The passage of small molecules through a water film supported by MgO(100):Transfer times from molecular dynamics simulations

The passage of small pollutant molecules (HCl, CO2) through a thin water fi lm supported on a MgO substrate at 300 K has been studied by constrained cl assical molecular dynamics simulations. The calculated free energy profile of the pollutants exhibits two minima, one at the gas/liquid film interface , and the other inside the film near the ionic substrate. Lifetimes of the pollutants in these two sites have been characterized by unconstrained simu lations. The residence times in these sites are in the range of a few tens of picoseconds. The transfer times from one site to the other, and the time s spent by the pollutants in the liquid and at the liquid/gas interface (si milar to one hundred ps) are always twice longer for CO2 than for HCl. This difference is interpreted in terms of correlated dynamics of HCl and H2O d ue to hydrogen bond interactions with water. The duration of the hydrogen b ond Cl-H ... O increases significantly (similar to 2 ps) at the film surfac e with respect to its value inside the film (< 1 ps). (C) 2000 American Ins titute of Physics. [S0021-9606(00)70727-6].