Insight into the Local Composition of the Wilson Equation at High Temperatures and Pressures through Molecular Simulations of Methanol–Water Mixtures

Abstract

Methanol–water mixtures are known to be microscopically inhomogeneous and show nonideal properties at ambient conditions and also at higher temperatures and pressures. To understand the nonideality of methanol–water mixtures, the behaviors of local compositions were studied through evaluating the local mole fractions by the well-known activity coefficient equation, Wilson equation, and by molecular dynamics (MD) simulation at three conditions: 25 °C–0.1 MPa, 300 °C–25 MPa, and 350 °C–25 MPa. The Wilson parameters were redetermined to generate the reference values for the comparison in this study. The deviation of local mole fraction from bulk mole fraction was selected as an indicator for local composition. The deviations by the Wilson equation were positive in all compositions, and its magnitude for water was much larger than that for methanol, which is principally independent of temperature and pressure and was completely supported by the results by MD simulation. The MD simulation provided the dependen...