表面活性剂胶束形状随浓度转变的核磁共振研究

运用核磁共振一维氢谱和自扩散实验方法研究了聚乙烯乙二醇异辛酚醚(TX-100)、十二烷基苯磺酸钠(SDBS)和十四烷基三甲基溴化铵(TTAB)三种不同类型的表面活性剂在重水溶液中的胶束形状转变, 发现它们在临界胶束浓度以上的各自相应浓度都有胶束形状的变化(由球状转变为椭球状或棒状). 在常温常压和没有其他添加剂的情况下, 表面活性剂溶液浓度高于其临界胶束浓度时, 球状胶束开始形成. 核磁共振一维氢谱和自扩散实验的结果显示, 当溶液浓度继续增加到一定程度时, 溶液中表面活性剂分子的化学位移和自扩散系数的变化速率都有明显的转折, 这说明溶液中球状胶束开始发生转变. 进一步通过仔细分析对比核磁共振一维氢谱中各基团谱峰, 发现表面活性剂胶束亲水表面上的质子的化学位移变化速率要远高于其疏水内核中的质子, 据此推测胶束形状很可能由球状转变为椭球状或棒状.

NMR Study of Surfactant Micelle Shape Transformation with Concentration

The micelle shape transformation of tert-octylpheny phenol polyethylene glycol ether(TX-100),sodium dodecyl benzene sulfonate(SDBS)and tetradecyl trimethyl ammonium bromide(TTAB)in heavy water solutions was studied by 1H nuclear magnetic resonance(NMR)spectroscopy,including an NMR self-diffusion experiment.These experiments showed that the surfactants formed several shapes of micelles(spherical,ellipsoidal,and rodlike)at the respective concentrations which were far above their critical micelle concentration(cmc).At ambient temperature,normal pressure and when the concentration of the surfactant was above the cmc without other additives,spherical micelles were formed.At higher surfactant concentrations,1H NMR spectra and self-diffusion experiments showed that abrupt changes occurred in their chemical shifts(δ)and in their self-diffusion coefficients(D).This indicated that spherical micelles transformed into larger micelles.By careful examination of every proton resonance peal in the 1H NMR spectra,we found that the protons near the head group experienced larger chemical shift changes than protons near the hydrophobic group.This suggests that the shape of surfactant micelles most likely transforms from spherical to ellipsoidal or rodlike.