SURFACE AND BULK INTERACTIONS OF ORGANIC-MOLECULES WITH CALIXARENE LAYERS

Modified calixarenes can be used as model compounds to study molecular recognition since their molecular cavities reversibly incorporate sma ll organic molecules. This effect has been used in chemical sensors wi th bulk and surface acoustic wave devices, coated with thin calixarene layers. These devices sensitively convert the mass changes during mol ecule/calixarene interactions into electronic signals. Thin films of m odified calixarenes were prepared with various side groups and various sizes by Knudsen sublimation under well-defined, ultra-high-vacuum co nditions. The interaction with perchloroethylene, chloroform, benzene, and toluene at constant temperatures, T, and partial pressures, p(i), was studied systematically for different film thicknesses, d, by mean s of mass changes DELTAm = f(T, p(i), d). From the thickness dependenc e of values, DELTAm(eq), obtained under thermodynamic equilibrium cond itions, surface and bulk effects during molecule calixarene interactio ns were separated. Pronounced excesses of surface concentrations of or ganic molecules have been found. Activation energies for molecular des orption from surface sites and for diffusion to subsurface sites were determined from mass spectroscopic results of thermal desorption behav iour. Experimental data of adsorption and desorption energies were con firmed by theoretical force field calculations.