Polymer, Water, and Salt Partitioning in Complex Coacervates Characterized by Femtosecond Stimulated Raman Microscopy

Complex coacervates can form through liquid–liquid phase separation in aqueous solutions containing oppositely charged macromolecules. This process results in macromolecule-rich droplets (coacervate phase) coexisting with a macromolecule-depleted supernatant phase. Here, femtosecond stimulated Raman microscopy (FSRM) is introduced as a tool to rapidly analyze both the supernatant and coacervate phases of complex coacervates. The well-known polyelectrolyte pair poly(diallyldimethylammonium chloride) and poly(4-styrenesulfonate) is investigated. Coacervate formation is induced by the addition of KBr and NH₄SCN as a Raman-active salt. For both salts, the partitioning of polymer and water between the coacervate droplets and the supernatant phase is quantified. For the Raman-active salt NH₄SCN, its partitioning between phases was also monitored. NH₄SCN was found to be enriched in the coacervate phase, as confirmed by FTIR spectroscopy. Overall, FSRM proves to be a valuable tool for collecting new data on coacervate composition requiring only low sample volumes and simple sample preparation, while offering convenient data acquisition.