Activity Measurement and Colloid Chemical Studies of Aqueous Sodium Tetradecyl Sulfate Solutions

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Abstract
A concentration cell using an ion-exchange semipermeable membrane as a septum for the measurement of the activities of ionic surfactants has been constructed, the method of measurement is here in described. The membranes used proved each to be exclusively permeable to one ion. The activities of both the tetradecyl sulfate ion (TS−) and the counterion (Na+) were determined. In the concentration region from 4.0×10−4 mol/1 to CMC, logf±=−(0.070logC+0.283) held for the mean activity coefficient, f±. The f± value at the CMC was found to be 0.81, considerably smaller than the value, 0.978, calculated by the Debye-Hückel equation taking account of the ionic size, 5.5 Å. Above the CMC the activity of the TS− ion decreased, that of the Na+ ion increased, and the mean activity slightly increased with the NaTS concentration. The Gibbs adsorption isotherm was applied for the adsorption of NaTS, using exclusively the observed data of the amount of adsorption, the surface tension, and the activities. A satisfactory agreement was obtained between the amount of adsorption observed and that calculated. The observed decrease in the surface tension above the CMC was confirmed by calculations using the Gibbs adsorption iostherm, the observed data of activity, and the amount of adsorption. The charged-phase separation model was verified as the mechanism of the micelle formation, and the degree of counterion attachment was calculated to be 0.81.

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