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 s tudy the transfer of 4-pentadecyl-pyridine (C15-4Py), an insoluble amphiphi lic surfactant, from the gas-solution (G-S) interface to the metal-solution IM-S) interface of a Au(lll) electrode. Neutron reflectivity experiments h ave demonstrated that C15-4Py forms a bilayer at the Au(111) electrode surf ace. Electrochemical experiments demonstrated that this bilayer is formed s pontaneously 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 t he M-S interface across the triple-phase boundary formed where the metal, s olution, and gas phases are in contact. Time-dependence experiments have sh own that the spreading process is irreversible. Having formed a bilayer or monolayer at the M-S interface, the C15-4Py surfactant molecules do not mov e 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 spr eading is a first-order surface reaction controlled by the activation barri er that the surfactant molecules have to overcome when crossing the triple- phase line.