Effect of Solvent Quality on the Phase Behavior of Polyelectrolyte Complexes

The role of polyelectrolyte-solvent interactions, among other non-Coulomb interactions, in dictating the thermodynamics and kinetics of polyelectrolyte complexation is prominent yet sparingly studied. In this article, we present systematic comparisons of the binodal phase behavior of polyelectrolyte complexes (PECs) comprising polyelectrolytes with varying quality of backbone-solvent interactions. Experimental phase diagrams of polyelectrolyte complexes with either a peptide or an aliphatic backbone highlight the influence of backbone chemistry on the compositions of complexes and their salt resistance. Corresponding theoretical phase diagrams, obtained from a framework combining the random phase approximation and the Flory-Huggins approach, reveal a transition from closed phase boundaries, with confined two-phase regions for PECs in good solvents, to open phase boundaries, wherein two-phase systems are predicted to exist even at very high salt concentrations, for PECs in poor solvents. These predicted trends compare qualitatively well with experimental observations of low salt resistance (∼1 M NaCl) of PECs comprising hydrophilic polyelectrolytes and persistence of complexes, stabilized by short-range hydrophobic interactions, even at very high salt concentrations (∼6 M NaCl) for PECs comprising hydrophobic polyelectrolytes.