Electrochemical and neutron reflectivity studies of spontaneously formed amphiphilic surfactant bilayers at the gold-solution interface

We have employed electrochemical and neutron reflectivity measurements to study the transfer of 4-pentadecyl-pyridine (C15-4Py), an insoluble amphiphilic surfactant, from the gas-solution (G-S) interface to the metal-solution (M-S) interface of a Au(111) electrode. Neutron reflectivity experiments have demonstrated that C15-4Py forms a bilayer at the Au(111) electrode surface. Electrochemical experiments demonstrated that this bilayer is formed spontaneously when the electrode surface is brought in contact with the film-covered G-S interface. The surfactant molecules can move from the G-S to the M-S interface across the triple-phase boundary formed where the metal, solution, and gas phases are in contact. Time-dependence experiments have shown that the spreading process is irreversible. Having formed a bilayer or monolayer at the M-S interface, the C15-4Py surfactant molecules do not move back to a film-free G-S interface. Three models were used to analyze the kinetics of spreading. Our results are best explained assuming that the spreading is a first-order surface reaction controlled by the activation barrier that the surfactant molecules have to overcome when crossing the triple-phase line.