CTAB/Water/Chloroform Reverse Micelles: A Closed or Open Association Model?

详细信息    查看全文

• 作者：L'ubica Kl铆膷ov谩 ; Peter 艩ebej ; Peter 艩tacko ; Sergey K. Filippov ; Anna Bogomolova ; Marc Padilla ; Petr Kl谩n
• 刊名：Langmuir
• 出版年：2012
• 出版时间：October 30, 2012
• 年：2012
• 卷：28
• 期：43
• 页码：15185-15192
• 全文大小：385K
• 年卷期：v.28,no.43(October 30, 2012)
• ISSN：1520-5827

文摘

The micellization of cetyltrimethylammonium bromide (CTAB) in chloroform in the presence of water was examined. Three scenarios of the reverse micelle formation, the closed, open and Eicke鈥檚 association models, were considered in the interpretation of the experimental data. The growth of the aggregates was observed through the changes of NMR signals of associated water, probing the microenvironment of the premicellar aggregates and the interior of reverse micelles. This technique if combined with isothermal titration calorimetry (ITC) revealed that hydrated surfactant premicellar aggregates are already present at 6 mM CTAB. NMR, ITC and conductometry were used to determine the critical micelle concentration (cmc) to be 40 mM CTAB. It is suggested that the variation of the cmc values reflects the fact that the NMR analysis indicated the beginning of the reverse micelle formation, whereas conductometry and ITC measurements provided the upper limit and an average value of a so-called apparent cmc, respectively. The cmc values were found to be unaffected by the water content. The presence of reverse micelles, the existence of multiple equilibria, and high polydispersity of the samples were evidenced by DOSY NMR spectroscopy. As a result, we validated Eicke鈥檚 association model, according to which cyclic inverse micelles are formed by a structural reorganization of linear associates within a narrow concentration range, called the apparent cmc. New experimental results have also been gained for micellization of cetyltrimethylammonium chloride (CTAC) in chloroform in the presence of water; a similar mechanism of reverse micelle formation has been suggested.