Salt Resistance as a Measure of the Strength of Polyelectrolyte Complexation

When mixed, solutions of positive and negative polyelectrolytes may spontaneously phase-separate into blended, hydrated complexes or coacervates (PECs). Charge-pairing interactions between oppositely-charged polyelectrolytes within PECs are weakened with the addition of salt MA. With a sufficiently high concentration of MA, the PEC may dissociate back into the individual polyelectrolytes, reversing the liquid-liquid phase separation induced by charge pairing and other interactions. This critical salt concentration (CSC), or "salt resistance," has been extensively used to compare the stability and strength of association in PECs. However, the CSC is not always observed, and it shows a strong dependence on the type of ions comprising MA. In addition, the CSC is more likely to be observed with PECs assembled from polycarboxylates, a type of weak polyelectrolyte. Here, it is shown that a lack of experimental CSC is correlated with the preferred role of ions M+ and A(-) in the PEC, counterion versus co-ion, or the specificity of a particular ion for a particular polyelectrolyte repeat unit, revealed by calorimetric measurements. The importance of the enthalpy of ionization of weak polyelectrolytes in providing an experimentally measurable CSC is quantitatively demonstrated.