Titration of Fatty Acids Solubilized in Cationic, Nonionic, and Anionic Micelles. Theory and Experiment

Abstract

The titration properties of a fatty acid solubilized in different types of micelles, cationic, anionic and nonionic, have been investigated experimentally and theoretically. The solution containing micelles, counterions, and added salt was treated as a dielectric continuum in the Monte Carlo simulations. The dielectric permittivity of the interior of the micelle can have a profound effect on the calculated pK shifts depending on how the dielectric discontinuity is chosen in the model. The simulated results are compared to data from Poisson−Boltzmann calculations and from experiments. The experimentally observed apparent pK a value changes from 7.6 to 4.9 when solubilizing lauric acid in anionic (SDS) and cationic (DoTAC) micelles, respectively. For the nonionic micelle it is found to be approximately 6.6. A significant ion specificity in the pK a value is observed for the DoTAC and DoTAB systems and surprisingly enough both systems show an upward pK shift.