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 Å-2. This corresponds to a lower bound thickness of the surface region of ∼6 Å, which we assume is a property of this region and not of the salt investigated. At salt concentrations ≲1 m, single-ion partition coefficients Kp,i, defined relative to Kp,Na+ = Kp,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 Kp,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 (Kp >1, e.g., Kp,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 (Kp,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 (Kp,GuH+ = 0.7), but is much less excluded than alkali metal cations (e.g., Kp,Na+ ≡ 0, Kp,K+ = 0.1). Hence, cation Kp values for the air−water surface appear shifted (toward exclusion) as compared with values inferred for interactions of these cations with protein surface.