By Yi Qin Gao
College of Chemistry and Molecular Engineering, Peking University
A simple theory of salt effects on water/air surface tension was formulated to take into account the interactions of individual ions with water. The theory predicts that the order of surface tension enhancement by salts is dependent on the solvation energy of cations and anions. The theory was then extended to understand how salts and other small molecules affect protein backbone solvation and protein secondary structure formation, with ion binding to the amide and ion pairing included. One of the most important predictions of the theory is that at low concentrations co-solutes rich in proton donors and those rich in proton acceptors have opposite effects and function as protein secondary structure denaturants and renaturants, respectively. For example, salts with strongly solvated hydrated cations and weakly solvated anions (e.g. MgI2 or even MgCl2) are expected to show strong salting-in effect, whereas salts with opposite cation versus anion properties, such as K2SO4, should have a strong salting-out effect. We also found that anion-cation cooperativity plays an important role in the understanding of the various experimental observations. MD simulations and thermodynamics calculations were then performed to understand ion-pairing in aqueous solutions, using a few simple salts as examples. In addition, it was found that the classical force fields with appropriately adjusted charge densities are capable of reproducing semi-quantatively the thermodynamic properties of electrolyte solutions.
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