Interpreting the H/D isotope fractionation of liquid water during evaporation without condensation

A theoretical model of liquid water evaporation has been developed to interpret results from a recent experimental investigation of isotope fractionation during free evaporation [Cappa et al. J. Phys. Chem. B 2005, 109 (51), 24391]. It is established that the free evaporation isotope fractionation factors (alpha(evap)) are primarily influenced by the nature of the intermolecular interactions between water molecules, namely, the condensed phase hindered translational and librational frequencies at the surface. The dependence of alpha(evap) on the isotopic composition of the liquid can be understood in terms of small variations in these frequencies with isotopic composition. This result suggests that the explicit nature of the solvation environment directly influences evaporation rates from liquids. The sensitivity of the calculated evaporation coefficient for liquid water to both temperature and isotope composition is also explored.