A simulation study of the kinetics of passage of CO2 and N-2 through the liquid/vapor interface of water

The rate of passage of molecules of carbon dioxide and nitrogen through the vapor-liquid interface of water at 300 K is studied by simulation. Previou s work has established the form of the free energy profile which has a mini mum when the solute molecule is on the surface and a barrier between this s tate and solution in the bulk liquid. In one set of simulations, trajectori es were initiated in the gas phase. From these, the average lifetime of mol ecules in the surface is determined to be considerably longer than the inve rse of the energy relaxation rate, so that the sticking coefficient is one and exiting molecules have no memory of their original velocities. However, most molecules do return to the gas phase rather than entering the bulk so lution. The rate of passage of molecules over the free energy barrier is st udied using the reactive flux method with trajectories initiated near the t op of the barrier. The results for nitrogen, in particular, give a good pla teau in the time-dependent transmission coefficient and hence a reliable ra te constant. The results from these two sets of simulations are combined to give an effective interface width which is used to determine the permeabil ity of thin water films. These results are compared to experimental permeab ilities of thin Newton black soap films. The rate-determining step for solu tion in bulk water is not passage through the few Angstroms width of the in terface we study, but rather the transport from the vicinity of the interfa ce into the bulk over the larger distance scale of mu m. (C) 1999 American Institute of Physics. [S0021-9606(99)71029-9].