Thermodynamic Characterization of Complex Coacervates of Oppositely Charged Biopolymers*

Complex coacervation occurs between two oppositely charged macromolecules which go into macroscopic phase separation and form two liquid phases: polyelectrolyte-rich (coacervate) and polyelectrolyte-poor (dilute) phase. There are several areas of application for coacervates in food, cosmetics, pharmaceuticals, and medical adhesives industries. Hyaluronic acid (HA) and chitosan (CH), which are semi-flexible biopolymers, are weak polyacids and polybases, respectively. In this study, isothermal titration calorimetry (ITC) and turbidity experiments were employed to understand the thermodynamics of complex formation and phase separation for this non-cognate biopolymer system. Parameters that affect coacervation (charge ratio of polyelectrolytes, pH, ionic strength, molecular weight of polyelectrolytes) were varied to determine binding constants, change in enthalpy, change in entropy, molar heat capacity, and stoichiometry of soluble complex formation and coacervation. By systematically changing these variables, we determined that HA/CH interaction was predominantly entropic as a result of counterion release.