Tuning Surface/Interfacial Properties of Ferrocene-Containing One-Head-Two-Tail Surfactant by a Redox Reaction

Exploring the structure-activity relationship of surfactants that respond intelligently to external stimuli remains a significant challenge. In this context, a novel cationic ferrocene-based surfactant featuring an amide functional group (FcAC₃NC_n = FcCONHC₃H₆N(CH₃)₂C_nH_2n+1) was successfully synthesized, and its surface/interfacial properties, as well as its wetting, foaming, emulsification, and antibacterial activities, were systematically investigated. Interestingly, compared to other linear-chain ferrocene-based surfactants, FcAC₃NC_n can reversibly switch between a one-head-two-tail structure (FcAC₃NC_n) and a two-head-one-tail structure (the more hydrophilic FcAC₃NC_n-Ox) upon oxidation and reduction. This switch alters the molecular configuration at the surface and interface, resulting in redox-controllable surface and interfacial properties. FcAC₃NC_n exhibits enhanced hydrophobicity compared to FcAC₃NC_n-Ox and a more compact packing arrangement, which leads to a lower critical micelle concentration (cmc) and surface tension (γ_cmc), a higher diffusion coefficient, improved wettability on hydrophobic PTFE surfaces, and superior foaming and emulsification capabilities, as well as increased antimicrobial activity. As the hydrophobic chain length increases, both the cmc and γ_cmc gradually increase, accompanied by a decrease in the diffusion coefficient as well as diminished wettability on the hydrophobic PTFE surface, reduced foaming and foam stability, impaired emulsification ability, and decreased antimicrobial activity. Notably, FcAC₃NC₁₀, at a concentration of 4 mM (near its cmc), decreased the water contact angle on the PTFE surface by 59.4° and successfully formed a stable octane-in-water emulsion, demonstrating reversible on/off switching behavior up to three times. Furthermore, it exhibited antimicrobial activity at concentrations as low as 15.63 μg/mL against S. aureus and 62.5 μg/mL against E. coli. These results provide valuable insights for the rational design of ferrocene-based surfactants, significantly enhancing their potential applications in various fields.