Quantifying accumulation or exclusion of H+, HO−, and Hofmeister salt ions near interfaces

Recently, surface spectroscopies and simulations have begun to characterize the non-uniform distributions of salt ions near macroscopic and molecular surfaces. The thermodynamic consequences of these non-uniform distributions determine the often-large ion-specific effects of Hofmeister salts on a very wide range of processes in water. For uncharged surfaces, where these non-uniform ion distributions are confined to the first few layers of water at the surface, a two-state approximation to the distributions of water and ions, called the salt ion partitioning model (SPM) has both molecular and thermodynamic significance. Here, we summarize SPM results quantifying the local accumulation of H+, exclusion of HO−, and range of partitioning behavior of Hofmeister anions and cations near macroscopic and molecular interfaces. These results provide a database to interpret or predict Hofmeister salt effects on aqueous processes in terms of structural information regarding amount and composition of the surface exposed or buried in these processes.