Properties of the Liquid-Vapor Interface of Water-Dimethyl Sulfoxide Mixtures. A Molecular Dynamics Simulation and ITIM Analysis Study

Molecular dynamics simulations of the liquid-vapor interface of water-dimethyl sulfoxide (DMSO) mixtures of nine different compositions, ranging from neat water to neat DMSO, are performed on the canonical (N,V,7) ensemble at 298 K. The surface molecular layer of the systems are identified and analyzed in detail in terms of the novel identification of the truly interfacial molecules (ITIM) method. The obtained results show that DMSO is adsorbed at the surface of such mixtures at every composition, but this adsorption is limited solely to the first molecular layer beneath the surface. Within the surface layer the minor component is always found to be preferentially located at the outer edge of the layer, close to the vapor phase. The preferred orientations of both molecules at the surface of their neat liquids are such that they can prevail upon addition of the other component, and the unlike molecules can form strong hydrogen bonds with each other in their preferred orientations. Thus, neither the water nor the DMSO molecules perturb considerably the local surface structure of the other component. On the other hand, similarly to the bulk liquid phase the like components show self-association behavior also at the surface of the mixed systems. Further, the lateral percolating hydrogen-bonding network of the surface water molecules, present at the surface of neat liquid water, is found to be broken already at the bulk phase DMSO concentration of 3.3 mol % (corresponding to the surface DMSO concentration of 25 mol %). This breakdown of the lateral percolating network of surface waters is found to be accompanied by a sudden increase of the mobility of the water molecules between the surface layer and the bulk-like part of the system.