Determination of acid–base properties of solid materials by inverse gas chromatography at infinite dilution. A novel empirical method based on the dispersive contribution to the heat of vaporization of probes

Abstract

We introduce a novel empirical method based on ΔH_vap^d, the dispersive component of the heat of vaporization of probes, to assess the acid–base properties of solid surfaces quantitatively by inverse gas chromatography (IGC) at infinite dilution. In this method, ΔG_a, the free energy of adsorption of probes, is related to ΔH_vap^d. As in the methods of Sawyer, Papirer, Schultz and Donnet we obtain a straight line for alkanes when ΔG_a is plotted vs.ΔH_vap^d. For polar probes interacting by both dispersive and specific forces, the experimental data lie above the alkane linear plot. ΔG_a^AB, the specific acid-base contribution to ΔG_a calculated by our new method, matches that determined by the four methods mentioned above. In particular, in the case of self-associated probes (e.g. tetrahydrofuran and ethyl acetate) it yields ΔG_a^AB values similar to those determined by the method of Donnet et al., whereas for non-self-associated probes (e.g. chloroform and diethyl ether)ΔG_a^AB values match those obtained by the methods of Papirer and of Sawyer. ΔH_vap^d for polar probes (for alkanes ΔH_vap^d=ΔH_vap) can easily be determined from the heat of mixing at infinite dilution in apolar solvents, as recommended by Fowkes.