The role of liquid penetration in detergency of long‐chain fatty acid

Abstract

AbstractThe detergency phenomena of a solid oily contaminant assessed with a quartz crystal microbalance (QCM) technique was discussed on the basis of surface energetics. Polyethylene, nylon 6, and cellulose acetate films were prepared on gold electrodes of the QCM by a spin‐coating method. Arachidic acid was deposited onto the QCM with and without polymer films by the Langmuir‐Blodgett (LB) technique. The QCM was then ultrasonically cleaned in aqueous solutions containing surfactant, ethanol, and alkali. The removal efficiency of the LB films of arachidic acid from the gold and polymer substrates was determined from the frequency change of the QCM due to cleaning. The efficiency was greatest for cellulose acetate, followed by nylon 6, gold, and polyethylene. For every substrate, the removal efficiency was found to increase with increasing surfactant or ethanol concentration. In the presence of sodium hydroxide, the removal efficiency further increased. The experimentally determined contact angles and surface free energies were influenced by surfactant and ethanol concentrations; however, they were not influenced by the addition of sodium hydroxide. This suggested the saponification of arachidic acid under alkali conditions was a major mechanism of fatty acid removal. The dependences of the kind of substrate and surfactant and ethanol concentrations on the removal efficiency were explained in terms of the free energy change resulting from penetration of the detergent solution between the arachidic acid film and the substrate in the zone of contact. Liquid penetration as well as saponification was found to be an oil removal mechanism in the present system.