Hydrogen ion titration of alkyldimethylamine oxides by 13C and 1H NMR and conventional methods

In the hydrogen ion titration of micelles, the degree of ionization of the micelle at a given pH has to be evaluated to obtain a pKa value of micelles (Ka being the proton dissociation constant) at the pH. We compared the degree of ionization obtained from 13C and 1H NMR spectra with that obtained from the stoichiometric method. We used dodecyldimethylamine oxide (C12DMAO) and hexyldimethylamine oxide (C6DMAO) to examine the titration behavior of micelles and monomers, respectively. We determined pKa values of amine oxides both in H2O and D2O. As to the monomer (C6DMAO), the degree of ionization from NMR, αNMR, coincided with that from the conventional stoichiometric method α. The difference of pK1 of amine oxide monomer between D2O and H2O was about 0.5: pK1(D) ≈ pK1(H) + 0.5. The difference was about the same as that for carboxylic acids. As to the C12DMAO micelle, αNMR did not coincide with α over a considerable range of α. The NMR chemical shift might be influenced by micellar structure changes induced by the ionization, such as the sphere-to-rod transition. The intrinsic logarithmic dissociation constants of the micelle were 5.9±0.1 for H2O, and 6.5±0.1 for D2O.