ABSTRACT
We have examined the effect of NaCl, Na(2)SO(4), Na(3)PO(4), and NaSCN on F127 solutions; properties examined were critical micellization temperature (cmt), cloud point, and solubilization of a model hydrophobic drug, propyl paraben. Static light scattering showed that the first three salts lower the cmt of F127 in the order Na(3)PO(4)>Na(2)SO(4)>NaCl. The extent of lowering depends on the salt concentration and can be ascribed to the water structure-making properties of these salts. NaSCN, a water structure breaker, was found to increase cmt. Pyrene fluorescence was used to study the changes in micellar interior in the presence of salts. We found that the micellar micropolarity is not significantly changed by salts, evidenced by a constant I(1)/I(3) ratio of pyrene. However, the I(e)/I(3) ratio changes significantly with salts, being lower for NaCl, Na(2)SO(4), and Na(3)PO(4) and higher for NaSCN. This is consistent with an increase in the total hydrophobic micellar domain, in micellar microviscosity, or both. Solubilization of propylparaben increases in the presence of Na(3)PO(4), consistent with a larger hydrophobic domain for solubilization. The thermodynamics of micellization continue to be entropically driven in the presence of salts, evidenced by a positive entropy overcoming an unfavorable enthalpy. Copyright 2000 Academic Press.
ABSTRACT
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.
