USE OF THE QUARTZ-CRYSTAL MICROBALANCE FOR THE STUDY OF ADSORPTION FROM THE GAS-PHASE

The quartz crystal microbalance has been used to study adsorption from the gas phase onto a gold surface. The commercial device used in our measurements has a resolution of 0.15 Hz, corresponding to 1.8 ng/cm2. Changes of frequency are caused by a change of mass, but factors such as the total pressure, the viscosity, the density, the surface roughness, the temperature, and the thermal conductivity of the medium may also cause a shift in frequency. A new method, which we call the supporting gas method (SGM), was developed to eliminate the effect of these factors on the frequency of the crystal. In this method the substance being studied is mixed with a large excess of an inert gas (Ar was mostly used, but H-2 and He yielded identical results). In the course of measurement, the total pressure is maintained constant, while the partial pressure of the substance being studied is varied. The adsorption isotherms of a number of substances were determined. A comparison between benzene and pyridine showed that the former is adsorbed flat on the surface, while the latter is attached to it in an upright orientation, through the nitrogen atom. The saturated homologues of these two compounds, cyclohexane and piperidine, respectively, do not exhibit monolayer adsorption. Water, methanol, and 1-propanol are all adsorbed, occupying the same number of sites per molecule on the surface. It may be concluded that these compounds are attached to the surface through the OH group, with the other atoms facing away. The SGM can be a powerful tool in the study of submonolayer adsorption from the gas phase. The sensitivity is high, on the order of a few percent of a monolayer, and can be increased further. The stability is excellent, and seems to be determined by the parameters of the system as a whole, and not by the inherent properties of the quartz crystal microbalance itself There are several sources of error which can lead to misinterpretation of the results. These can be reduced to an insignificant level by making measurements at a constant total pressure, employing an inert gas in large excess.