The passage of gases through the liquid water vapour interface: a simulation study

We have used atomistic simulation to measure the free energy profiles for a number of molecules crossing the air/liquid water interface at room temperature. The principal molecules studied were CO2 and N-2, both common molecules with quadrupole moments; in addition some results were obtained for CH3CN, an example of a dipolar molecule, and Ar which has no electrostatic interaction with water. Our aim is to establish these profiles in order to provide a foundation for understanding the kinetics of gas uptake across the vapour/liquid interface and to understand them in terms of local structure at the molecular level. We found that there is a free energy minimum corresponding to a surface-adsorbed site in each case and that for N-2 and CO2 there is a free energy barrier to passage from the bulk solution to the surface-adsorbed site. We discuss these results in terms of the local structure and in relation to some theoretical models of gas uptake.