Infrared study of the effects of thermal treatment on montmorillonite-benzidine complexes III. Mg-, Ca- and Al-montmorillonite

Mg-, Ca-, and Al-montmorillonite were saturated with benzidine and gradually heated under vacuum to 200°C. IR spectra were recorded at various temperatures. X-ray diffractions were recorded before and after the thermal treatment. In Mg and Ca clays three types of association between the clay and the organic molecule (A, B, and D) were identified. In type A the benzidine is bound to the exchangeable metal cation through a water molecule bridge. Hydrogen bonds between water and benzidine molecules are obtained by H₂O-proton donation to N. In type B NH₂ group of benzidine is also bound to the metal cation through a water molecule bridge, but the hydrogen bonds are obtained by NH₂-proton donation to water-O. In type D benzidinium cation, produced by protonation of benzidine in the interlayer space, is bound to the clay by electrostatic forces. Before the thermal treatment type A predominates. During heating water is evolved, a direct coordination of the benzidine to dehydrated metal cation is obtained (type C), and more benzidinium is obtained in the interlayer space. Ammonium-amine is also obtained (types E). In Al clay types D and E predominate before the thermal treatment and persist at 200°C. More benzidine is protonated with the rise in temperature. In Al, during the thermal treatment a very small fraction of the benzidine is oxidized to semiquinone and quinoidal cation.