Molecular-weight-driven partitioning in polyelectrolyte complexes

Complexation of oppositely charged polymers is influenced by a range of molecular parameters, including molecular weight (MW), polarity, stoichiometry, etc. Relatively little is known of how disperse polyelectrolytes are partitioned between the complex and supernatant phases. We investigate MW-driven partitioning by mixing polyanions of different MWs with homologous polycation. Using three uniquely-dye-labeled polyelectrolytes, we find that high-MW polyanions are enriched in the complex phase with partition coefficients that depend on the ratio of MWs. These observations are attributed to the translational entropy of polyanions, and the experimental trends are semi-quantitatively captured by a theoretical analysis that balances the translational entropy and electrostatic correlation free energy.