Dissociation of Micelles and Intermicellar Concentrations of Aqueous Solutions of Calcium and Sodium Decyl Sulfates

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Abstract
Activities of the surfactant ions and the counterions were calculated from the electromotive forces of concentration cell with ion-exchange membranes for the aqueous solutions of calcium (Ca(DeS)2) and sodium (NaDeS) decyl sulfates. Moreover, the mean activities and the activity coefficients were obtained. From these activities, the valence effects of the counterions on the dissolved state of micellar solutions were studied. The following results were obtained: (1) The mean activities were not constant, but increased with the increase in the concentration of the two surfactants above the CMC; the slope of curve of the mean activity for Ca(DeS)2 against its concentration was smaller than that for NaDeS. (2) The CMCs were 6.4 for Ca(DeS)2 and 32.8 mmol dm−3 for NaDeS, respectively. (3) For the mechanism of micelle formation, the phase separation model was set up in both cases. (4) From this model, the degrees of dissociation of the micelles were calculated as 0.12 for Ca(DeS)2 and 0.28 for NaDeS, respectively. (5) The intermicellar concentrations of the Ca2+, the Na+, and the decyl sulfate (DeS−) ions were calculated; those of the counterions increased but those of the DeS− ions decreased with the increase in their surfactant concentrations. Moreover, those of the DeS− for Ca(DeS)2 changed slowly while those for NaDeS changed steeply.