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Electrical conductivities of sodium dodecyl sulfate (SDS) in aqueous sodium acetate (NaAc) and sodium propionate (NaPr) solutions of different concentrations were measured at 25 °C. An anomalous behavior in the form of minima in the plots of critical micelle concentration (cmc) of SDS versus concentration of NaAc and NaPr was observed, thereby indicating the effect of acetate and propionate co-ions on micellization. The conductivity data were analyzed using a conductivity equation derived on the basis of a mixed-electrolyte model for surfactant solutions wherein ion-ion interactions were accounted for by the Debye-Hu ¨ckel-Onsager equation. Such an analysis provided a method to estimate the values of cmc, counterion binding constant, and aggregation number directly from conductivity data. It was observed that ionic micelles contribute to electrical conductivity but not to ionic strength of surfactant solution. The surface potentials of SDS micelles in NaAc and NaPr solutions were computed by solving the Poisson-Boltzmann equation. The values of total free energy, electrostatic free energy, and hydrophobic free energy of micellization were estimated. The electrostatic free energy seems to control the decrease in cmc with added electrolyte concentration. On the other hand, the increase in cmc with NaAc and NaPr concentration appears to be due to the penetration of SDS micelles by the acetate and propionate co-ions.

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