Computer simulation and ITIM analysis of the surface of water-methanol mixtures containing traces of water

Molecular dynamics computer simulation of the surface of water-methanol mixtures containing only traces (i.e., 3%, 5% and 10%) of water are performed on the canonical (N,V,T) ensemble at 298 K. The properties of the surface are analyzed in terms of the novel ITIM method. The obtained results show that the vicinity of the vapor phase only affects the properties of the first molecular layer, and vanishes beyond this layer. No region of water adsorption/methanol depletion is observed. The methanol molecules are found to be in an excess number at the surface (relative to the bulk liquid phase), and are found to stay considerably longer at the surface layer than the water molecules. In their preferred alignment surface methanols are found to stay perpendicular to the macroscopic plane of the surface, sticking their CH3 group straight to the vapor phase.