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-n-11 ^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 (˜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.