Molecular Simulation and Spectroscopy of a Strong Dipolar Fluid*

Acetonitrile is an amphiphilic molecule with a large dipole moment whose complex behavior in the liquid state belies its relatively simple molecular structure. Here we present both molecular simulations of the bulk and interfacial liquids and experiments using neutron scattering and optical Kerr effect spectroscopy to probe the bulk and nanoconfined liquids. Using angularly resolved radial distribution functions, we identify a complex microscopic structure in the simulated bulk liquid in which most liquid molecules are associated with one or more neighboring molecules in antiparallel or head-to-tail configurations. This structural picture is associated with pairing times longer than the typical rotational and translation time constants. In contrast to the bulk liquid, acetonitrile is known to adopt an interdigitated, bilayer-like organization at hydroxylated silica interfaces. We present initial results of molecular simulations on such systems and spectroscopic results on nanoconfined acetonitrile, including putative signatures of hydrogen bonding at the liquid/silica interface.