Effect of salts on the micellization, clouding, and solubilization behavior of pluronic F127 solutions

We have examined the effect of NaCl, Na2SO4, Na3PO4, 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 Na3PO4 > Na2SO4 > 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, Na2SO4, and Na3PO4 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 Na3PO4, 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. (C) 2000 Academic Press.