A Study of the Temperature-Dependent Micellization of Pluronic F127.

We have examined the temperature-dependent micellization of the pharmaceutically important PEO-PPO-PEO copolymer, Pluronic F127, using static light scattering and various aspects of the pyrene fluorescence spectrum (monomer intensity, excimer formation and the I1/I3 ratio). All techniques gave essentially the same value for the critical micellization temperatures (cmt) of various F127 solutions, and our results agreed with those reported in the literature. Cmt values decrease with increasing F127 concentration. We observed significant solubilization of pyrene in F127 solutions below the cmt, which was also reflected in the measured I1/I3 ratios. The thermodynamics of the micellization process were studied and gave different results at low and high F127 concentrations. In the low F127 concentration range (up to approximately 50 mg/mL), we obtain DeltaH = 312 kJ mol-1 and DeltaS = 1.14 kJ mol-1 K-1. Above 50 mg/mL we obtain DeltaH = 136 kJ mol-1 and a DeltaS = 0.54 kJ mol-1 K-1. This discontinuity in thermodynamic behavior can be due to a change in aggregation number with temperature and/or a change in the micellization process at higher concentrations. Copyright 1999 Academic Press.