A comparative Molecular Dynamics study of water-methanol and acetone-methanol mixtures

Two binary mixtures with methanol as common component, namely water-methanol and methanol-acetone are studied by Molecular Dynamics simulations. Thermodynamical properties such as enthalpies, excess enthalpies, volumes and excess volumes are compared. Structural properties are studied through the various site-site pair correlation functions and the associated Kirkwood-Buff integrals. While the thermodynamical properties are relatively well calculated, we show that structural properties are affected by two severe problems. The first is an inherent property of the asymptote of the correlation function in finite systems, which affects their integrals, the second is the micro-heterogeneous structure of these hydrogen bonding mixtures, which affects the medium-to-long-range part of the distribution functions. These two problems conspire to make computer simulation unreliable to study this precise part of the structural properties of such mixtures. We propose pathways to correct this situation and demonstrate how it serves to considerably improve the calculated Kirkwood-Buff Integrals. Finally, we compare the two mixtures relative to their tendencies to form local heterogeneity. The analysis demonstrates that the micro-structure of neat methanol is better preserved in acetone than in water.