John H. Dillon Medal Award (2022): Toward quantitative modeling of polyelectrolyte complexation

Recent activities in polyelectrolyte complexation (PEC) have been spurred by the advancements in theoretical descriptions of polyelectrolytes, and by the need to control PEC behavior in a wide range of applications. The basic principles behind PEC are well understood. However, quantitative PEC models have not achieved the same accuracy as models for neutral polymer solutions. A survey of new theoretical and experimental developments on thermodynamics of PEC suggests that electrostatic interactions, though not the only important factor, are essential for successful modeling of PEC. We show that the modern molecular theory for polyelectrolytes captures PEC behavior calibrated from carefully designed experiments. The effects of backbone polarity and mixing stoichiometry are presented as examples. An unexpected reentrant behavior revealed in non-stoichiometric solutions is highlighted, and is shown to originate from the interplay of the entropy of neutralizing counter-ions and the strength of electrostatic correlations. Finally, some open questions on accurate PEC modeling are discussed.