Chemical sensors for solvent vapors: Enthalpic and entropic contributions to host-guest interactions

Chemical sensors, based on the mass-sensitive quartz micro balance (QMB) or the surface acoustic wave device (SAW), that are coated with thin cyclopha ne layers allow the detection of harmful organic vapors. The sensor signal of these supramolecular analyte-receptors can be predicted by a method that uses estimated free energies of the host-guest complex formation. From MM3 force field calculations, the reaction enthalpies Delta H degrees of host- guest interaction between the macrocycle and the analyte can be calculated, whereas the entropy changes are taken from condensation data. The validity of this condensation model is proven by an excellent linear correlation of the logarithm of the experimental equilibrium constant with the estimated Gibbs energy Delta G degrees. In this way promising sensor materials can be selected, even before they are synthesized.