Effect of electrolyte concentration on interfacial and bulk solution properties of ferrocenyl surfactants with anionic headgroups

We report the effect of electrolyte concentration on the surface and bulk solution properties of a redox-active ferrocenyl surfactant, Fc(CH₂)₁₁SO₃ ^-Na⁺, where Fc is ferrocene. Electrochemical oxidation of the electrically neutral Fc moiety to the ferrocenium cation reversibly transforms the anionic surfactant (Fc(CH₂)₁₁SO₃^-) into a zwitterionic species (Fc⁺(CH₂)₁₁SO₃^-). Whereas the effect of an increase in electrolyte concentration on the surface activity of Fc(CH₂)₁₁SO₃^- and Fc⁺(CH₂)₁₁SO₃^- is similar to that for classical anionic (surface tension decreases) and zwitterionic surfactants (surface tension invariant), respectively, the bulk solution behavior is not. We measured oxidation of Fc(CH₂)₁₁SO₃^- to Fc⁺(CH₂)₁₁SO₃^- to lead to the spontaneous transformation of either monomers or globular micelles into vesicle-like assemblies (R_g/R_h = 1.0 ± 0.3) with hydrodynamic diameters of 90 ± 10 nm in 1 mM Li₂SO₄. We observed the addition of salt to Fc⁺(CH₂)₁₁SO₃^- to lead to a modest decrease in the size of the vesicular aggregates (to 60 ± 7 nm at 100 mM Li₂SO₄) followed by formation of a precipitate (0.7 M). This result contrasts with past studies of mixtures of anionic and cationic surfactants which have reported the addition of high concentrations of salt to lead to the conversion of vesicular solutions into micellar ones. This difference in phase sequence likely reflects constraints on the ionic composition of assemblies formed from Fc⁺(CH₂)₁₁SO₃^- that do not exist when using mixtures of anionic and cationic surfactants. This paper also reports briefly on the effect of a change in oxidation state on dynamic surface tensions of solutions of this class of surfactant. These results suggest that the kinetics of adsorption of Fc⁺(CH₂)₁₁SO₃^- plays an important role in regulating the time dependence of the surface tension.