Département de chimie and CERMA, Université Laval, Québec, Pavillon A.-Vachon, 2240-C, G1 V 7P4.
Langmuir. 2010 May 4;26(9):6250-5. doi: 10.1021/la904033e.
The (1)H NMR spectrum of IgepalCO520 in ternary mixtures containing water and cyclohexane shows a complex dependence on water content. This is in part because of rapid exchange between surfactant molecules within the micelles and free surfactant dissolved in the continuous phase. The analysis of this two-state system is further complicated by the fact that the chemical shifts of both free and micellar surfactants vary with micelle size. We demonstrate that the relative quantities of free and micellar surfactants can be determined from the NMR spectra if the data are compared within sample sets of constant micelle size but differing global composition. By fixing micelle size, the spectra of both surfactant states remain constant within a given series and only the relative populations of the free and micellar species change with overall composition. This method of analysis allows for the determination of free surfactant concentration as a function of micelle size. Results are presented for the water/IgepalCO520/cyclohexane system and indicate that the free surfactant concentration is far from negligible and strongly dependent on micelle size. The free surfactant concentration increases with decreasing micelle size, reflecting the lower stability of the smaller micelles. Similar behavior can be expected for other reverse micellar systems based on non-ionic surfactants.
IgepalCO520 在含有水和环己烷的三元混合物中的 1H NMR 谱显示出对含水量的复杂依赖性。这部分是由于胶束内表面活性剂分子之间以及溶解在连续相中的游离表面活性剂之间的快速交换。对这种两态体系的分析进一步复杂化，因为游离和胶束表面活性剂的化学位移都随胶束大小而变化。我们证明，如果在具有恒定胶束大小但不同总体组成的样品组内比较数据，则可以从 NMR 谱中确定游离和胶束表面活性剂的相对量。通过固定胶束大小，给定系列内两种表面活性剂状态的光谱保持不变，只有游离和胶束物种的相对丰度随总体组成而变化。这种分析方法允许确定作为胶束大小函数的游离表面活性剂浓度。给出了水/IgepalCO520/环己烷体系的结果，表明游离表面活性剂浓度远非可以忽略不计，并且强烈依赖于胶束大小。游离表面活性剂浓度随胶束尺寸的减小而增加，反映出较小胶束的稳定性较低。基于非离子表面活性剂的其他反胶束体系也可以预期类似的行为。
