Metachromasy in clay mineral systems - Spectrophotometric and calorimetric study of the adsorption of crystal-violet and ethyl violet by Na-montmorillonit and by Na-kaolinite

Adsorption of the metachromic (colour depends on concentration) cationic dyes crystal violet (CV) and ethyl violet (EV) by Na-kaolinite and Na-montmorillonite has been studied by visible spectroscopy and titration calorimetry. In the kaolinite system, metachromasy stems from association of the dye cations on the external surface of the clay. Enthalpies of adsorption at small dye/clay ratios are -3.5 and -3.8 kJ/mol for CV and EV, respectively. In the montmorillonite system, which has been studied more thoroughly, metachromasy stems from the π electron interactions between the dyes and the oxygen plane of the alumino-silicate. Enthalpies of adsorption (more exothermic than for kaolinite) depend on the dye/clay ratio in ways that are consistent with the spectra and the derived picture of dye/clay interactions.