Water-Methanol Mixtures: Simulations of Mixing Properties over the Entire Range of Mole Fractions

Numerous experimental and theoretical investigations have been devoted to the hydrogen bond in pure liquids and mixtures. Among the different theoretical approaches, molecular dynamics (MD) simulations are predominant in obtaining detailed information, on the molecular level, simultaneously on the structure and the dynamics. Water and methanol are the two most prominent hydrogen-bonded liquids, and they and their mixtures have consequently been the subject of many studies; we revisit here the problem of the mixtures. An important first step is to check whether a classical potential model, the components of which are deemed to be satisfactory for the pure liquids, is able to reproduce the known thermodynamic excess properties of the mixtures sufficiently well. We have used the available BJH (water) and PHH (methanol) flexible models because they are by construction mutually compatible and also well suited to study, in a second step, some dynamic property characteristic of hydrogen-bonded liquids. In this article we show that these models, after a slight reparametrization for use in NpT simulations, reproduce the essential features of the excess mixing and molar properties of water-methanol mixtures. Furthermore, in the pure liquids, the agreement of the radial distribution functions with experiment remains as satisfactory as before. Similarly, the translation self-diffusion coefficients D are modified by less than 10%. In the mixtures, they evolve nonmonotonously as a function of mole fraction.