Study of adsorption isotherms in the detection of acetone and isopropyl alcohol using QCM sensor with chitosan sensing layer

The adsorption isotherm model requires extensive study in promoting a high-performance sensor, especially where adsorption mechanisms adhere. In this work, Langmuir and Freundlich adsorption isotherms are analyzed for isopropyl alcohol (IPA) and acetone gas detection using quartz crystal microbalance with chitosan as the sensing layer. Langmuir adsorption isotherm presumed the equilibrium rate constants and monolayer adsorption formed on the surface of the adsorbent. Meanwhile, Freundlich adsorption isotherm predicted a multilayer adsorption phenomenon as the adsorbates and adsorbents are interacting with one another. Finite element simulation gives the detection response in terms of frequency shift which is then translated to mass change via Sauerbrey's equation. The relationship between mass change and the adsorption isotherm models were analyzed by constructing the regression plot in accordance with the defined adsorption equations. For both IPA and acetone, the regression values obtained from the Freundlich adsorption model fitting are at 0.98971 and 0.99572. The regression values close to 1 is demonstrating a precise estimation following the Freundlich adsorption isotherm. Thus, the adsorption of IPA and acetone gas on the surface of chitosan is compelled to multilayer adsorption rather than monolayer adsorption promoted by the Langmuir adsorption isotherm.