Metachromasy in clay minerals. Spectroscopic study of the adsorption of crystal violet by laponite

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Abstract
The adsorption of crystal violet (CV) on laponite has been investigated by normal and comparative electronic spectroscopy. The following spectroscopic phenomena have been observed: with degrees of saturation below 44 mmol CV per 100 g clay, band α showed a bathochromic shift, indicating the adsorption of monomeric CV into the interlayer space; with degrees of saturation between 44 and 130 mmol CV per 100 g clay, metachromasy and flocculation, indicating the adsorption of dimeric CV species. Since metachromasy appeared together with flocculation it was assumed that the adsorbed dimers were located in the interparticle space of the floc. With higher saturation laponite repeptized and band α again appeared, but was not shifted to longer wavelengths as before the flocculation. At this stage the monomeric cations were adsorbed in excess of the cation-exchange capacity. Consequently, the clay was peptized and the adsorbed cations were located at the water/solid interface.

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