Detachment behavior of Langmuir–Blodgett films of arachidic acid from a gold surface studied by the quartz crystal microbalance method

The detachment process of an oily contaminant from a gold surface was investigated using a model system constructed by the application of the quartz crystal microbalance (QCM) method and the Langmuir–Blodgett (LB) technique. Long-chain fatty acid, arachidic acid, was used as a model oily contaminant. The solid-condensed monolayers of arachidic acid on water were transferred onto the QCM by the vertical dipping method. The frequency change of the QCM at each immersion–withdrawal cycle indicated that the monolayers were successfully deposited on the gold surface of the QCM as Y-type multilayers. The QCM having the LB films of arachidic acid was cleaned in aqueous solutions containing sodium chloride, sodium hydroxide, surfactants and/or ethanol by applying ultrasonic waves. Surfactants used were sodium dodecyl sulfate (SDS) and n-octyl β-d-glucopyranoside (OGP). The detachment efficiency of the LB films from the gold surface was evaluated from the frequency change of the QCM due to the detachment. The efficiency increased with time and showed saturation at 20–30 min. The detachment was found to increase with increasing concentrations of sodium hydroxide, SDS, OGP, and ethanol. At the critical micelle concentrations of SDS and OGP or at ca. 50 vol.% of ethanol, the LB films were perfectly removed from the gold surface. The detachment efficiencies obtained in all systems were correlated with the free energy change due to liquid penetration between the LB films and the gold surface, which was calculated from the experimentally determined contact angles and surface free energies. The saponification of arrachidic acid by alkali as well as the liquid penetration was found to contribute to the detachment of the LB films. The exposure of the 172 nm ultraviolet excimer light prior to the ultrasonic washing was effective for the removal of the LB films.