Benzidine oxidation on cationic clay surfaces in aqueous suspension monitored by in situ resonance Raman spectroscopy

In situ resonance Raman spectroscopy was employed to correlate the blue, yellow, purple, and orange-brown colors with the organic species that appear in aqueous suspensions of benzidine and montmorillonite clay (BZ–MMT system) at several pHs. From pH 3 to 9, the radical cation of BZ (BZ+), which is responsible for the blue suspension color (λmax ∼ 600 nm), is the predominant species. At pH lower than 3, the dication species BZ2+ (yellow color, λmax ∼ 440 nm) is the main compound. When the blue BZ–MMT slurry is dehydrated, giving a yellow color, the resonance Raman technique indicates the presence of BZ+ and BZ2+ species. At pHs higher than 9, the orange-brown color is identified, for the first time, as poly(benzidine) (PBZ). A purple color developed after standing the blue BZ–MMT aqueous suspension for 3 weeks (so far not identified) comes from a mixture of BZ+ and BZ2+ species together with PBZ. The BZ oxidation on the synthetic Syn1 and pillared MMT clays in aqueous suspensions at different pHs was also investigated. In these acidic clays, BZ2+ species was not identified maybe due to the predominance of the less reactive protonated BZ+ on clay surfaces. At pHs above 9, these clay suspensions seem play a role of catalyst in the benzidine polymerization through the reaction between unprotonated BZ+ radical cations.