Adsorption of alizarinate-micelle complexes on Na-montmorillonite

The adsorption on Na-montmorillonite of the monovalent anion alizarinate and its complexes with micelles of cationic surfactants was investigated. Micelles were prepared from octadecyltrimethylammonium (ODTMA) and benzyldimethylhexadecyl-ammonium (BDMHDA). Alizarinate-micelle complexes are much more efficient for the adsorption of the dye than alizarinate alone, e.g., from 0.05 mM alizarinate solution 12.5 mmol per 100 g clay was adsorbed in the presence of micelles, whereas only 4 mmol per 100 g clay was adsorbed in their absence. However, unlike the adsorption of alizarinate on the clay, where its adsorbed fraction increased with the concentration of the clay, in the presence of micelles, the concentration of the clay should be optimized for achieving maximal adsorption of alizarinate. This effect is due to monomer adsorption on excessive clay sites, which leads to the decomposition of micelles, and consequently, the adsorption of alizarinate is reduced. Visible spectra of alizarinate-micelle complexes in water and adsorbed on montmorillonite were recorded. The absorption band of alizarinate in aqueous solutions at 522 nm was red-shifted in the presence of ODTMA and BDMHDA micelles due to the effect of the micelle microenvironment. Further shifts were observed in the presence of montmorillonite indicating that the alizarinate-micelle complexes were adsorbed on the clay. Significant spectroscopic changes were observed after the adsorption of alizarinate-BDMHDA-micelle complex on the clay indicating that the adsorption leads to changes in the micelle. This observation suggests that the dye may serve as a probe in visible-spectroscopy study of the adsorption of micelles on clay minerals. (C) 2003 Elsevier B.V. All rights reserved.