Free energy of micellization of polyoxyethylene surfactants
Résumé
Micellar properties of four homogeneous polyoxyethyleneglycol n-dodecyl ethers C12H25(OCH2CH2)iOH (i = 3, 5, 7 and 9) were examined in the temperature range 15-80°C. For each surfactant, the variation in the critical micelle concentration (c.m.c.), deduced from surface tension measurements, exhibited a minimum as a function of temperature. The hydroxyl group contribution to the free energy of micellization was found to be higher than that of the ethylene oxide group, especially at the lowest temperatures. When temperature increased, the hydroxyl group contribution decreased as a result of a dehydration, while the ethylene oxide contribution remained practically constant. As an illustration of the possible uses of the contributions determined, enthalpies of micellization were calculated for surfactants of the same family but having different chain lengths. These enthalpies were in reasonable agreement with values directly derived from calorimetric measurements.