Critical micelle concentrations (cmc) of cetylpyridinium chloride (CPC) in aqueous sodium bromide and tetrabutylammonium bromide (TBAB) solutions were determined at 30 °C from surface tension and conductance methods. Bromide decreases cmc of CPC more than chloride. A modified form of Corrin-Harkins equation has been used to describe the variation of cmc with added electrolyte concentration in the presence of mixed counter ions. From the slope of this equation the lower limit to the value of total counter ion binding constant (β) can be known and binding constants of the individual counter ions (β1 and β2) can be determined provided β is known. It is demonstrated that the mixed-electrolyte-model (MEM) of Shanks and Franses (J. Phys. Chem. 1992, 96, 1794) can be applied to analyze the conductance data of surfactant solution containing mixed counter ions. The value of β obtained from the MEM was then used to determine β1 and β2. By this treatment we could show that (i) bromide preferentially binds to CPC micelle by replacing chloride counterion, (ii) bromide binding is more than chloride and (iii) aggregation number of CPC is higher in the presence of bromide. These observations are in accordance with the reported results of SANS and counter ion trapping studies
