PHOTOPHYSICAL INVESTIGATION OF THE DEGREE OF DISPERSION OF AQUEOUS COLLOIDAL CLAY

The photophysical properties of water-soluble tetrakis(N-methylpyridyl)porphyrin (TMPyP) are altered upon adsorption to the surface of a smectite clay mineral in an aqueous dispersion. In particular, the Soret absorption band shifts from 421 nm in aqueous solution to 452 nm when the molecule is adsorbed on the external face of a clay sheet in contact with the bulk solution. Upon intercalation between clay sheets, the Soret band further shifts to 488 nm. The net Soret absorption band observed in a particular clay sample is a superposition of contributions from intercalated versus externally adsorbed porphyrin and provides a convenient method to monitor the degree of dispersion or extent of deflocculation of a clay dispersion to produce primary particles or single sheets. Further, the spectral changes can be utilized to assess the effect of chemical and physical modification to the clay dispersion on the extent of face-to-face particle aggregation. Shifts in the fluorescence spectrum parallel those of the Soret band. A mechanism which could account for the substantial spectral changes observed is proposed. © 1990, American Chemical Society. All rights reserved.