol becomes relatively more volatile at higher benzene concentrations. Furthermore, at the azeotropic composition and temperature, both benzene and methanol attain nearly equal compositions at the liquid-vapor interface, indicating that the volatilities of the two components become comparable. This molecular-level observation provides a microscopic explanation for the simultaneous boil- ing of both components, which is the defining characteristic of azeotropic behavior. Overall, this study demonstrates that molecular dynamics simulations can successfully capture the interfacial structure, molecular organization, and phase coexistence characteristics of the benzene-methanol azeotrope, providing valuable insight into the molecular mecha- nisms governing azeotropic mixtures.