Coexistence of micelles and crystallites in solutions of potassium myristate: Soft matter vs. solid matter

Here, we investigate the coexistence of surfactant micelles and acid-soap crystallites in solutions of potassium myristate (n-tetradecanoate) to determine the micelle composition and charge, and the stoichiometry of the acid soaps. We carried out parallel pH, conductivity, and solubilization measurements, which indicate that micelles are present in the potassium myristate solutions at the higher concentrations, in contrast with the results for sodium myristate, where no micelles were detected at room temperature. Theoretical expressions describing the concentration dependences of conductivity and pH of the micellar carboxylate solutions are derived. Diagrams, showing the concentrations of all species in the solution, are constructed. The comparison of theory and experiment indicates that the undissociated fatty acid is incorporated in acid-soap crystallites, i.e. it behaves as initiator of crystallization. The rest of dissolved carboxylate forms micelles that are composed only of carboxylate anions and bound potassium counterions. Above 2-3 times the critical micellization concentration (CMC), the main mass of the carboxylate is in micellar form, despite the presence of coexisting acid-soap crystallites. Surface tension isotherms are obtained and interpreted on the basis of the results for the bulk composition. The adsorption layer is composed mostly of fatty acid and 1:1 acid-soap molecules. Not only the appearance of micelles, but also the change in the stoichiometry of the acid soaps in the solution leads to kinks, and even jumps, in the surface tension isotherm. The results for acid-soap stoichiometry have been confirmed by independent analysis of crystals collected from the solutions.