ION-BINDING AND ION SPECIFICITY - THE HOFMEISTER EFFECT AND ONSAGER AND LIFSHITZ THEORIES

Present theories of colloid science do not account for specific ion ef fects as exemplified by the Hofmeister effect, surface tension of elec trolyte solution interfaces, binding to micelles, effective charge in double layer interactions, and attractive interactions in low Hamaker constant systems. It is argued that specificity emerges naturally and can be rationalized if dispersion interactions acting on ions are incl uded in the theory. These are in principle accessible from bulk soluti on properties. Specific ion adsorption due to dispersion interactions can be dominant even at charged interfaces especially at high salt con centrations (similar to 0.1M). The effects can be qualitatively differ ent at air-water and oil-water surfaces. That part of extended Lifshit z theory for low Hamaker constant systems, in which the forces are mai nly due to temperature and salt dependent interaction, is re-examined. It is shown to be at the same level of approximation as, and precisel y equivalent to, the Onsager limiting law for the interfacial tension change due to dissolved salt at a single interface, i.e., to lineariza tion of the Poisson-Boltzmann distribution, and restriction to electro static potentials as the sole determinant of adsorption excesses. The usual description of interactions into separate electrostatic, and dis persion interaction, is invalid, even at the level of continuum (primi tive model) theories.