Effect of position of ester group and temperature on critical micelle concentration of ester containing surfactants

Abstract

AbstractThe critical micelle concentrations of sodium sulfoalkyl alkanoates, CnH2n+1 COO(CH2)mSO3Na (n=9, 10, and 11; m=2, 3, and 4), in aqueous solution were measured by an electro conductivity method from 15 to 50 C. Plots of critical micelle concentrations versus temperature show a minimum for all of the surfactants. Increasing the number of methylene groups between the ester and sulfonate groups in the surfactant molecule lowered the temperature at which the minimum critical micelle concentration was observed. When the percentage deviation of critical micelle concentration values obtained at different temperatures from critical micelle concentration values at 30 C was plotted as a function of the temperature, deviation in the range below 30 C increased with length of the hydrocarbon chain in the fatty acid portion of the surfactant, and decreased with the increase in length of the hydrocarbon chain situated between ester and sulfonate groups. These phenomena suggest that the change of interaction between the ester group and water molecule with the temperature is largely dependent on the position of the ester group in the alkyl chain of the surfactant. On the other hand, deviation values above 30 C increased with length of the hydrocarbon chain in the fatty acid portion and of the hydrocarbon chain situated between ester and sulfonate groups. These results indicate that interaction between the ester group and water molecule decreases with rise in temperature. The enthalpies, free energies, and entropies of micelle formation were calculated from temperature variation of critical micelle concentrations, and are consistent with current theories of the role of solvent in micelle formation.