Hofmeister Effects

Abstract

Hofmeister effects, also known as specific ion effects, are variations in interactions between particles and surfaces or other particles in salt solutions that depend on the specific identity of the ions comprising the salt. Na+ is the dominant cation in physiological fluids, Li+ is used to treat bipolar disorder, K+ is used in lethal injections. The differences matter, and yet these ions are identical if considered solely as simple point charges. Hofmeister effects are ubiquitous, observed in protein aggregation, surface tension, bubble coalescence, pH. The origin of specific ion effects can be understood as a consequence of electron structure, both in ion size and in polarizability of the electron cloud. Not only the intrinsic size of the ion but also recent theoretical developments suggest the formation of the cavity in the solvent, in which the ion resides, plays a key role in determining these effects, alongside ionic van der Waals interactions driven by ion polarisability.