A Study of the Behavior of α-Cyclodextrin with Single Solutions of Hydrogenated and Fluorinated Surfactants and Their Mixtures

The interactions between α-cyclodextrin (α-CD) and an equimolar mixture of the anionic hydrogenated and fluorinated surfactants sodium decyl sulfate (SDeS) and sodium perfluorooctanoate (SPFO), respectively, were studied by 1H and 19F NMR spectroscopy. It was directly proved that α-CD preferentially included the hydrogenated surfactant in such a surfactant mixture, although SDeS and SPFO showed almost the same hydrophobicity. The added α-CD formed 1:1 and 2:1 complexes with SDeS, but SPFO showed only a weak association with α-CD after the near saturation of inclusion of SDeS. Such selectivity of α-CD is contrary to that of β-CD, as shown in our previous work in which β-CD preferentially included the fluorinated surfactant in an SDeS/SPFO mixture. This selectivity of α-CD could be ascribed to the better fit between the cavity of α-CD and the hydrocarbon chain. It is consistent with the equilibrium constants (K) for the inclusion complexes in the solution. A peculiar trend of chemical shift variation was noted in mixtures of α-CD with the single surfactant SDeS: the chemical shift of β-CH2 increased rapidly while ω-CH3 showed a smaller ascent to declivity, until both of them reached a plateau. The binding constant for the formation of the 2:1 α-CD/SDeS complex (K2) was found to be relatively large, as determined by the NMR investigation.