Kakehashi Rie, Shizuma Motohiro, Yamamura Shingo, Maeda Hiroshi
Osaka Municipal Technical Research Institute, Morinomiya, Osaka 536-8553, Japan.
J Colloid Interface Sci. 2005 Sep 15;289(2):498-503. doi: 10.1016/j.jcis.2005.03.090.
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, alpha(NMR), coincided with that from the conventional stoichiometric method alpha. The difference of pK1 of amine oxide monomer between D2O and H2O was about 0.5: pK1(D) approximately pK1(H) + 0.5. The difference was about the same as that for carboxylic acids. As to the C12DMAO micelle, alphaNMR did not coincide with alpha over a considerable range of alpha. 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.
在胶束的氢离子滴定中，为了获得胶束在给定pH值下的pKa值（Ka为质子解离常数），必须评估胶束在该pH值下的电离程度。我们将通过13C和1H NMR光谱获得的电离程度与通过化学计量法获得的电离程度进行了比较。我们分别使用十二烷基二甲基氧化胺（C12DMAO）和己基二甲基氧化胺（C6DMAO）来研究胶束和单体的滴定行为。我们测定了氧化胺在H2O和D2O中的pKa值。对于单体（C6DMAO），NMR得到的电离度α(NMR)与传统化学计量法得到的α一致。氧化胺单体在D2O和H2O中的pK1差值约为0.5：pK1(D)≈pK1(H)+0.5。该差值与羧酸的差值大致相同。对于C12DMAO胶束，在相当大的α范围内，αNMR与α不一致。NMR化学位移可能受到电离诱导的胶束结构变化的影响，例如球-棒转变。胶束的固有对数解离常数在H2O中为5.9±0.1，在D2O中为6.5±0.1。
