New approach to characterise physicochemical properties of solid substrates by inverse gas chromatography at infinite dilution: III. Determination of the acid–base properties of some solid substrates (polymers, oxides and carbon fibres): a new model

In this paper, the inverse gas chromatography (IGC) technique was used to calculate the acid–base superficial characteristics of some solid substrates such as oxides, polymer and polymer adsorbed on oxide. The acid–base constants were calculated for many solids: Monogal, MgO, ZnO, SiO2 and Al2O3, four different carbon fibres and polymers such as poly(methyl methacrylate) (PMMA) at various tacticities adsorbed on alumina or silica. The determination, by IGC, of the specific free enthalpy ΔGspa of adsorption corresponding to the specific interactions of polar molecules with the solid, as a function of the temperature, allowed to obtain the specific enthalpy ΔHspa and specific entropy ΔSspa. Knowing ΔHspa of the various polar molecules, we were able to determine the acidic constant KA and basic constant KD, the two constants characterizing solid substrates like PMMA, PMMA–SiO2 or PMMA–Al2O3, using the following classical expression:−ΔHspa=KADN+KDANwhere DN and AN are respectively the electron donor and acceptor numbers of the solid substrates. This study showed an important effect of the tacticity on the acid–base properties. On the other hand, we proved that the previous relation was not correct in many cases and especially for some oxides (as MgO, ZnO and Monogal) and carbon fibres. We proposed a new relationship by adding a third parameter K reflecting the amphoteric character of the solid according to:−ΔHspa=KADN+KDAN−KDN×AN.