6 - The extension of the Lifshitz theory to include electrolytes and Hofmeister effects

Summary

Inclusion of electrolytes and Hofmeister effects in the theory

We have seen that the classical theories of electrolytes are limiting laws that are strictly valid, if at all, at very low concentrations. That is so for the Born self energy, the correlation free energy of Debye–Hückel theory, the interfacial tension of electrolytes represented by Onsager's theory, and the double-layer theory of interactions between charged colloidal particles. These theories ignore water structure, hydration, induced by ions. They also ignore dispersion interactions between ions and between ions and surfaces. In reality, with moderate salt concentrations of interest in biology (> 0.1 M) strong specific ion effects emerge. These effects can be taken into account by introducing more (unquantified) parameters that attempt to build in hard core interactions, local ‘water structure’ and overlap of hydration shells. But even with such extensions, the parameters vary from one situation to another. For example, the double-layer forces between the cationic bilayers of Chapter 4 differ by more than an order of magnitude with sodium bromide or sodium acetate at concentrations as low as 10⁻³ M! This is entirely unsatisfactory. A theory of electrolytes that deserves the name ought to work predictively for all electrolytes.

As a first step to throwing some light on the problem, we here address how to build a theory that includes the missing dispersion forces together with electrostatics.