Effect of alcohols on the heat of micellization of Pluronic F88 aqueous solutions

The effects of addition of alcohols (ethanol, 1-propanol, and 1-butanol) to Pluronic F88 aqueous solutions on heat of micellization of Pluronic F88 are carefully examined by using high sensitivity differential scanning calorimeter and dynamic light scattering. With addition of ethanol to Pluronic F88 aqueous solutions, the critical micelle temperature (CMT) increases, the heat of micellization significantly decreases, and the F88 concentration region of the unusual positive heat capacity change (attributed to impurity aggregates) occurs at higher Pluronic F88 concentrations. The addition of ethanol increases the solubility of copolymer and impurities and, hence, hinders the formation of impurity aggregates. Introduction of butanol to F88 aqueous solutions would significantly reduce the CMT but only slightly change the heat of micellization, indicating exothermic interactions of 1-butanol with poly(propylene oxide) blocks. With addition of 1-propanol, both CMT and heat of micellization decrease. With addition of 5 wt.% 1-propanol would substantially expand the F88 concentration region of the unusual positive heat capacity change, indicating a significant dissociation of impurity aggregates into micelles. In addition, the effect of alcohols on the cloud points of Pluronic F88 solutions is also carefully examined and discussed. The enthalpy-entropy compensation phenomenon for the micellization of Pluronic F88 over the concentration regions of negative/positive heat capacity changes is discussed.