Simultaneous determination of the concentration of methanol and relative humidity based on a single Nafion(Ag)-coated quartz crystal microbalance Original Research Article

A quartz crystal microbalance (QCM) coated with Nafion® film recast from Nafion®(Ag) complex solution has been used to investigate the interaction between methanol, water and Nafion®(Ag) and to determine simultaneously the concentration of methanol and water (relative humidity). The frequency shift caused by methanol and water adsorbed onto Nafion®(Ag) was in the order: water>methanol, due to larger association constant of water than methanol. Binding rate constant analysis showed that methanol demonstrates larger binding and dissociation rate constants than water due to larger vapor pressure of methanol. Molecular mechanics calculation gave binding energy change between Nafion®(Ag) and methanol or water molecules. Artificial neural networks (ANNs) method was utilized to process the frequency response data of the single piezoelectric crystal at different times in the light of different adsorption dynamics of methanol and water on the Nafion®(Ag) coated QCM. Concentrations of methanol and water can be estimated simultaneously with the ANNs.