Charge Density and Hydrophobicity-Dominated Regimes in the Phase Behavior of Complex Coacervates

We investigate the phase behavior of coacervates formed from a library of 54 acrylamide-based copolymers with charge densities ranging from 40-100% and nonionic aliphatic sidechains ranging from zero to twelve carbons in length. The phase behavior of the coacervates is characterized by optical turbidity, revealing three distinct regimes of complex coacervate behavior. For polymers with hydrophilic sidechains, the salt resistance of the polymers decreases with decreasing charge density, while for polymers with the longest hydrophobic sidechains, the salt resistance increases with increasing hydrophobic content. For polymers with intermediate length hydrophobic sidechains, however, there is a critical charge density at which the phase behavior switches from being dominated by the charge density to being dominated by the hydrophobic interactions. Comparison with the solubilities of the component polyelectrolytes suggests that the phase behavior of these materials cannot be understood simply in terms of changes in the polymer-water interaction parameter, motivating an urgent need for models of coacervate phase behavior that accurately reflect the molecular-scale interactions at play.

Funding: American Chemical Society Petroleum Research Fund (58034-DNI7)