Calorimetric study of the effect of urea, guanidinium salts, and alcohols on the micellization of dimethyldecylamine oxide

Abstract

A thermal titration calorimeter, suited to rapid and accurate measurement of heats of micellization of surfactants, is described. The critical micelle concentration (CMC) and standard heat of micellization ( ΔH m o ) for dimethyldecylamine oxide are determined in solutions of urea, guanidinium salts, and alcohols of varying concentrations. The entropy and enthalpy of micellization are reduced in all solutions, relative to water, with the exception of 1-butanol solutions. The results can be interpreted as a reduction of the amount of ordered water released during micellization of the surfactant—an effect associated with the involvement of hydrophobic bonding in the process. Urea and guanidinium salts weaken these hydrophobic bonding contributions to micellization. Alcohols appear to play a dual role of weakening the surfactant's hydrophobic contributions while favoring micelle formation at high enough alcohol chain length owing to probable incorporation in the micelle. This is inferred from the increase in ΔH m o values in 1-butanol solutions and at high concentrations of methanol, ethanol, and 1-propanol. With ethanol at high concentration, the CMC is not sharp.