Multivalent ions accumulate in interpolyelectrolyte complexes, also on the wrong side of the Donnan potential

Recently, interpolyelectrolyte complexes have been shown as good candidates for sequestration of ionic solutes, ranging from small ions to proteins.
A theoretical understanding of the mechanism of sequestration is desired in order to exploit this observation to be exploited in applications.
However, theories often fail to predict the partitioning of ions between interpolyelectrolyte complexes and the supernatant, even in the simple case of monovalent ions. In this work, we present a combination of simulations and experiments that allow us to follow the partitioning of monovalent and divalent ions in interpolyelectrolyte complexes with a variable excess of polycations or polyanions. We show that the concentration of monovalent ions in the polyelectrolyte complex is slightly higher than in the supernatant, in line with previous observations. In contrast with that, the concentration of divalent ions in the complex is much higher than in the supernatant. Our simulations using a simple coarse-grained model very well predict the experimentally observed trends in the partitioning of small ions.
Specifically, the partitioning of monovalent ions as a function of IPEC stoichiometry (charge ratio) can be rationalized by the Donnan theory whereas the divalent ones prefer the IPEC phase even in cases when Donnan theory predicts the opposite. Thus, we conclude that electrostatic interactions dominate the ion partitioning controlled by the valency, whereas ion-specific effects cause only slight deviations from this generic trend. Our results thus provide a conceptual basis for the future design of systems for sequestration of ions, based on interpolyelectrolyte complexes.