Cabaleiro-Lago C, García-Río L, Hervella P
Universidad de Vigo, Lagoas-Marcosende s/n, 36310 Vigo, Spain.
Langmuir. 2007 Sep 11;23(19):9586-95. doi: 10.1021/la701051h. Epub 2007 Aug 16.
A kinetic study was carried out on various solvolytic reactions in water/ NH4OT /isooctane microemulsions. The NH4OT surfactant is a derivative of the sodium salt of bis(2-ethylhexyl) sulfosuccinate (NaOT or AOT), where the Na+ counterion has been replaced by NH4+. The counterion substitution effects the phase diagram of the system, and therefore, NH4OT-based microemulsions with high water content reaching values of W = 350 (W = [H2O]/[NH4OT]) can be obtained. The presence of high W values suggests a transition in the microemulsion microstructure from water-in-oil (w/o) to oil-in-water (o/w), as was confirmed by conductivity and 1H NMR self-diffusion measurements. The interpretation of the kinetic studies in terms of pseudophase formalism allows us to analyze the effect of the microemulsion on chemical reactivity, regardless of its microstructure. It has been confirmed that the values of the solvolytic rate constants at the interphase of oil-in-water microemulsions are similar to those obtained for aqueous SDS systems, showing that the hydration degree of the interphase of the oil-in-water microemulsions is independent of W. The influence of the surfactant counterion on the solvolytic rate constants was analyzed by comparing HOT-, NaOT-, and NH4OT-based microemulsions. An important influence on the rate constants caused by the changes in the structural properties of water has been observed as was confirmed by the water 1H NMR signals.
对水/ NH₄OT /异辛烷微乳液中的各种溶剂解反应进行了动力学研究。NH₄OT表面活性剂是双（2-乙基己基）磺基琥珀酸钠（NaOT或AOT）钠盐的衍生物，其中Na⁺抗衡离子已被NH₄⁺取代。抗衡离子的取代影响了体系的相图，因此，可以获得具有高含水量且W值达到350（W = [H₂O]/[NH₄OT]）的基于NH₄OT的微乳液。高W值的存在表明微乳液微观结构从油包水（w/o）向水包油（o/w）转变，这已通过电导率和¹H NMR自扩散测量得到证实。根据假相形式对动力学研究进行解释，使我们能够分析微乳液对化学反应性的影响，而不管其微观结构如何。已经证实，水包油微乳液界面处的溶剂解速率常数的值与在SDS水溶液体系中获得的值相似，表明水包油微乳液界面的水合程度与W无关。通过比较基于HOT、NaOT和NH₄OT的微乳液，分析了表面活性剂抗衡离子对溶剂解速率常数的影响。正如水¹H NMR信号所证实的，已经观察到水结构性质的变化对速率常数有重要影响。
