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The critical micelle concentrations (CMC) of nine commercial nonionic surfactants (Tween 20, 22, 40, 60, and 80; Brij 35, 58, and 78) and two pure nonionics [C 12 (EO) 5 and C 12 (EO) 8 ] were determined by surface tension and dye micellization methods. Commercially available nonionic surfactants (technical grade) usually contain impurities and have a broad molecular weight distribution owing to the degree of ethoxylation. It was shown that the surface tension method (Wilhelmy plate) is very sensitive to the presence of impurities. Much lower CMC values were obtained with the surface tension method than with the dye micellization method (up to 6.5 times for Tween 22). In the presence of highly surfaceactive impurities, the air/liquid interface is already saturated at concentrations well below the true CMC, leading to a wrong interpretation of the break in the curve of surface tension (γ) vs. concentration of nonionic surfactant (log C). The actual onset of micellization happens at higher concentrations, as measured by the dye micellization method. Furthermore, it was shown that when a commercial surfactant sample (Tween 20) is subjected to foam fractionation, thereby removing species with higher surface activity, the sample yields almost 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 indicates that precaution should be taken when determining the CMC of commercial nonionic surfactants by the surface tension method, as it indicates the surface concentration of all surface-active species at the surface only, whereas the dye method indicates the presence of micelles in the bulk solution. FIG. 1. Ultraviolet-visible absorbance spectrum of eosin Y in aqueous surfactant solution. The wavelength maximum (λ max ) shifts from 518 nm in the absence of surfactant to 538 nm as the surfactant concentration increases. The rise is most significant at 542 nm. 54 A. PATIST ET AL. FIG. 2. Critical micelle concentration (CMC) determination of Tween 20 (CMC = 0.042 mM) using the dye micellization method (absorbance at 542 nm). Eosin Y concentration: 0.019 mM. Horizontal dashed line represents dye absorbance in water in the absence of surfactant.

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