Surface Reactions of 3,3',5,5'-tetramethyl Benzidine on Hectorite

-The adsorption and oxidation of 3,3',5,5'-tetramethyl benzidine (TMB) on hectorite has been investigated using X-ray powder diffraction, ultraviolet-visible spectroscopy, electron spin resonance, and infrared spectroscopy. The molecule adsorbed by cation exchange at low adsorption levels and oxidized to the monomeric radical cation (yellow). At higher adsorption levels, intercalation of TMB occurred in amounts greater than the cation-exchange capacity of the hectorite, and the ~r-~r charge-transfer complex (blue) became much more evident. The TMB monomers appeared to lie fiat in the layer silicate interlayers, whereas the molecules in the charge-transfer complexes assumed a near-vertical orientation relative to the surface. The oxidation of adsorbed TMB was probably due to diffusion of O2 to the surface, because the structural Fe 3+ content of the hectorite was too low to facilitate a significant quantity of direct Fe 3+TMB electron transfer. Key Words--Benzidine, Charge-transfer complex, Color, Electron spin resonance, Hectorite, Infrared spectroscopy, Ultraviolet-visible spectroscopy. I N T R O D U C T I O N Various aromatic amines react with smectites to generate strongly colored clay-organic complexes (Hakusui et al., 1970; Tennakoon et aL, 1974b). Studies of the interaction of benzidine with smectites to form a blue complex on the hydrated surfaces have been particularly numerous (see, for example, Lahav and Raziel, 1971). The oxidation mechanism evidently involves electron transfer from adsorbed benzidine to structural Fe 3+ in the octahedral layer of clays (Tennakoon et al., 1974a) to form the monovalent radical cation. Dehydration of the clay reversibly shifts the equilibrium in favor of the yellow quinoidal divalent cation (Hakusui et al., 1970). Although oxidation of benzidine on smectites having structural Fe 3+ is rapid, slow oxidation occurs in the presence of 02 on smectites with no significant Fe 3+ content (McBride, 1979). Benzidine adsorption initially occurs via ion exchange as the protonated monovalent and divalent ions (Furukawa and Brindley, 1973), but intercalation of benzidine in excess of the cation-exchange capacity can also occur on smectites (Tennakoon et al., 1974a). Certain aspects of the reactions of benzidine and related molecules with smectites remain to be clarified, most particularly the interlamellar alignment and intermolecular interactions of the amines and their oxidation products. The present study was undertaken to determine the types of clay-organic complexes formed over a range of adsorption levels on the clay. A methylated derivative of benzidine, 3,3',5,5'-tetramethyl benzidine (TMB), was used for the study because it is much less mutagenic than benzidine, and because prel iminary experiments showed it to react with smectites to form colored complexes similar to those of benzidine. Copyright 9 1985 The Clay Minerals Society MATERIALS AND METHODS Natural hectorite obtained from the Baroid Division of NL Industries, with a previously determined cationexchange capacity (CEC) of 71 meq/100 g (McBride, 1979), was treated with 1 M Na-acetate (pH 4.8) to remove any free carbonates, then with citrate-bicarbonate-dithionite to dissolve Fe oxide impurities. The hectorite was then saturated with Na + using 3 washings containing excess NaC1 and dialyzed to remove free salt. The dihydrochloride salt of 3,3',5,5'-tetramethyl benzidine (TMB. 2HCI) with the chemical structure: H3C CH 3 o ~ .__/ -o *H3n ~ NH~ CC