Sorption of water investigated with the photoacoustic effect

Abstract

Water adsorbed in the bulk of a sample essentially modifies the heat capacity, thermal conductivity, and, therefore, the transport of heat. At small concentrations of adsorbed water, this effect governs the photoacoustic (PA) signal measured in a gas–microphone cell. At higher concentrations, temperature-induced periodical adsorption and desorption mainly determine the PA signal. This effect is described formally by extending a model of oscillating mass diffusion, developed earlier to explain the photoacoustic effect on liquids, to the process of sorption. Its contribution to the PA amplitude is proportional to the sorption isotherm multiplied by the heat of sorption. Measurements with zeolite as an adsorbent in equilibrium with water vapour confirm the model. In addition, qualitative measurements on a sheet of brass, on filter paper, and on pine wood are included.