POLAR MIXTURES UNDER NANOCONFINEMENT

LARIA, DANIEL HECTOR

Brno

Conferencia; 8th Liblice Conference on the Statistical Mechanics of Liquids; 2010

Czech Academy of Science

POLAR MIXTURES UNDER NANOCONFINEMENTWe present results from Molecular Dynamics experiments describing structural and dynamical characteristics of water (W)-acetonitrile (ACN) mixtures confined in three different environments of nanometric dimensions.        The first one corresponds to two silica walls separated at distances d= 0.6, 1 and 1.5 nm with        different hydrophobic characteristics. For fully hydrophobic environments, we found that, at the        shortest interplate distance examined, the confined region is devoid of water molecules. At        interplate distance of the order of 1 nm, water moves into the confined region, although in all        cases, there is a clear enhancement of the local concentration of ACN in detriment of that of W.        Dynamical characteristics of the solvent mixture are analyzed in terms of diffusive and rotational        motions in both environments.        In the second case, the confinement is provided by a membrane of (16,16) carbon nanotubes connecting        two reservoirs, initially filled with W and ACN. Within the hydrophobic nanotube cavities, the        equilibrium concentrations contrast sharply to those observed at the reservoirs, with a clear        enhancement of ACN, in detriment of W. From the dynamical side, the relaxation involves three well        differentiated stages: the first one corresponds to the equilibration of individual concentrations        within the nanotubes. An intermediate interval with Fickian characteristics follows, during which the        overall transport can be casted in terms of coaxial opposite fluxes, with a central water domain        segregated from an external ACN shell, in close contact with the tube walls. We also found evidence of a third, much slower, mechanism to reach equilibration, which involves structural modifications of        tightly bound solvation shells, in close contact with the nanotube rims.        The third environment correspond to cylindrical silica nanopores with diameter close to 3 nm. In        these cases our analysis will focus on how the structural and dynamical characteristics of the        confined liquids are modified by the functionalization operated at the pore walls.