Polar mixtures under nanoconfinement

JAVIER RODRIGUEZ, M. DOLORES ELOLA AND DANIEL LARIA

Viena

Conferencia; 8TH Liquid Matter Conference; 2011

European Physical Society

We present results from molecular dynamics simulations describing structural and dynamicalcharacteristics of equimolar mixtures of water and acetonitrile, confined between two silica wallsseparated at inter-plate distances d = 0.6, 1 and 1.5 nm. Two different environments were investigated:a first one, where wall-solvent dispersion forces prevail (hydrophobic confinement) and asecond one, in which the terminal O-atoms at the silica surface are transformed into silanol groups(hydrophilic confinement). For the former case, we found that, at the shortest interplate distanceexamined, the confined region is devoid of water molecules. At interplate distance of the order of 1nm, water moves into the confined region, although in all cases, there is a clear enhancement of thelocal concentration of acetonitrile in detriment of that of water. Within hydrophilic environments,we found clear distinctions between a layer of bound water lying in close contact with the silicasubstrates, and a minority of confined water that occupies the inner liquid slab. The bound aqueouslayer is fully coordinated to the silanol groups and exhibits minimal hydrogen bonding with thesecond solvation layer, which exclusively includes acetonitrile molecules. Dynamical characteristicsof the solvent mixture are analyzed in terms of diffusive and rotational motions in both environments.Compared to bulk mixtures, we found significant retardations in all dynamical modes,being the most dramatic, those ascribed to water molecules bound to the hydrophilic plates.