Stable surfactant monolayers at the electrochemical interfacial double layer across a wide potential range

Cetyltrimethylammonium bromide (CTAB) has been used to enhance the selectivity of CO2 electrochemical reduction. Traditionally, this selectivity was attributed to repulsion of water molecules due to a CTAB self-assembled monolayer, which forms under negative potential and disassembles at positive voltage due to electrostatic repulsions. In this report, using in operando interface sensitivity sum frequency generation spectroscopy, we investigated the self-assembly behavior of CTAB across a broad electrochemical potential range. We observed that CTAB molecules form a stable monolayer at the Stern layer over the entire potential scan, even when the electrodes are positively charged. Rather than disassembling, the CTAB molecules reorient themselves to balance the electrostatic interactions and the non-covalent hydrophobic effects, the latter being the primary driving force maintaining the monolayer at a positive potential. This finding contrasts the traditional view that CTAB monolayers are absent when the electrodes are positively charged, indicating a stable and ordered monolayer with respect to the electrostatic repulsions at liquid/electrode interfaces. The balance between non-covalent and electrostatic interactions offers a facile and reversible electrochemical method to control the local environment and dominating interactions at the Stern layer of the electrode surface, thus providing a means for engineering a micro-electrochemical environment.