Application of Quartz Crystal Microbalance as a Tool for Detergency Evaluation of Fatty Acid Contamination in Aqueous...

Removal process of a fatty acid from four substrates was monitored using a quartz crystal microbalance (QCM). Model substrates included carbon, silica, and gold sputtered electrodes and a polymer film prepared on a gold sputtered electrode. Stearic acid (SA), a model solid oily contaminant, was deposited on the substrates as an ultra-thin layer using the Langmuir-Blodgett (LB) technique. Cleaning tests of the SA-LB films were performed in aqueous solutions containing sodium chloride (NaCl), sodium hydroxide (NaOH), and/or sodium dodecyl sulfate (SDS). The removal efficiency was calculated from the QCM frequency vs. time curve obtained during the cleaning process. The neutralization by NaOH was effective for removing the SA-LB film from all substrates, although the reaction was slow. In the absence of NaOH, minor amounts of the SA-LB film was removed from the substrates, with the exception of silica, even in the SDS solution. To increase the removal efficiency of the SA-LB film in the absence of NaOH, the SA-LB film deposited on the substrates was exposed to atmospheric-pressure plasma before the cleaning process. This treatment promoted the removal of the film in the NaCl and/or SDS solutions, which we interpreted to be due to the hydrophilization of both the surfaces of the substrates and the SA-LB film.