Liquid-Liquid Equilibria of Binary Water/1-Butanol and Water/THF Mixtures Studied By Molecular Simulation

Aqueous mixtures can exhibit a large variety of phase behavior. The water/1-butanol mixture exhibits only an upper critical solution temperature (UCST) below the critical points of both components and can be categorized as a type II phase diagram following van Konynenburg and Scott [1].  In contrast, the water/tetrahydrofuran (THF) mixture shows a type VI phase diagram, characterized by an UCST and a lower critical solution temperature (LCST) and, hence, a closed-loop miscibility gap. Although thermodynamic descriptions of both types of mixtures are well known, the microscopic origins of such behavior remain unclear. In this work, Gibbs ensemble Monte Carlo simulations using the TraPPE force field and the TIP4P water model were performed to directly model the coexisting fluid phases at atmospheric and elevated pressures. The calculated liquid-liquid coexistence curves are in good agreement with experimental data. The unfavorable entropy of mixing is explored in terms of hydrogen-bond speciation, aggregate distribution, and number integrals.

[1] van Konynenburg, P. H. and Scott, R. L., Phil. Trans. A298, 495 (1980)