On the measurement of critical micelle concentrations of pure and technical-grade nonionic surfactants

The critical micelle concentrations (CMC) of nine commercial nonionic surfa ctants (Tween 20, 22, 40, 60, and 80; Triton X-100; Brij 35, 58, and 78) an d two pure nonionics [C-12(EO)(5) and C-12(EO)(8)] were determined by surfa ce tension and dye micellization methods. Commercially available non ionic surfactants (technical grade) usually contain impurities and have a broad m olecular weight distribution owing to the degree of ethoxylation. It was sh own that the surface tension method (Wilhelmy plate) is very sensitive to t he presence of impurities. Much lower CMC values were obtained with the sur face tension method than with the dye micellization method (up to 6.5 times for Tween 22). In the presence of highly surface active impurities, the ai r/liquid interface is already saturated at concentrations well below the tr ue CMC, leading to a wrong interpretation of the break in the curve of surf ace tension (gamma) vs. concentration of nonionic surfactant (log C). The a ctual onset of micellization happens at higher concentrations, as measured by the dye micellization method. Furthermore, it was shown that when a comm ercial surfactant sample (Tween 20) is subjected to foam fractionation, the reby removing species with higher surface activity, the sample yields almos t the same CMC values as measured by surface tension and dye micellization methods. It was found that for monodisperse pure nonionic surfactants, both CMC determination methods yield the same results. Therefore, this study in dicates that precaution should be taken when determining the CMC of commerc ial nonionic surfactants by the surface tension method, as it indicates the surface concentration of all surface-active species at the surface only, w hereas the dye method indicates the presence of micelles in the bulk soluti on.