Micellization and Counterion Binding Behavior of Cetylpyridinium Chloride in Aqueous Solutions of Sodium Bromide and Tetrabutylammonium Bromide

Critical micelle concentrations (CMC) of cetylpyridinium chloride (CPC) in aqueous sodium bromide and tetrabutylammonium bromide (TBAB) solutions were determined at 30 °C by conductivity and surface tension measurements. Bromide counterions reduce CMC of CPC more than the chloride. The modified form of the Corrin-Harkins (CH) equation describes the variation of CMC of CPC with the concentration of added electrolyte containing different counterions. It is demonstrated that the mixed-electrolyte-model (MEM) of Shanks and Franses can be applied to analyze the conductivity data of the surfactant solution containing mixed counterions, which was otherwise used for a surfactant solution containing single counterion only. The value of the total counterion binding constant (β) obtained from the MEM was then used to separate from the slope of the modified CH equation the counterion binding constants for chloride (β₁) and bromide (β₂) ions. This data analysis showed that binding of bromide counterion to cetylpyridinium ionic micelle is stronger than that of the chloride counterion, which is in accordance with the positions of chloride and bromide ions in the Hoffmeister series and the results reported from other studies.