Comparison of the surface activity and bulk aggregation of ferrocenyl surfactants with cationic and anionic headgroups

We report the synthesis and aqueous solution properties of a novel redox-active surfactant, Fc(CH2)(11)SO3-Na+, where Fc is ferrocene. Electrochemical oxidation of Fe to the ferrocenium cation (Fc(+)) reversibly transforms the anionic surfactant (Fc(CH2)(11)SO3-) into a zwitterionic species (Fc(+)(CH2)(11)SO3-). When dissolved at concentrations between 0.15 and 0.4 mM in aqueous solutions containing 10 mM Li-2-SO4, oxidation is accompanied by the reversible transformation of a solution of monomeric species of Fc(CH2)(11)SO3- into vesicle-like (D-h = 70 +/- 8 nm, R-g/R-h = 1.0 +/- 0.3) surfactant assemblies of Fc(+)(CH2)(11)SO3-. In contrast, at concentrations above 0.4 mM, oxidation reversibly transforms a solution of globular (Dh = 6 +/- 2 nm) micelles into vesicle-like assemblies of Fc+(CH2)11SO3-. These various oxidation-induced changes in aggregation are reflected in the interfacial properties of this surfactant system, which are substantially different from those of ferrocenyl surfactants with cationic headgroups (Fc(CH2)(11)N+(CH3)(3)).