COMPARISON OF CRYSTALLINE (H-MAGADIITE) AND AMORPHOUS SILICAS USING INVERSE GAS-CHROMATOGRAPHY AT FINITE CONCENTRATION CONDITIONS

Two amorphous silicas prepared by hydropyrogenation or by precipitation, two crystalline silica H-magadiite samples and a modified sample obtained by intercalation of quaternary ammonium surfactant, were characterised using inverse gas chromatography at finite concentration conditions, with n-octane as probe. The influence of crystallinity on the isotherm shape of n-octane was clearly observed. Crystalline silica (H-magadiite) exhibits a higher capacity of adsorption, at low pressure, than the amorphous silicas, a property that is certainly closely connected with the existence of high-energy sites on the lateral surfaces of the crystal. Modification of the crystalline silica by the insertion of surfactant into the interlayer space, induces a strong change in the capacity of the latter, due to the surfactant-probe interaction. Distribution functions of the adsorption energy, computed from the isotherm using the condensation approximation, confirm the previous interpretation that crystalline silica shows a much higher surface heterogeneity than amorphous silicas.