Scattering Evidence of Charge Correlations in Polyelectrolyte Coacervates

Polyelectrolyte complex coacervation plays a crucial role in biological processes. Rational design of coacervate-based materials/assemblies requires understanding the internal structure of the complexes. Despite broad consensus among different theoretical methods, the existence of positional charge correlations in complex coacervates remains a hypothesis that has not been confirmed experimentally. Small angle scattering (SAS) experiments with X-rays (SAXS) or neutrons (SANS) are employed to study the internal structure of polyelectrolyte complex coacervates. Past SAS profiles have confirmed the structural similarity of polyelectrolyte complex coacervates to semidilute solutions of neutral polymers where the resultant total scattering function exhibits an Ornstein-Zernike (OZ) form. However, these studies have not provided insight into positional correlations of charges within the complexes. To access charge correlations, scattering from either the polyanions or polycations exclusively is required. We have achieved this through deuterium labeling of one type of polyelectrolyte and solvent contrast matching. The scattering function of polyanions exhibits the scattering peak at the correlation length scale, which gradually disappears and reconstitutes OZ form at high salt concentrations. Experimental scattering results are consistent with the RPA-based theoretical analysis and coarse-grained molecular dynamics simulations.