Sorption and condensation phenomena of volatile compounds on solid-state metalloporphyrin films

Metalloporphyrins have been in use since a decade as sensitive layers for thickness shear mode resonator gas sensors. In spite of several successful demonstrations of sensitivity properties a complete comprehension of the sensing mechanisms is still far to be reached also if it is rather clear that metalloporphyrins molecular structure determines peculiar physico-chemical interactions with guest molecules from gas phase. At the same time, the solubility properties are mostly regulated by the film architecture. In this paper a first attempt in this direction is presented through a detailed analysis of isotherm curves, typical for each volatile compounds, and related to a wide concentration range. Results show that a general mathematical model based on BET isotherms, fits adequately the experimental data. Fitted isotherm parameters show the different sensitivity features of metalloporphyrins aggregates, pointing out the role of both the surface structure and the diffusion processes through the film volume.