Titration of fatty acids solubilized in cationic, nonionic, and anionic micelles.: Theory and experiment

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.