A study of the temperature-dependent micellization of pluronic F127

We have examined the temperature-dependent micellization of the pharmaceuti cally important PEO-PPO-PEO copolymer, Pluronic F127, using static light sc attering and various aspects of the pyrene fluorescence spectrum (monomer i ntensity, excimer formation and the I-1/I-3 ratio). All techniques gave ess entially the same value for the critical micellization temperatures (cmt) o f various F127 solutions, and our results agreed with those reported in the literature. Cmt values decrease with increasing F127 concentration. We obs erved significant solubilization of pyrene in F127 solutions below the cmt, which was also reflected in the measured I-1/I-3 ratios. The thermodynamic s of the micellization process were studied and gave different results at l ow and high F127 concentrations. In the low F127 concentration range (up to similar to 50 mg/mL), nle obtain Delta H = 312 kJ mol(-1) and Delta S = 1. 14 kJ mol(-1) K-1. Above 50 mg/mL we obtain Delta H = 136 kJ mol(-1) and a Delta S = 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 cha nge in the micellization process at higher concentrations, (C) 1999 Academi c Press.