Hydrogen-Bonded Clusters on the Vapor/Ethanol-Aqueous-Solution Interface

Abstract

The structure of the vapor/ethanol-aqueous-solution interface has been carefully investigated focusing on an intermolecular hydrogen bond (HB) and molecular clusters bound by HBs. This paper is a continuation of our previous molecular dynamics (MD) study (Langmuir 2005, 21, 10885), and all analysis was performed based on five independent adsorption-equilibrated configurations of a slab of ethanol solution at 298.15 K, where the ethanol mole fraction of the solution, chi(e), is 0.0052, 0.012, 0.024, 0.057, and 0.12, respectively. The geometrical definition of HB enabled the detection of the HB between ethanol-ethanol, ethanol-water, and water-water molecules. The variations of the density of HB and the coordination number of HB across the vapor/solution interface were analyzed. Analysis on the density of HB reveals that a monolayer of adsorbed ethanol can be classified into two parts where ethanol molecules prefer to form HBs with each other and ethanol molecules prefer to form HBs with water molecules. Despite chi(e), the coordination number of ethanol-ethanol HB monotonically increases toward the vapor region, while those of ethanol-water and water-water HBs monotonically decrease. In addition, the variation of the mean size of both ethanol one-component clusters and ethanol/water binary clusters across the interface were analyzed. The mean size of an ethanol one-component cluster and that of an ethanol/water binary cluster are expressed as a maximum at the interface. These behaviors are linked with the size distributions of both one-component and binary clusters. A relatively large system in this calculation also enables detailed discussion about the molar dependency of the bulk structural properties of an ethanol solution.