Synthesis and viscoelastic properties of gemini surfactants containing redox-active ferrocenyl groups

A type of electro-responsive threadlike micelle (TLM) was formed using new cationic gemini surfactants (Fc)11-n11(Fc) with redox-active ferrocenyl groups in the gemini alkyl chains. By varying the alkyl spacer length, we synthesized several kinds of (Fc)11-n-11(Fc)-based gemini surfactants with n = 3, 4, 6, and characterized their viscoelastic behavior, surface activity, and redox properties in aqueous solutions through the use of oscillatory rheology, surface tensiometry, cyclic voltammetry, and UV-vis spectroscopic measurements. The (Fc)11-4-11(Fc) solution was found to form highly viscous ((similar to)10(3) Pa.s) TLMs. Unlike the monomeric surfactant, the (Fc)11-4-11(Fc) micellar solution maintained its high viscosity even after adding extra builder and/or electrolyte agents. The surface-tensiometric measurements indicate that the effective molecular area of (Fc)11-n-11(Fc) at the air/water interface systematically increased with the alkyl spacer length. Using the critical packing parameters estimated with the molecular area, (Fc)11-4-11(Fc) is expected to be the most suitable for rod-like micellar growth. Importantly, the viscosity of (Fc)11-4-11(Fc) micellar solution could be decreased to that of pure water as a response to electrochemical oxidation reaction, and recovered by the subsequent re-reduction reaction. Hence, the gemini surfactants prepared in this study are a promising class of candidates for potential smart soft material applications.