Use of a surface acoustic wave device for investigation of water adsorption on NaCl

A new method for measurement of water adsorption on thin films of inorganic salts using a surface acoustic wave (SAW) device is described. Mass changes on the SAW chip were detected as a phase shift in an 83 MHz acoustic wave, measured using an electrical delay line. It was shown that coating of the sensor with polycrystalline NaCl can be achieved without appreciable loss of sensitivity for detection of the small changes in mass loading due to water adsorption. Reproducible adsorption isotherms were measured which agreed quantitatively with isotherms measured by conventional methods. There are indications that the adsorption of water on thin polycrystalline films at high humidities differs from that on bulk and single surface crystals. The SAW device can in principle be used to characterise a whole range of surfaces of interest to atmospheric chemistry, such as sea salt, sulfates, mineral dust or low volatile organics, in a relatively simple fashion, provided the substrate can be deposited in a suitable thin film or layer.