Accommodation of Alcohols by the Liquid/Vapor Interface of Water:  Molecular Dynamics Study

Molecular dynamics computer simulations have been used in conjunction with statistical mechanical perturbation theory to examine the process by which water, ethanol, and ethylene glycol (1,2-ethanediol) molecules are transported from the vapor phase into bulk water. The calculated energetics for solvation in the bulk liquid and surface properties such as the surface tension of water and orientations of ethanol and ethylene glycol adsorbed at the water interface agree well with the corresponding experimental data. Currently, the uptake of trace species by water droplets is generally modeled by decoupling the process of mass accommodation at the interface from gas- and liquid-phase diffusion and then coupling the independent processes using an electrical resistance model. In the resistance model, the mass-accommodation coefficient is a measure of the competition between the kinetics of solvation into the bulk liquid and the kinetics of desorption back into the gas phase. Interpreting experimental uptake rates of a variety of solute molecules by water using the resistance model requires mass-accommodation coefficients that are less than 0.5. Mass-accommodation coefficients less than 0.5 imply that the rate of desorption is greater than solvation, thus suggesting that the free energy of activation for desorption is less than that for solvation. The calculated equilibrium free-energy curves for transporting water, ethanol, and ethylene glycol molecules across the liquid/vapor interface and into bulk water exhibit barriers to solvation that are considerably smaller than those implied by the resistance model. Nonequilibrium solvation or dynamical solvent effects on the calculated activation free energies have also been estimated and are shown to be too small to account for the large difference in comparison with the resistance model. In addition, the temperature dependence of this barrier for ethanol has been calculated. Although this dependence agrees with that predicted by the resistance model, the heights of the calculated barriers are again much lower than those predicted by the model.