Temperature Effects on Salt Dependent Phase Separation in Complex Coacervate Systems

Complex coacervate systems utilize various forces to induce charge driven phase separation under unique conditions. Using the system of strong polyelectrolytes, poly(styrene sulfonate sodium salt) (PSS) and poly(diallyldimethyl ammonium chloride) (PDADMAC), we investigated how temperature impacted phase separation alongside more well known salt curves. Temperature based complex coacervation studies are hoped to lead to future discoveries supporting easier vaccine storage, which became extremely relevant recently with the COVID-19 mRNA vaccines expiring after short times at room temperature. These PSS/PDADMAC coacervates follow lower critical solution temperature (LCST) behavior, separating into two phases with increasing temperature; a dense polymer-rich coacervate phase and a polymer-poor supernatant phase. Water interactions influence the LCST behavior. The goal of this study is to take advantage of the salt-water interactions, and observe the changes it may have on the LCST behavior. We created ion-free polyelectrolyte complexes (PECs) to start with a clean slate in which new salts can be introduced. Salts introduced into the system were NaCl, KBr, NaBr, and KCl. UV-Vis spectroscopy is used to determine the PSS concentration in previously separated and centrifuged samples. These samples are forced back into solution phase by oversaturating the coacervate phase with salt, allowing for the back calculation of the original concentration of the temperature changed sample solution to be observed. These coacervate systems are often studied in the frame of holding and releasing materials through phase separation and solution. We hypothesize that by understanding the role water has on coacervation, we can better understand the driving forces that govern complex coacervation and the effect the salt identity has. This understanding can further the field of materials and polymers for use in temperature sensitive systems.