Probing charge inversion in model colloids: electrolyte mixtures of multi- and monovalentcounterions

Abstract

Under certain conditions ion–ion correlations play a crucial role in the description of theelectrical double layer of colloidal particles. In fact, in many instances, the inclusion of theshort range correlations between ions in the study of the ionic distribution leads to quitedifferent results with respect to the classical treatment (where ions are assumed to bepoints). In particular, these discrepancies become more noticeable for highly chargedparticles in the presence of moderate or highly multivalent counterion concentrations.Moreover, it can be shown that the existence of an electrolyte mixture consisting ofmulti- and monovalent counterions may cause that system to become overcharged,a feature that cannot be predicted from a classical point of view based on theBoltzmann distribution function. Precisely this aspect has recently produced anenormous interest in the field of biophysics since small variations in the physiologicalconditions of biocolloidal systems (e.g. the addition of a multivalent salt) can induceimportant changes in their behaviour. In order to determine the relevance ofion correlations in electrolyte mixtures, we present some experimental resultson the electrophoretic mobility of latex particles in the presence of different 1:1and 3:1 salt mixtures. Likewise, these results are analysed within the so-calledhypernetted-chain/mean spherical approximation where ion size correlations are taken intoaccount.