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

We introduce a novel empirical method based on DELTAH(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, DELTAG(a), the free energy of adsorption of probes, is related to DELTAH(vap)d. As in the methods of Sawyer, Papirer, Schultz and Donnet we obtain a straight line for alkanes when DELTAG(a) is plotted vs. DELTAH(vap)d. For polar probes interacting by both dispersive and specific forces, the experimental data lie above the alkane linear plot. DELTAG(a)AB, the specific acid-base contribution to DELTAG(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 DELTAG(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) DELTAG(a)AB values match those obtained by the methods of Papirer and of Sawyer. DELTAH(vap)d for polar probes (for alkanes DELTAH(vap)d=DELTAH(vap)) can easily be determined from the heat of mixing at infinite dilution in apolar solvents, as recommended by Fowkes.