Structure formation in polyelectrolytes induced by multivalent ions

We review recent investigations of aqueous solutions of polyelectrolytes in presence of multivalent ions. In particular, we look into the interaction of di- and trivalent counterions with star polyelectrolytes and spherical polyelectrolyte brushes. In presence of monovalent ions these systems are dominated by the marked osmotic pressure of the confined counterions. Higher-valent counterions lead to a collapse of these structures and to precipitation if the counterion concentration is raised beyond the critical coagulation concentration. This effect can be used to switch the structure of star polyelectrolytes between an extended and a dense conformation. No overcharging occurs in these systems. This is in contrast to the behavior of bare colloids where overcharging in presence of multivalent ions is a common phenomenon. A precise monitoring of the coagulation kinetics shows that the repulsive interaction between spherical polyelectrolyte brushes can be finely tuned by addition of trivalent ions. All experimental data obtained so far are well-understood by theory, at least in a semi-quantitative fashion.