Thermodynamic insight into the thermoresponsive behavior of chitosan in aqueous solutions: A differential scanning calorimetry study

Thermoresponsivity of chitosan induced by beta-glycerophosphate (GP) in diluted aqueous solutions has been first studied by high-sensitivity differential scanning calorimetry. It has been found that the GP solutions of chitosan undergo a first-order phase transition upon heating. The onset of this transition coincides with the cloud point of the system. This allows one to identify the thermoresponsivity of chitosan as a macroscopic demonstration of the phase separation transition. The transition temperature, enthalpy, heat capacity increment, and width were determined as functions of GP and chitosan concentrations, and the dielectric constant of the solvent. Based on this data, we suggested that GP binds cooperatively to the chitosan matrix at low temperatures. The standard free energy of GP binding (Delta(b) g(int) = -6 +/- 1 kJ mol(-1)) was estimated from the DSC data. It was shown that the Okada-Tanaka model of cooperative hydration of polymers adequately describes the thermogram of the GP induced phase transition of chitosan.