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

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Authors

KLÍČOVÁ Ľubica ŠEBEJ Peter ŠTACKO Peter FILLIPOV Sergey K. BOGOMOLOVA Anna PADILLA BARRIENTO Marc KLÁN Petr

Year of publication 2012
Type Article in Periodical
Magazine / Source Langmuir
MU Faculty or unit

Faculty of Science

Citation
Web http://pubs.acs.org/doi/abs/10.1021/la303245e
Doi http://dx.doi.org/10.1021/la303245e
Field Organic chemistry
Keywords Reverse micelles; CTAB; NMR; ITC
Description 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’s 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’s 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.
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