UC Irvine

Chemical reactions occur ubiquitously at the air/water interfaces of oceans and rivers, as well as on aerosols in the atmosphere. Presently, these occurrences are not well understood. In recent years, however, this field has drawn a wave of interest, due to the emergence of experimental techniques sensitive to the liquid surface and more powerful computing architectures that permit the simulations of large systems. The importance of understanding molecular transfer through the liquid/vapor interface to better interpret atmospheric chemical processes cannot be overstated. Reported here, are molecular dynamics simulations of acetonitrile-water binary solutions at concentrations of 0.05--0.6 mole fraction. Evaluating systems of low bulk acetonitrile concentration reveals an enhanced population of the solute near the liquid/vapor interface. These surface-bound acetonitrile molecules exhibit a specific preferential anisotropy, where temporally averaged probability distributions of molecular orientation have indicated interfacial acetonitrile laying nearly flat along the surface with the terminal methyl group directed away from the condensed phase. Upon increasing the bulk concentration, the formation of acetonitrile domains are promoted by interactions between hydrophobic methyl moieties. Dipole-dipole interactions facilitate a pseudonematic, antiparallel pairing of near-neighbor molecules, a behavior present in both bulk solution and near the liquid/vapor interface. In the latter, the preferred orientation of acetonitrile flattens further to accommodate antiparallel pairing of neighbor molecules, such that the methyl group remains above the solution, protected from unfavorable hydration. This study offers an interpretation of a binary liquid solution that manifests behavior similar to liquid-crystals through preferred orientations and pseudonematic antiparallel pairing.