Miscibility and phase behavior in bulk and micellar systems of surfactant-polymer coacervate mixtures

Coacervates have become the focus of great recent attention due to their observation in living systems in the form of membraneless organelles. One of the interesting features observed for both cellular and in vitro coacervates is the immiscibility among some of them, leading to formation of so-called multiphase coacervates. This study addresses the issue as to whether two coacervates formed by a polyanion (polyacrylate, PAA) with a polycation (poly(diallyldimethylammonium, PDDMA⁺) or a cationic surfactant (C_nTA⁺) would be miscible in bulk or confined in complex core coacervate micelles, C3Ms, using an EO-PAA block copolymer for the latter.

Their mixtures were evaluated visually for bulk miscibility, or with DLS or cryo-TEM analyses, for their C3Ms. SAXS analyses were employed for characterization of the liquid crystalline structures, in both systems.

Our primary observation is that, although these coacervate systems exhibit similar interaction strengths, they remain immiscible when a surfactant liquid crystalline phase is present, both in bulk and within the micellar core. However, when this surfactant mesophase is disrupted, such as through the addition of salt, both systems become miscible. These findings emphasize the critical role of molecular ordering in the miscibility of coacervates, consistent with recent evidence highlighting the differences between liquid-liquid and liquid-liquid crystalline phase separation.