Electrochemical, neutron reflectivity and in situ PM-FT-IRRAS studies of a monolayer of n-octadecanol at a Au(1 1 1) electrode surface

The horizontal touch and Langmuir-Blodgett techniques have been used to transfer a monolayer of n-octadecanol from the gas-solution interface of a Langmuir trough onto the metal-solution interface of a Au(1 1 1) electrode. Chronocoulometry has been used to determine the charge density at the electrode surface covered by the film of n-octadecanol. The surface pressure of this film was calculated from the charge density data and was found to be controlled by the electrode potential. We have demonstrated that by dialing the potential applied to the electrode via a potentiostat the monolayer adsorbed on the surface can be compressed or decompressed. Two states of the monolayer were observed. The transition between these states took place at a film pressure ∼12 mN m-1. Neutron reflectometry and polarization modulation Fourier transform infrared reflection absorption spectroscopy have been employed to determine the nature of the two states. The results show that octadecanol molecules form a two-dimensional solid film at all film pressures. At film pressures larger than 12 mN m-1, the film has low compressibility and n-octadecanol molecules assume a small tilt angle with respect to the surface normal. At film pressures lower than 12 mN m-1 a compressive film is formed in which the tilt angle progressively increases with decreasing surface pressure. We have demonstrated that the properties of a monolayer of n-octadecanol at the metal-solution interface display many similarities to the properties of that film at the air-solution interface