Investigation of the Liquid-Vapor Interface of Water-Formamide Mixtures by Computer Simulation and Intrinsic Surface Analysis

Molecular dynamics simulations of the liquid-vapor interface of water-formamide mixtures of different compositions, spanning the entire composition range, have been performed in the canonical (N, V, T) ensemble at 300 K. The layer of the surface molecules has been identified and analyzed in detail in terms of the intrinsic identification of the truly interfacial molecules (ITIM) method. The obtained results reveal the strong lateral hydrogen-bonding ability of the surface molecules in these systems. Thus, the surface molecules form a percolating lateral hydrogen-bonding network in every case, which makes the surface layer noticeably more compact than the subsequent subsurface molecular layers. However, the molecules mix with each other even on the molecular scale. Neither strong adsorption, nor lateral self-association of any of the components in the surface layer has been observed, although formamide exhibits a slight ability of being accumulated in the surface layer. Further, in contrast with the aforementioned percolating lateral hydrogen-bonding-mixed network of the surface water and formamide molecules, no such network of the like molecules is observed, even in the case of their large excess at the surface of the mixed systems. The orientational preferences of the surface molecules are also governed by the requirement of maximizing their hydrogen bonds. The main preference of both molecules is found to be the parallel alignment with the macroscopic plane of the liquid surface. The dynamics of the surface molecules is also dominated by the hydrogen bonding of unlike pairs. Thus, water stabilizes the stay of the formamide molecules at the liquid surface, whereas formamide immobilizes the water molecules within the surface layer, preventing them from a considerable lateral diffusion during their entire stay at the liquid surface.