Investigation of Self-Assembly and Micelle Polarity for a Wide Range of Ethylene Oxide-Propylene Oxide-Ethylene Oxide Block Copolymers in Water

Critical micelle concentrations (cmc) were determined for aqueous solutions of 15 EO-PO-EO (EO = ethylene oxide, PO = propylene oxide) block copolymers by using three different techniques: spectral changes of an iodine/iodide mixture, methyl yellow solubilization, and surface tension measurements. Two polymers showed no sign of micelle formation up to phase separation: one reverse (PO-EO-PO) and the other with low EO/PO ratio. The obtained cmc's were consistent for the different techniques employed and allowed the determination of the ΔG values associated with the micellization. These values were analyzed in terms of the EO and PO contributions, confirming the leading role of the hydrophobic PO units and revealing a small, but favorable, contribution of the hydrophilic EO chains, in contrast to the usual behavior of small nonionic surfactants. The polarity of the micelle solubilization sites was determined using the solvatochromic dye ET30, revealing environments similar to those observed for micelles of hydrocarbon surfactants. These findings, allied to the thermodynamic similarity between the micellization and the phase separation processes for the copolymers and PPO, support the view that the micellization is accompanied by a large dehydration of the PO units.