Molecular dynamics simulations of water transport through butanol films.

It is well-known that well-formed monolayers of long-chain alcohols on water dramatically reduce the rate of water evaporation. Recent experiments, however, have shown that the short-chain surfactant butanol has a negligible effect on the rate of water evaporation from a solution of supercooled sulfuric acid. We used molecular dynamics simulations to examine the condensation of water through a layer of butanol coating water. From those simulations, we observed a reduction in the rate of water condensation by a factor of 3 which would imply a similar reduction in the rate of evaporation. It was observed that a vapor molecule would condense only if it formed a long-lived hydrogen bond with another molecule and that there was no dependence on the initial velocity of the vapor molecule. The ability of the surfactant to participate in hydrogen bonding was found to facilitate the condensation of water into the bulk.