The Role of Hydrophobic Chains in Self-Assembly at Electrified Interfaces: Observation of Potential-Induced Transformations of Two-Dimensional Crystals of Hexadecane by In-situ Scanning Tunneling Microscopy

Insoluble alkanes have been observed by electrochemical scanning tunneling microscopy (STM) to form ordered monolayers at the Au(111)/0.1 M HClO4 solution interface. Hexadecane molecules self-assemble into ordered layers that are stable over the potential range from 0.15 VSCE to 0.55 VSCE, on both reconstructed and unreconstructed Au(111) surfaces under electrochemical control. The hexadecane molecules appear as 2.2 nm long and 0.45 nm wide rods, suggesting an extended conformation. STM images show that a reversible order−disorder transition can be induced by moving the electrode potential positive or negative of the stable potential region (0.15 VSCE to 0.55 VSCE). The hexadecane molecules aggregate immediately after lifting of the surface reconstruction by a positive potential step to 0.65 VSCE. Reversible disappearance of the ordered alkane structure is also observed when the potential is stepped below 0.15 VSCE. The molecules resume an ordered lamellar structure upon return of the substrate to the sta...