Layer by Layer Characterization of 1-Octadecanol Films on a Au(111) Electrode Surface. “In Situ” Polarization Modulation Infrared Reflection Absorption Spectroscopy and Electrochemical Studies

Abstract

Bilayers of 1-octadecanol were transferred from the gas-solution interface onto a Au(111) electrode surface using either the horizontal touch or the Langmuir-Blodgett (LB) technique. The horizontal touch method gives an X-type film in which the molecule in the bottom layer is oriented with the tail toward the metal and the head toward the tail of the molecule in the top layer (head to tail orientation). The LB technique allows one to build not only X but also a Y bilayer, in which the molecules in the two leaflets are oriented tail to tail with the polar heads facing the metal and the solution. The properties of these films were characterized by electrochemistry and polarization modulation infrared reflection absorption spectroscopy (PM IRRAS). The freshly prepared X bilayer is defect free and is characterized by a low capacity of 0.84 microF cm(-2). The bilayer may be desorbed from the electrode surface at E < -0.6 V and readsorbed again at more positive potentials. The potential-driven readsorption gives a Y film with a higher capacity of 2.2 microF cm(-2). The IR studies of a mixed bilayer composed of one hydrogenated leaflet and a second deuterated leaflet showed that 1-octadecanol molecules assume a more vertical orientation in the leaflet facing the solution than in the leaflet in contact with the metal. Upon desorption of the X film, the molecules in the bottom layer flip-flop and the readsorbed film assumes the Y structure. Smaller tilt angles are observed in the X film as opposed to the Y film.