Admicellar CTAB Layers on Carbon Electrodes Serving as Interfacial Ion Exchangers Probed by Ferricyanide-Mediated Ascorbic Acid Oxidation

The structure and molecular dynamics of surfactant-driven assemblies at the electrode-electrolyte interface remain an active area of research, particularly for mimicking biomembrane-like systems and developing novel application platforms. The structural and dynamic characteristics of these assemblies are strongly influenced by both the solution-phase and electrode-surface parameters. In this study, a thin layer of the cationic surfactant cetyltrimethylammonium bromide (CTAB) was adsorbed on a glassy carbon electrode (GCE) using an ethanol-assisted modification process. This modification yielded highly ion-exchangeable organized assemblies, which were investigated by using ferricyanide ions as a probe via cyclic voltammetry in an aqueous medium. The resulting ferricyanide-intercalated CTAB assembly, denoted as GCE/{CTAB} - Fe(CN)₆^3-, exhibited a well-defined and stable redox couple with a formal potential (<i>E</i>°) of 0.06 V vs Ag/AgCl and a surface excess value of 21.5 × 10^-9 mol cm^-2. When alternative solvents such as DMF, THF, or CHCl₃ were employed during the modification process, either no voltammetric response or a significantly reduced signal was observed. Similarly, when other electrode substrates Pt, Au, or indium tin oxide (ITO) were used instead of GCE or a screen-printed carbon electrode, the ion-exchangeable layer formation was substantially diminished. Physicochemical, electrochemical, atomic force microscope (AFM) and scanning electrochemical microscopy (SECM) analyses revealed that CTAB adsorption under electrified conditions on GCE leads to the formation of adlayer and admicelles-like multilayered organized assemblies. These structures are stabilized by non-Coulombic interactions between the hydrophobic domains of the CTAB aggregates and the hydrophobic basal planes of the GCE, facilitating subsequent electron-transfer reactions. As a practical demonstration, in situ formation of Prussian Blue (PB) was achieved using the {CTAB} - Fe(CN)₆^3- assembly as a template in the presence of Fe²⁺ ions. Furthermore, the modified surface, {CTAB} - Fe(CN)₆^3- enabled selective and sensitive heterogeneous mediated oxidation of ascorbic acid via the confined ferricyanide/ferrocyanide redox couple. Overall, the findings of this study provide a new perspective on surfactant-adlayer-modified electrode surfaces, highlighting their potential for real-time electrochemical applications.