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(+center dot)), which is responsible for the blue suspension color (lambda(max) similar to 600 nm), is the predominant species. At pH lower than 3, the dication species BZ(2+) (yellow color, lambda(max) similar to 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(+center dot) and BZ(2+) 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(+center dot) and BZ(2+) 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, BZ(2+) species was not identified maybe due to the predominance of the less reactive protonated BZ(+center dot) 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(+center dot) radical cations. (c) 2006 Elsevier B.V. All rights reserved.