| Abstract: | The light absorption spectra of monoionic montmorillonite suspensions with Li, Na, K, Rb, NH4, Cs, Mg, Ca and Ba as adsorbed ions were studied in the wavelength range 200–800 mμ. Theoretical analysis of the spectra showed that in the visible range, only scattering is responsible for light intensity attenuation. The scattering was found to depend on the type of adsorbed ion and to increase systematically along the series from Li to Ba. This was attributed to increased particle size of the clay caused by parallel plate condensation to form tactoids. A geometrical model of a montmorillonite tactoid was assumed. The general dependence of the properties of such tactoids, and of their suspensions, on the number of plates per tactoid was formulated. Using the measured values of light absorbance index, and the theoretical model, the relative sizes, measured by the number of plates per tactoid, in the different ionic forms were calculated. The relative sizes thus estimated were found to agree favorably with relative sizes calculated from negative adsorption surface area measurements. A typical peak of specific light absorption was observed at 240–245 mμ, for both the suspensions and their clay-free supernatant solutions. Though the compound responsible for this peak is unidentified yet, its quantity in the various suspensions could be roughly estimated. The quantity was found to be negatively correlated with the relative size of the tactoid. It was concluded that the average particle size of montmorillonite in suspension markedly and systematically depends on the type of adsorbed ion. The implications of this finding are discussed. |
