Evaluation of surfactant ion activity in micellar solutions of dodecylpyridinium chloride from partition data and counter ion activity measurements

The partition isotherm of dodecylpyridinium chloride in the water-1,2-dichloroethane system, measured over a broad concentration range, served for the evaluation of the critical micelle concentration and of the effective concentration and mean activity of the surfactant ions in the micellar concentration range. On the basis of the charged phase separation model for the micelles and EMF measurements of counter-ion activity, the surfactant cation activity and the apparent degree of counter ion association to the micelle () of 0.69 were estimated. The results indicate a gradual decrease of surfactant cation activity with increasing total concentration in the micellar range and support literature data derived from EMF measurements that apply surfactant selective plastic exchange membrane electrodes.