The micellar properties of 1-dodecylpyridinium chloride as studied by the ion-selective electrodes.

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The surfactant ion and counterion activities of aqueous 1-dodecylpyridinium chloride with and without KCl were determined by using ion-selective electrodes. In the concentration range above the critical micelle concentration (cmc), the surfactant ion activity (a+) was found to decrease gradually with increasing surfactant concentrations, while the reverse change was found for the counterion activity (a−). As a result, the mean activity increased slightly but regularly with increasing concentrations. The concentration dependences of a+ and a− became progressively smaller as the salt concentration increased. An application of the charged phase separation model led to an apparent degree of counterion association to the micelle (β) of 0.70 regardless of the salt concentration. On the basis of these observations, the estimations were made for the concentration of monomeric surfactant ion (C+) and the activity coefficients of counterion (γ−) and surfactant ion (γ+) above the cmc. The values of C+ and γ+ decreased appreciably, while that of γ− increased with increasing concentrations. The value of β was also estimated from the concentration dependence of a− and the validity of this method was discussed.
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