Three-dimensional structure in water-methanol mixtures

The diverse properties of hydrogen-bonded liquids and solutions must manifest their unique local structures. An unambiguous three-dimensional picture of the local ordering in these liquid systems is not accessible through radial distribution functions, the usual outputs of computer simulation, or experimental studies. In this work we employ spatial distribution functions to analyze the three-dimensional local structure in water(-) methanol solutions. Molecular dynamics simulations are performed at room temperature for five water(-) methanol liquid mixtures scanning the entire range of compositions. The effects of the alcohol on water structure and water on methanol structure are considered in detail. The results are compared to previous simulations and discussed from the point of view of various solvation models. Large structural changes are observed, many of which are not apparent from simple radial analysis. In water-rich solution we confirm a high degree of ordering, characterized by a very strong preference for tetrahedral arrangements, where the water molecules appear most highly localized around the hydroxyl group of the methanol solute. Strongly hydrated methanol molecules adopt rather specific relative positions that most readily accommodate the ordering within their hydration cages. In methanol-rich solution the local structure very closely resembles that of pure methanol. We find that rather long equilibration periods appear to be necessary to obtain accurate structural information in computer simulations of these complex systems.