Effect of Alcohols on the Micelle Formation of Nonionic Surfactants in Aqueous Solutions

Abstract

The variation of critical micelle concentrations (CMC) of polyoxyethylene lauryl ethers having various ethylene oxide chain lengths \barn(\barn=6, 11, 20, and 31) on addition of short chain alcohols to the surfactant aqueous solutions was determined at 20 °C. Thermodynamic quantities of micelle formation in methanol– and ethanol–water mixtures and in pure water were obtained in connection with the heat of micelle formation determined by direct calorimetry. Methanol and ethanol showed a CMC-increasing effect only, whereas the addition of n-propanol, n-butanol, and n-pentanol showed a CMC-decreasing effect. The overall CMC-increasing effect is interpreted as being mainly due to the fact that methanol and ethanol weaken the hydrophobic bond. However, the effect also has a secondary effect of lowering the CMC slightly on the polyoxyethylene portion of a surfactant. Several factors are suggested for explaining this secondary effect. The CMC-decreasing effect is explained by a decrease in the free energy of mixing resulting from the solubilization of alcohol molecules into the micelle.