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.