Interactions between β-cyclodextrin and an amino acid-based anionic gemini surfactant derived from cysteine

Abstract

The interaction between β-cyclodextrin (β-CD) and an amino acid-based anionic gemini surfactant derived from cysteine (C8Cys)2 was studied by three independent techniques: electrical conductivity, UV–Vis spectral displacement technique using phenolphthalein as probe, and 1H NMR spectroscopy. The data obtained indicated the formation of a 1:1 inclusion complex between β-CD and the gemini surfactant studied and allowed for the determination of the binding constant, K1, by considering this stoichiometry. Electrical conductivity, spectral displacement technique, and NMR chemical shift measurements, obtained for aqueous β-CD–surfactant systems, yielded consistent K1 values in the order of 102dm3mol−1, typical of a weakly bound β-CD–surfactant complex. The influence of the presence of the inclusion complex on the micellization process of the gemini surfactant has also been studied and the apparent critical micelle concentration (cmc∗) has been obtained. Increasing β-CD concentration was found to shift the cmc∗ to higher values, as complexed surfactant monomers are not available to form micelles and aggregation takes place only when all β-CD cavities are occupied.