Structure of the Liquid−Vapor Interface of Water−Acetonitrile Mixtures As Seen from Molecular Dynamics Simulations and Identification of Truly Interfacial Molecules Analysis

• Published in: Journal of physical chemistry. C Vol. 113; no. 42; pp. 18173 - 18183
• Main Authors: Pártay, Lívia B, Jedlovszky, Pál, Horvai, George
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22.10.2009
• Subjects: C: Surfaces, Interfaces, Catalysis
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp901832r

The surfaces of water−acetonitrile mixtures of four different compositions have been studied by molecular dynamics computer simulation. In analyzing the molecular level properties of these surfaces, the full list of the molecules that are indeed at the surface (i.e., at the boundary of the liquid and vapor phases) have been determined by means of the novel method for the identification of truly interfacial molecules (ITIM). The obtained results show that the acetonitrile molecules are strongly adsorbed at the surface of these solutions. Further, besides the surface layer, in systems of high enough acetonitrile contents, the second and sometimes even the third molecular layer beneath the surface contains acetonitrile in a considerably higher mole fraction than the bulk liquid phase. Acetonitrile molecules are found to stay also much longer than waters at the surface of these systems. The preferred surface orientation of the acetonitrile molecules is found to depend on the local curvature of the surface as well as on the acetonitrile mole fraction at the surface layer. Nevertheless, these orientational preferences are largely determined by the requirement of sticking as many apolar CH3 groups out to the vapor phase as possible. Further, the orientational preferences of acetonitrile in the second molecular layer beneath the surface is found to be the opposite of what is found at the surface layer, reflecting the strongly dipolar character of the acetonitrile molecule as well as its ability of forming π−π pairs with its nearest neighbors. The orientational preferences of the surface water molecules are also found to be governed by the dipolar interactions with the neighboring acetonitriles. Finally, a strong ability of the like molecules for lateral self-association is found at the surface layer of the systems studied.