Enhanced Concentration Fluctuations in Model Polyelectrolyte Coacervate Mixtures along a Salt Isopleth Phase Diagram

Abstract

A temperature (T)-versus-polymer concentration (cp) representation leads to nonoverlapping coexistence curves prepared from different initial polymer concentrations along a salt isopleth of aqueous mixtures of charge-stoichiometric, oppositely charged polydisperse polyelectrolytes. This effect was explained by an unequal salt concentration (cs) between equilibrating phases that improves the relationship between the common cs–cp representation and T–cp along a salt isopleth in terms of a T–cs–cp phase envelope. Further quantification of the cloud points and spinodal temperatures predicts regions of metastability and instability with critical points on a salt isopleth phase diagram. As the cloud point is approached, the correlation length for concentration fluctuations far exceeds the chain dimensions with evidence of a crossover from mean field to fluctuation regime. The effective critical exponents for the divergence in the osmotic compressibility, via scattered intensity to zero angle and correlation length, γeff and νeff, respectively, deviates from the 3D Ising model. These static light-scattering measurements illustrate that concentration fluctuations are enhanced by polyelectrolyte chain association near the lower critical solution temperature.