A nanostructural model of ethanol adsorption in thin calixarene films

In order to better understand and describe the adsorption phenomena in porous materials, parallel measurements of physical parameters by means of two independent methods were conducted with four different calixarene films (tert-butyl-calix[4,6,8]arenes and tetra-pentyl-calix[4]resorcinarene) during the adsorption of volatile ethanol molecules at partial pressures ranging from zero to saturated vapor. Measurements of the films thickness and refraction index were performed by means of reflected light interference spectroscopy with a colorimetric readout system based on the digital registration of RGB color components. The mass of adsorbate molecules in the sample films was estimated using quartz crystal microbalance measurements, in accordance with Sauerbrey equation. For mathematical description of the adsorption process in the composite material, the generalized Bruggeman relation was applied. A physical model of the porous calixarene film structure is proposed, with the film being comprised of calixarene material with stochastically distributed nanocavities where adsorption takes place, and the adsorbate layer covering the inside surface of cavities. Assuming a spherical shape for the nanocavities, the pores concentration, radius, and the effective thickness of adsorbed ethanol layer were estimated for C[6,8]A and C[4]Re films. Based on the obtained quantitative data, some conclusions are made on the character of ethanol adsorption in the investigated samples.