Time-dependent spectrophotometric study of the interaction of basic dyes with clays - III. Mixed dye aggregates on SWy-1 and Laponite

The interactions of a pair of different cationic dyes (methylene blue and acridine orange) in the presence of dilute aqueous suspensions of the montmorillonite SWy-1 and Laponite clays were studied using spectrophotometric techniques. A standardized procedure was developed to prepare the samples and perform the spectrophotometric measurements of the clay-dye suspensions. The observed nonadditivity of the absorption spectra of the mixture compared with those of the individual dyes is attributed to the formation of mixed aggregates on the clay particles. Significant spectral variations with time were observed: for suspensions containing the montmorillonite SWy-1 they were mainly due to the reorganization of the dye molecules on the clay particles. For systems containing Laponite, the spectral changes were ascribed to processes of approximation-association of the clay particles. At longer times (24 h) the association processes prevail, even for the SWy-1 suspensions. After sufficient time, both the SWy-1 and the Laponite suspensions showed similar spectra, indicating that the processes occurring in both systems lead to similar configurations. Schemes are proposed to describe the temporal variations occurring in these systems. (C) 1998 Academic Press.