Stimuli-Induced Non-Equilibrium Phase Transitions in Polyelectrolytesurfactant Complex Coacervates

Polyelectrolyte-surfactant complexes (PESCs) are important soft colloids with applications in the field of personal care, cosmetics, pharmaceutics and much else. If their phase diagrams have long been studied under pseudo-equilibrium conditions, and often inside the micellar or vesicular regions, understanding the effect of non-equilibrium conditions, applied at phase boundaries, on the structure of PESCs generates an increasing interest. In this work we cross the micelle-vesicle and micelle-fiber phase boundaries in an isocompositional surfactantpolyelectrolyte aqueous system through a continuous and rapid variation of pH. We employ two microbial glycolipid biosurfactants in the presence of polyamines, both systems being characterized by their responsiveness to pH. We show that complex coacervates (Co) are always formed in the micellar region of both glycolpids’ phase diagram and that their phase behaviour drives the PESCs stability and structure. However, for glycolipid forming single-wall vesicles, we observe an isostructural and isodimensional transition between complex coacervates and a multilamellar walls vesicle (MLWV) phase. For the fiber-forming glycolipid, on the contrary, the complex coacervate disassembles into free polyelecrolyte coexisting with the equilibrium fiber phase. Last but not least, this work also demonstrates the use of microbial glycolipid biosurfactants in the development of sustainable PESCs.