Thermodynamics of micellization and oil solubilization in block copolymers P85

Thermophysical properties of aqueous pluronic solutions of P85 and P85–P123 systems with concentrations varying between 0.1 and 11 mass% for P85 and equimolar solution of 11 mass% P85-10 mass% P123 were measured in the absence and presence of solubilized lavender oil at temperatures below and above their critical micellization temperature in the temperature range of 288.15 K to 323.15 K. Maximum oil solubility was 3% (v/v) in the case of 11 mass% P85 and 2% (v/v) for mixed P85–P123 system. From density and sound speed measurements thermodynamic parameters like apparent molar volume, isentropic compressibility and expansibility were derived to evaluate the interactions of these block copolymers below and above the micellar region in pure and mixed micellar states. Apparent molar isentropic compressibility exhibited unusual variation with concentration which could be explained in light of pre-micellar aggregate formation. Isothermal titration calorimetry (ITC) was used to evaluate the energy changes involved in the aggregation processes for these systems. Critical micelle concentration (cmc) for P85 was found to be 0.74 mass% and pre-micellar aggregation was observed at 0.276 mass% from ITC study. Enthalpy of pre-micellar aggregation and micellization for oil-solubilized P85 solution were found to be 60 kJ mol−1 and 15 kJ mol−1 in the absence of oil. Micellization enthalpy for P85–P123 system was obtained as 72 kJ mol−1 and 142 kJ mol−1, respectively, in the absence and presence of solubilized oil. No pre-micellar aggregation was observed in mixed micelle system. DLS study was carried out to corroborate the findings obtained from densimetry and calorimetry.