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 temper ature. The principal molecules studied were CO2 and N-2, both common molecu les with quadrupole moments; in addition some results were obtained for CH3 CN, an example of a dipolar molecule, and Ar which has no electrostatic int eraction with water. Our aim is to establish these profiles in order to pro vide a foundation for understanding the kinetics of gas uptake across the v apour/liquid interface and to understand them in terms of local structure a t the molecular level. We found that there is a free energy minimum corresp onding to a surface-adsorbed site in each case and that for N-2 and CO2 the re is a free energy barrier to passage from the bulk solution to the surfac e-adsorbed site. We discuss these results in terms of the local structure a nd in relation to some theoretical models of gas uptake.