Micellar aggregation for ionic surfactant in pure solvent and electrolyte solution: Nonextensive thermodynamics approach

We show in this study that the concepts of nonextensive thermodynamics introduced and applied in a series of previous studies can be used to describe the behaviour of ionic surfactant solutions at concentrations higher than the critical micelle concentration (cmc) in pure solvents and in the presence of electrolytes. We supposed that the aggregated amphiphiles and their counter ions form two interpenetrated nonextensive phases of the same thermodynamic dimension, m  , characterised by two parameters AamAam and ACIACI related to the aggregated amphiphile and the counter ion, respectively. Our experimental results and those published in the literature indicate that logarithms of the activities of the amphiphile and of its counterion vary with the quantity of aggregated monomer according to a power law. Thus, we demonstrate a linear relation between the logarithms of the activities of the two ions beyond the cmc in pure solvents (“micellization product”). An original relation, different from the Corrin–Harkins relation, can thus be established to describe the effects of salt on the cmc of ionic surfactants. According to this relation the cmc of charged surfactant in some systems can increase in the presence of an electrolyte with a common ion.

In this approach, amphiphilic ions and their counterions are supposed to form two nonextensive phases of the same dimension, m. They are in equilibrium with the monomers in the solution at the temperature T. The interfaces with the surrounding medium are “fuzzy interfaces.”Figure optionsDownload high-quality image (66 K)Download as PowerPoint slide