Orientation and Structure of Acetonitrile in Water at the Liquid–Vapor Interface: A Molecular Dynamics Simulation Study

Abstract

We report molecular dynamics simulations of acetonitrile–water binary solutions at concentrations of 0.032–0.59 mole fraction. We find that at low bulk concentration acetonitrile has an enhanced population near the liquid/vapor interface. The surface-bound acetonitrile molecules exhibit anisotropic orientations and lie nearly flat along the solution surface with their terminal methyl groups directed toward the vapor. Upon increasing the bulk concentration, the formation of acetonitrile domains is promoted by interactions between hydrophobic methyl moieties. Dipole–dipole interactions facilitate a pseudonematic, antiparallel pairing of near-neighbor molecules both in the bulk solution and near the liquid/vapor interface. Near the interface the preferred orientation of acetonitrile flattens further to accommodate antiparallel pairing of neighboring molecules such that the methyl group remains above the solution. This study paints a surprisingly complex picture of a binary organic–water solution that manifests behavior similar to liquid crystals through preferred orientations and pseudonematic antiparallel pairing.