Molecular thermodynamics of complex coacervate systems. Part II: Measuring and modeling of the phase envelope using pePC-SAFT

Abstract

Complex coacervation is an associative liquid-liquid phase separation (LLPS) observed in aqueous solutions of oppositely charged polyions. Coacervates are relevant systems in biology, chemistry, food and cosmetics industry, medicine as well as in engineering e.g. as extracting agents, for drug delivery or as gelling, foaming or stabilizing agents. Unfortunately, accurate experimental data on equilibrium compositions of complex coacervates are still scarce in the literature. Here, the LLPS of the coacervate-forming system water-Na₂NADH-protamine sulfate was measured at T = 298.15 K and p = 1.013 bar and at different polycation/polyanion ratios. Qualitative features of the experimental phase envelope are carefully discussed based on molecular interactions in this system. Compared to equilibrium data of the system water-Na₂NADH-poly-l-lysine HBr, the system water-Na₂NADH-protamine sulfate revealed a larger miscibility gap, suggesting a strong contribution of non-coulombic interactions to the phase behavior of this coacervate system. Experimental data were successfully modeled using the recently developed pePC-SAFT (Ascani et al., Part 1, Fluid Phase Equilibria, under review). The pePC-SAFT predicted phase envelope was in very good agreement with the measured experimental points. To the best of our knowledge, this is the first time that a physically sound model was used to model the phase envelope of a biologically relevant complex coacervate system.