Surface Reactions of 3,3′,5,5′-Tetramethyl Benzidine on Hectorite

Abstract

Abstract--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.

Intercalation Characteristics of 1,1′-Diethyl-2,2′-Cyanine and other Cationic Dyes in Synthetic Saponite: Orientation in the Interlayer

“…The favorable 7r-~r interactions and balanced electrostatic interactions of the cationic dye molecules are produced by the alternating vertical orientation as shown in Figure 1 l c instead of any combination of stacking of horizontal molecules. Similar results for protonated tetramethyl benzidine on hectorite were reported by McBride (1985). Such a topology permits the adsorption of colored benzidine as monomeric cations with an orientation that is horizontal to the surface, followed by intercalation of the cations as paired species oriented vertically to the surface.…”

Oxidation of Benzidine on Low-Iron Smectites: A Proposed Mechanism

“…Adsorption of the TMBZ and BZ cations at the planar smectite surface may favor a flattened configuration of the molecule as opposed to the twisted configuration expected in solution (Figure 1). X-ray powder diffraction of moist hectorite films after equilibration with aqueous acid TMBZ solution has revealed both collapsed (12.6 /l) and expanded (19.1 ~) interlayer spacings (McBride, 1985). Electrostatic and van der Waals attractive forces combined with the motional restrictions imposed by narrow interlayer spacings may overcome the energy barrier (several kilocalories/mole) to a planar configuration.…”

“…Either of these species, if aligned fiat on the surface, can be accommodated in the narrow interlayer of TMA § This explanation of results based on site geometry is not the only possibility, inasmuch as the suppression of formation of charge-transfer complexes can also be explained by strong competition ofTMA § for exchange sites, thereby preventing a high surface loading of TMBZ. McBride (1985) noted that low adsorption densities of TMBZ on hectorite tend to inhibit the formation of the charge-transfer complex.…”

“…Benzidine (BZ) and its derivative compounds, such as 3,3',5,5'-tetramethylbenzidine (TMBZ), adsorb instantly on smectites at low pH by cation exchange (McBride, 1979(McBride, , 1985. For smectites which possess octahedral Fe 3 § in significant quantities (such as Upton, Wyoming, montmorillonite), an immediate electron transfer between adsorbed BZ or TMBZ and structural Fe occurs, with the oxidation of the BZ and TMBZ molecules to form a blue charge-transfer complex (McBride, 1985).…”

“…For smectites which possess octahedral Fe 3 § in significant quantities (such as Upton, Wyoming, montmorillonite), an immediate electron transfer between adsorbed BZ or TMBZ and structural Fe occurs, with the oxidation of the BZ and TMBZ molecules to form a blue charge-transfer complex (McBride, 1985). This reaction consumes little or no molecular oxygen from solution at least initially, although evidence exists for a small 02 consumption after long equilibration times, which may reflect a slow reoxidation of structural Fe 2 § by 02 (unpublished data).…”