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

J. A.M. Sondag-Huethorst, L. G.J. Fokkink

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Abstract

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 (CnH2n+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 (θared) decreases from 74° to 49°, the latter being the advancing contact angle of the oxidized Fc+ cation, θaox. Upon renewed reduction the meniscus recedes and the receding contact angle, θrred, becomes 56° and can be continuously stepped between θrred and θaox. 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 θrred 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.

Original languageEnglish
Pages (from-to)4380-4387
Number of pages8
JournalLangmuir
Volume10
Issue number11
DOIs
Publication statusPublished - 1 Nov 1994
Externally publishedYes

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