Surface-chemical properties of ferrocene surfactants were investigated in the reduced and oxidized states of their ferrocene moiety at the air-water and oil-water interfaces. N,N-dimethylferrocenylmethylalkylammonium bromides, F(II)CnAB, were synthesized by the reaction of N,N-dimethylaminomethylferrocene with l-alkylbromides: CnBr (n = 8, 10, 12, 14 and 18). N,N-dimethylferroceniummethylalkylammonium nitrates, F(III)CnAN, which corresponded to the oxidized states of F(II)CnAB, were prepared in aqueous solution by oxidizing F(II)C n AB with AgN0 3 . The redox states of these ferrocene surfactants were confirmed by visible and Mossbauer spectra.
Solubility, surface tension lowering, surface activity, foaming and emulsion stabilities were measured in the reduced and oxidized states of ferrocene surfactants. As a result, it was found that when the ferrocene surfactants had been oxidized, their ferrocene moieties were converted from a hydrophobic to a hydrophilic character, and then became less surface active at the oil-water interface as well as at the air-water interface. The difference between ferrocenyl and ferrocenium moieties in surface activity corresponds to a difference of about two carbon atoms in the long alkyl chain. Remarkably different features of the redox states were observed in studies of foaming height and emulsion stability. These phenomena of the ferrocene surfactants were explained by conformational differences in the ferrocene moieties of the surfactants at the interface.