Redox-Induced Backbiting of Surface-Tethered Alkylsulfonate Amphiphiles: Reversible Switching of Surface Wettability and Adherence

The synthesis and characterization of electrode-supported poly­(ferrocenylsilane) (PFS) films bearing iodopropyl (PFS-I) and undecanesulfonate (PFS-SO₃^–) surface moieties are presented. The redox responsiveness of these PFS films allows for controlled and repeatable switching of the surface energy of the PFS-I and PFS-SO₃^– layers under electrochemical control. Static water/surface contact angle measurements showed a change in contact angle values for PFS-I from 80° (reduced state) to 70° (oxidized state) over repeated cycles. However, an opposite change in wettability was observed for PFS-SO₃^–, where the values observed varied from 59° (reduced state) to 77° (oxidized state). Nanoscale adherence was assessed with colloid probe AFM. The adhesive forces between these surfaces and a polystyrene (PS) colloid probe in water alternated between 130 nN (reduced state) and 30 nN (oxidized state) for PFS-I layers and between 75 nN (reduced) and 180 nN (oxidized) for the PFS-SO₃^– films. The reversed response of PFS-I films to oxidation compared to that of PFS-SO₃^–, in both contact angles and adhesive forces, suggests a different underlying mechanism for switching. As PFS-I is tuned from the reduced to the oxidized state, positively charged ferrocenium (Fc⁺) centers that formed in the film increase its wettability and reduce its adherence to the hydrophobic colloid probe. For PFS-SO₃^– in the reduced state, the exposed alkanesulfonate moieties increase the hydrophilicity of the surface. When oxidized, the Fc⁺ units attract the negatively charged sulfonate groups, which results in a bending of the sulfonate groups toward the PFS surface, exposing the undecyl spacer. This alteration of the surface chemistry reduces the surface energy and increases the adherence between the bent alkyl chains and the hydrophobic PS colloid in water. The attraction of the charged sulfonate group to Fc⁺ is in competition with the counterions present in the electrolyte solution. Therefore, the backbiting of the chain can be achieved only in electrolytes where the affinity of Fc⁺ for the ions is lower than for the sulfonate group, in agreement with the Hofmeister series