Molecular dynamics simulation and identification of the truly interfacial molecules (ITIM) analysis of the liquid-vapor interface of dimethyl sulfoxide.

The liquid-vapor interface of dimethyl sulfoxide (DMSO) is investigated by molecular dynamics computer simulation and by the novel method of the identification of the truly interfacial molecules (ITIM). With this method, it is possible to consider in the analysis specifically those molecules that are located right at the boundary of the two phases. The obtained results show that the orientation of the surface molecules is driven by the requirement that these molecules should be able to maintain their strong dipole-dipole and pi-pi interactions with each other, such as in the bulk liquid phase. This requirement leads to the preference of the polar S=O double bonds for laying parallel with the surface, and of the apolar CH(3) groups for pointing rather flatly out of the liquid phase. As a result, the surface of liquid DMSO is only mildly corrugated, in terms of the molecular size. Similarly to the bulk liquid phase, the DMSO molecules are also found to form long dipole chains of head-to-tail oriented neighbors even within the surface layer. The results also indicate that, at least from the point of view of the dynamics of the individual molecules, the division of the system beneath the truly surface layer into further subsurface molecular layers is already physically meaningless.