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

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