Hofmeister salt effects on surface tension arise from partitioning of anions and cations between bulk water and the air-water interface

We apply a recently developed surface-bulk partitioning model to interpret the effects of individual Hofmeister cations and anions on the surface tension of water. The most surface-excluded salt (Na2SO4) provides a minimum estimate for the number of water molecules per unit area of the surface region of 0.2 H2O angstrom(-2). This corresponds to a lower bound thickness of the surface region of similar to 6 angstrom, which we assume is a property of this region and not of the salt investigated. At salt concentrations less than or similar to 1 m, single-ion partition coefficients K-p,K-i, defined relative to K-p,K-Na+ = K-p,K-SO42- = 0, are found to be independent of bulk salt concentration and additive for different salt ions. Semiquantitative agreement with surface-sensitive spectroscopy data and molecular dynamics simulations is attained. In most cases, the rank orders of K-p,K-i for both anions and cations follow the conventional Hofmeister series, qualitative rankings of ions based on their effects on protein processes ( folding, precipitation, assembly). Most anions that favor processes that expose protein surface to water (e.g., SCN-), and hence must interact favorably with (i.e., accumulate at) protein surface, are also accumulated at the air-water interface (K-p > 1, e.g., K-p,K-SCN- = 1.6). Most anions that favor processes that remove protein surface from water (e.g., F-), and hence are excluded from protein surface, are also excluded from the air-water interface (K-p,K-F- = 0.5). The guanidinium cation, a strong protein denaturant and therefore accumulated at the protein surface exposed in unfolding, is somewhat excluded from the air-water surface (K-p,K-GuH+ = 0.7), but is much less excluded than alkali metal cations (e.g., K-p,K-Na+ = 0, K-p,K-K+ = 0.1). Hence, cation K-p values for the air-water surface appear shifted ( toward exclusion) as compared with values inferred for interactions of these cations with protein surface.