Polymer Complexation: Partially Ionizable Asymmetric Polyelectrolytes
Studies of the thermodynamics of complex coacervation of pairs of symmetric, strongly ionizable, oppositely charged polyelectrolyte chains are abundant. To generalize such understanding to asymmetric chain lengths and variable ionizability (chemical charge density), frequently observed in experiments, we present a theoretical framework to analyze the effective charge and size of the complex and the thermodynamics of complexation of two polyions as a function of such asymmetries. The free energy ensuing from the Edwards' Hamiltonian undergoes variational extremization, and explicitly accounts for the screened Coulomb and non-electrostatic interactions among monomers within individual polyions and between two polyions. Assuming maximal ion-pair formation of the complexed part, the system free energy comprising configurational entropy of the polyions and free-ion entropy of the small ions is minimized. The thermodynamic drive for complexation is found to increase with the ionizability of the symmetric polyions and to be maximum for symmetric chain lengths for equally ionizable polyions. The effective charge and size of the complex increase with asymmetry in charge density, where the size can be substantially larger than a collapsed globule found for symmetric chains. The regimes of enthalpy- and entropy-driven complexation are found, respectively, for low and high Coulomb strengths. The crossover strength is found to be strongly dependent on the dielectric environment and salt, but marginally dependent on the charge density, thus implying an entropy-driven process at moderate strengths. The key results match the trends in simulations and experiments, and are expected to provide insight for asymmetric complexation in real systems.