Potential-Dependent Wetting of Electroactive Ferrocene-Terminated Alkanethiolate Monolayers on Gold

The electrochemical and electrowetting behavior of ferrocene (Fc)-terminated alkanethiol (FcCO₂C₁₁H₂₂SH) monolayers adsorbed on gold is studied in 1 M HClO₄. The concentration of the ferrocene groups in the monolayer is varied by diluting the adsorbed ferrocenethiol with alkanethiol (C_nH_2n+1SH with n = 8, 12, 16, or 22). The Fc group of FcCO₂C₁₁H₂₂SH can be oxidized to the Fc⁺ cation while leaving the monolayer intact. Upon oxidation, the wettability of the surface increases: the advancing contact angle of the FcCO₂C₁₁H₂₂SH monolayer with the electrolyte (θ_a^red) decreases from 74° to 49°, the latter being the advancing contact angle of the oxidized Fc⁺ cation, θ_a^ox. Upon renewed reduction the meniscus recedes and the receding contact angle, θ_r^red, becomes 56° and can be continuously stepped between θ_r^red and θ_a^ox. Due to contact angle hysteresis this potential-dependent change in contact angle is only partly reversible. Within the number of applied steps, a decrease in θ_r^red is observed due to disordering of the monolayer on continuous oxidation/reduction. This disruptive effect is reduced for a mixed FcCO₂C₁₁H₂₂SH/C₁₂H₂₅SH monolayer. The C₁₂H₂₅SH thiol increases the distance between the bulky Fc groups and consequently the monolayer becomes more stable. For a mixed monolayer with 60% FcCO₂C₁₁H₂₂SH and 40% C₁₂H₂₅SH, the contact angle can reversibly be switched between 49° and 59°. We suggest that the change in wettability after oxidation/reduction is due to specific interactions of the charged Fc⁺ ion with anions from the electrolyte. Due to the hydrophilic nature of the ClO₄⁻ anion, the wettability of the surface increases.