NMR-STUDY OF CYCLODEXTRIN INCLUSION OF FLUOROCARBON SURFACTANTS IN SOLUTION

The association of alpha-, beta-, and gamma-cyclodextrins with sodium perfluorobutanoate, sodium perfluoroheptanoate, sodium perfluorooctanoate, and sodium perfluorononanoate has been studied by F-19 NMR. First, the results suggest that the association of alpha-cyclodextrin with various fluorocarbon surfactants is fairly weak. The cavity of the host cyclodextrin is too small to accommodate the fluorocarbon chain to form an inclusion complex, but there exists an appreciable association between the tail of the surfactant and alpha-cyclodextrin. Secondly, the cavity of beta-cyclodextrin is large enough to accommodate the fluorocarbon chain, but the latter is forced to adopt an extended, possibly all-trans conformation to fit into the cavity. For perfluorosurfactants having a short fluorocarbon chain, a 1:1 complex is formed. When the fluorocarbon chain is longer, 2:1 complexation becomes significant for higher concentrations of beta-cyclodextrin. Thirdly, gamma-cyclodextrin forms a 1:1 complex with fluorocarbon surfactants. The complexation is stronger than that in the alpha-cyclodextrin/surfactant system but weaker than that in the beta-cyclodextrin/surfactant system. The association constants for the 1:1 complexes have been calculated from the dependence of the F-19 NMR chemical shift on the concentration of cyclodextrin.