Electron spin resonance study of the pH-induced transformation of micelles to vesicles in an aqueous oleic acid/oleate system
Electron spin resonance study of the pH-induced transformation of micelles to vesicles in an aqueous oleic acid/oleate system
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DOI：
10.1021/la0100338
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发表时间：
2001-07-10
期刊：
影响因子：
3.9
通讯作者：
Walde, P
中科院分区：
文献类型：
--
作者：
Fukuda, H;Goto, A;Walde, P
将脂肪酸自旋探针（16-羟基硬脂酸，16-DS）掺入油酸和油酸分子组成的水表面活性剂体系中，在10 ~ 50℃至油酸+油酸总浓度为50 mM范围内测量了电子自旋共振（ESR）光谱。根据总浓度和pH值的不同，形成了不同类型的油酸/油酸聚集体。在pH范围的两端（高于pH 10.4和低于pH 6.4）， 16-DS的ESR光谱高度对称，可以计算出表面活性剂聚集体中的微粘度分别为4 cP和6 cP。在高pH范围内，观察到的聚集体为胶束。另一方面，在低pH范围内，微粘度明显低于纯油酸（测量值为11 cP），表明所获得的乳液体系不是由纯油酸液滴组成的。我们假设表面活性剂分子在低pH下形成层状双分子层的凝聚聚集体。在pH 6.4 ~ pH 10.4的中间pH范围内，获得了不对称高场ESR谱线。这表明探针分子以两种不同的物理聚集状态存在。基于显微镜和光散射技术的观察，我们将这两种聚集态分别定义为囊泡和非层状聚集体（很可能是非球形胶束）。利用改进的Bloch方程对ESR谱线进行光谱模拟分析，支持在整个中间pH范围内囊泡和非层状聚集体共存；这两种聚集形式的相对数量主要取决于pH值、温度和浓度。此外，光谱模拟表明，在pH值8.5左右形成了特别稳定的油酸/油酸囊泡，其中质子化和电离的物质以化学计量比存在。
ESR (electron spin resonance) spectra of a fatty acid spin probe ( 16-doxylstearic acid, 16-DS) incorporated into an aqueous surfactant system composed of oleic acid and oleate molecules were measured between 10 and 50 degreesC up to a total oleic acid + oleate concentration of 50 mM. Depending on the total concentration and the pH, different types of oleic acid/oleate aggregates formed. At the two ends of the pH range investigated (above pH 10.4 and below pH 6.4), the ESR spectra of 16-DS were highly symmetric, enabling calculation of the microviscosities in the surfactant aggregates to be 4 cP and 6 cP, respectively. In the high pH range, the observed aggregates are micelles. On the other hand, in the low pH range the microviscosity was considerably lower than that of neat oleic acid (measured to be 11 cP), indicating that the obtained emulsion system was not composed of pure oleic acid droplets. We postulate that the surfactant molecules at low pH form condensed aggregates of lamellar bilayers. Asymmetric high-field ESR lines were obtained at intermediate pH between pH 6.4 and pH 10.4. This indicates that the probe molecules were present in two physically different aggregation states. We assigned the two aggregation states to be vesicles and nonlamellar aggregates (most likely nonspherical micelles), based on the observation made by microscopy and light scattering techniques. The analysis of the ESR lines by spectral simulation using a modified Bloch equation supports the coexistence of vesicles and nonlamellar aggregates through the entire intermediate pH range; the relative amount of the two aggregation forms depends critically on pH, temperature, and concentration. Furthermore, the spectral simulation indicated that particularly stable oleic acid/oleate vesicles are formed around pH 8.5, where the protonated and ionized species exist in a stoichiometric ratio.