Anion Fractionation and Reactivity at Air/Water:Methanol Interfaces. Implications for the Origin of Hofmeister Effects

Anions are selectively enriched in interfacial layers. This universal phenomenon, first identified in connection with protein precipitation 120 years ago, underlies fundamental processes. Its physical causes, however, remain conjectural. It has been speculated that the more polarizable anions should have larger affinities for air/liquid interfaces, and that their reactivities toward gaseous species would be affected by whether the liquid is capped by hydroxyl groups, as in water itself, or by hydrophobic layers of organic contaminants. These issues are particularly relevant to the composition and fate of atmospheric aerosols. Recently, we found that fractionation factors, f_(X^−), of simple anions at the air/water interface increase exponentially with ion radius, a_(X^−). In this paper, we report new experimental results on a set of anions that include the large PF_6^− and the highly polarizable IO_3^− species. A strict ln f_(X^−) ∝ a_(X^−) correlation is confirmed. Experiments performed in {x_wH_2O + (1 − x_w)MeOH} mixtures show that f_(X^−) is almost independent of x_w. Furthermore, O_3(g) oxidizes I^− at virtually identical rates on H_2O and MeOH.