On the Nature of the Ion-Specific Effects

Abstract

This article reports the effect of the counter-ions on the ionic surfactant adsorption layer and its relation to the stability of colloidal dispersions. The adsorption theory of Davies about the ionic surfactant monolayer is revisited and explored. Adsorption at the water|gas interface is compared to the water|oil interface, and the applicability of Langmuir’s concepts for distributing pressure and liquid expanded film is shown. The detailed analysis clarifies the effects of the medium and the surfactant structure (hydrocarbon chain length, the nature of the container) on the adsorption. Extra accent is put on the Hofmeister effect of the counter-ion on the adsorption of ionic surfactants. The simple model of the van der Waals interaction of an ion with the interface was revisited, so taking into account the quantitative interpretation of the Hofmeister effect on the adsorption parameters. The core of this model stands a quantity called ion specific adsorption energy ui0, which is related straightforwardly to basic ion characteristics, and is calculated for the counter-ions typically used in practice. An extended comparison with tensiometric and other experimental data demonstrates the usefulness of the theoretical models. The universality of the concept for the ion specific adsorption energy ui0 is further demonstrated by considering the role of the Hofmeister effect in two other phenomena, related to ionic surfactants: the disjoining pressure on thin liquid films and the stability of foams and emulsion. Unfortunately, the experimental data on the stability of foams and emulsions shows an opposite trend to our theoretical expectation – the stability increases upon the increase of the absolute value of the of the specific adsorption energy of the counter-ions in the water|gas or water|oil interfaces. It was shown abnormal effect of the potassium counter-ion on the stability of foams. The later either act as a foamer or de-foamer depending on its
concentration. To clarify the nature of this effect further investigations are required.