Evaluation and Interpretation of the Experimental Data

Of primary interest in gas-phase radiochemistry are the energies (enthalpies) of desorption of molecular entities from various surfaces. For a new element only single chromatographic measurement of the adsorption equilibrium constant becomes available, and the van’t Hoff equation cannot be applied. The only way is to calculate the entropy change from the first principles. Usually, one has assumed smooth homogeneous surface and mobile adsorption. For known compounds their desorption energies from silica correlate with the vaporization (or sublimation) energies; moreover, the values differ little. The finding is difficult to rationalize for smooth bare surfaces and stimulates considering real surfaces. Those of silica and metals are inherently rough and heterogeneous. In the chromatographic experiments with halides, they become covered (modified) by bonded fragments of the reagent molecules. Then a molecule adsorbed at the possible sites in the form of nanoscale wells (pockets) needs similar energies to escape as to leave its own condensed phase. However, accounting for the mostly localized adsorption in the entropy calculations further raises the above “experimental” desorption energy. Meanwhile, heterogeneity makes the latter be an effective value, necessarily less than the maximum in the spectrum. Hence, the highest desorption energy may be considerably larger than the sublimation energy. Possibly the source of it, as well as of the considerable scatter of the data, is incomplete and poorly reproducible modification of the surface. Gas-solid chromatography, besides mere adsorption℃desorption of unaltered molecules, may be based on chemical mechanisms; some of them are evidenced and characterized.