The Density and Rôle of Ions in Polyelectrolyte Complex Coacervates

The ion pairing responsible for the spontaneous formation of polyelectrolyte complex coacervates, PECs, may be partially reversed by adding small ions (“salt”). As pairs between positive, Pol⁺, and negative, Pol^-, polyelectrolyte repeat units are broken, the material becomes significantly more fluid-like. These “sticky interactions” may be reformed simply by washing out the salt ions with water. Quantitative equilibrium expressions may be derived for the distribution of salt ions inside and outside the PECs. The equilibrium is a balance between entropy (difference in ion density inside versus outside the PEC phase) and enthalpic effects such as ion specificity. Entropy may be calculated using the Donnan equilibrium, and enthalpy may be measured using sensitive calorimetry. This talk will cover a range of PEC/salt systems to illustrate a consistent picture for the rôle of ions in complexed polyelectrolytes.