The interactions of surfactant viologens with cyclodextrins

N-ethyl-N′-hexadecyl-4,4′-bipyridinium bromide (C₁₆VBr₂) and N-ethyl-N′-octadecyl-4,4′-bipyridinium bromide (C₁₈VBr₂) were used as electroactive probes to assess the interactions between surfactants and cyclodextrins. Cyclic voltammetry, visible spectroscopy, fluorescence spectroscopy and surface tension techniques were used to detect the formation of complexes between the surfactant viologen probes and α- and β-cyclodextrins. The voltammetric results suggest the formation of inclusion compounds in which the hydrophobic tail of the surfactant viologens penetrate the cyclodextrin cavity. The dimerization of the viologen cation radicals is essentially suppressed by the presence of α-cyclodextrin (ACD) while no effects are observed in the presence of β-cyclodextrin (BCD). The observed results are best explained by the relative solubility in aqueous media of each of the inclusion complexes in the several accessible viologen oxidation states.