Can salting-in/salting-out ions be classified as chaotropes/kosmotropes?

Abstract

Attempts to understand the Hofmeister Series at the molecular level has yielded numerous hypotheses, many of which refer to the way different salts modify the structural and dynamical properties of water. The most famous, and at the same time the most controversial, is the classification of cosolutes and ions as structure-breakers (chaotropes) or structure-makers (kosmotropes), and their identification as salting-in and salting-out agents, respectively. In this paper, we present results from molecular dynamics simulations correlating the ion-induced changes of the structural and dynamical properties of water and the ability of these ions to alter the magnitude of the hydrophobic interaction. Although most of the properties of water in the salt solutions change monotonically with the ability of the salt to increase/decrease the hydrophobic interaction, none of them is able to predict the transition from salting-in to salting-out, a prediction that was observed by the preferential binding/exclusion analysis. In addition, we find that the use of the terms, kosmotropes and chaotropes, is very misleading since the so-called kosmotropes can actually reduce the structure of water, whereas, the so-called chaotropes can increase the structure of water. Specifically, we find that the ability of the ions to reduce the hydrophobic interaction (a property attributed to chaotropes), correlates with their ability to increase the structure between the water molecules, including the number and strength of hydrogen bonds, and as a consequence, the water-water interaction energy (features attributed to kosmotropes). Nevertheless, the viscosity (as well as the rotational decay rate) of the water molecules decreases (increases) due to weaker binding to the ions. Thus, it is not the ion-induced structural ordering between the water molecules that affect the dynamical properties of water, but the strength of the ion-water interaction. Our results indicate that attempts to understand and predict salt-induced modulation of hydrophobic interactions only through the binary, salt-water, system is not possible.