Molecular dynamics computer simulations have been used to explore the structural and dynamical properties of water's liquid/vapor interface using the simple extended point charge (SPC/E) model. Comparisons to the existing experimental and simulation data suggest that the SPC/E potential energy function provides a semiquantitative description of this interface. The orientation of H2O molecules at the interface is found to be bimodal in nature. The self-diffusion constant of water is calculated to be larger at the surface than in the bulk.

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. Massaccommodation 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.

Molecular dynamics computer simulations are utilized to study the structural and thermodynamic properties of the liquid/vapor interface of aqueous ethanol solutions as a function of concentration. In addition, the free energy profile for inserting a single ethanol molecule into a 0.059 mole fraction aqueous ethanol solution is calculated using statistical mechanical perturbation theory. The calculated free energy for solvation of an ethanol molecule in the bulk solution, the surface tension as a function of ethanol concentration, and the average orientation of ethanol molecules at the solution/vapor interface are in agreement with the corresponding experimental data. The calculated equilibrium free-energy profile, however, exhibits a barrier to solvation that is considerably smaller than that predicted by the resistance model for mass accommodation.