Structure of a salt–amphiphile–water solution and the mechanism of salting out

Abstract

Salting out of amphiphiles from aqueous solution is a process of central importance in aqueous solution chemistry and of major application in the concentration of macromolecules and the crystallization of proteins. Despite its importance, our understanding of the mechanism is poor, there being no direct experimental evidence to support the several competing explanations in the literature. Using neutron diffraction with isotope substitution, we report a structural study of the effect of adding a simple salting out agent (NaCl) to a dilute solution of an amphiphile (t-butanol). The results show clearly that the anion is central to driving the changes in association of the amphiphile through the formation of an anion bridge between the polar ends of neighboring alcohol molecules. This further exposes the nonpolar surface of the amphiphile and suggests that further association may then occur through hydrophobic interaction. The mechanism uncovered is both different from those currently in the literature and relatively simple, and offers a possible route forward to understanding the variation in salting out efficacy of different ions as indicated by the Hofmeister or lyotropic series.