Ion-specific effects
Project of Research Lab of Nadine Schwierz
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Reversed Hofmeister series—The rule rather than the exception
Ion specific effects are ubiquitous. The term denotes the fact that most aqueous physicochemical processes not only depend on ion concentration and valency, but also on the ion type. Early on, it was recognized that anions and cations can be ordered reproducibly according to their efficiency to precipitate proteins from solution into the Hofmeister series.
The identification of the mechanism underlying this series and the implementation into a generally applicable theory remain challenging. Moreover, the universality of the Hofmeister series is called into question by state-of-the art experiments which reveal a whole spectrum of direct, altered and reversed series.
Our work focuses on describing this diverse spectrum of experimentally observed phenomena. Primary insight into the origin of the Hofmeister series and its reversal is gained from simulation-derived ion–surface interaction potentials at surfaces containing non-polar, polar and charged functional groups for halide anions and alkali cations. In a second step, the detailed microscopic interactions of ions, water and functional surface groups are incorporated into Poisson–Boltzmann theory. This allows us to quantify ion-specific binding affinities to surface groups of varying polarity and charge, and to provide a connection to the experimentally measured long-ranged electrostatic forces that stabilize colloids, proteins and other particles against precipitation. Based on the stabilizing efficiency, the direct Hofmeister series is obtained for negatively charged hydrophobic surfaces. Hofmeister series reversal is induced by changing the sign of the surface charge from negative to positive, by changing the nature of the functional surface groups from hydrophobic to hydrophilic, by increasing the salt concentration, or by changing the pH. The resulting diverse spectrum reflects that alterations of Hofmeister series are the rule rather than the exception and originate from the variation of ion-surface interactions upon changing surface properties.