Studies of the Dissolved State of Sodium Tetradecyl Sulfate in Ethanol–Water Mixtures by Measurements of the Activity

The details of electromotive-force measurement by means of a concentration cell consisting of sodium tetradecyl sulfate (NaTS) in ethanol–water mixtures and of anionic and cationic ion-exchange membranes as septams were described. The membranes used proved to be permeable exclusively to the surfactant (TS−) and Na+ ions respectively. The activities of the TS− and Na+ ions were measured separately, and then the mean activity was computed as a function of the ethanol content in the solvent. Below the CMC, the activities for TS− and Na+ ions and the mean activity increased with an increase in the NaTS concentration, but decreased with an increase in the ethanol concentration. Above the CMC, the activity of the TS− ion decreased, whereas that of the Na+ ion increased; the mean activity also slightly increased with an increase in the NaTS concentration. The values of CMC obtained were 1.5×10−3, 1.7×10−3, and 2.2×10−3 mol/dm3 for 5, 10, and 15 vol % ethanol concentrations respectively. The mechanism of mixed micelle formation was also expressed by a charged phase-separation model in an ethanol–water mixture much as in the case of a simple aqueous solution. The degrees of counterion attachment were found to be 0.73, 0.68, and 0.62 for 5, 10, and 15 vol % ethanol respectively. The intermicellar concentrations of TS−, (CTS), and Na+ ions, (CNa), were calculated. With an increase in the NaTS concentration, CNa monotonously increased, but CNa decreased. At a given NaTS concentration, CNa increased with an increase in the ethanol concentration, while CNa was approximately constant.