Time-dependent spectrophotometric study of the interaction of basic dyes with clays .2. Thionine on natural and synthetic montmorillonites and hectorites

The behavior of thionine in the presence of Wyoming (SWy-1) and Barasym montmorillonite and hectorite and laponite clays were studied by recording the time evolution of the visible spectrum of the dye. Peaks corresponding to monomers, dimers, and higher aggregates were detected initially at 608, 565, and 525 nm, respectively. The montmorillonite dyes also presented a peak around 695 nm, ascribed to the acid form of the monomeric dye. The evolution of the spectra with time shows that there is a migration of monomers to the internal domains of the clays, as well as a deaggregation of the higher aggregates. Only monomers and protonated monomers are found in those regions, with absorption bands at 620 and 698 nm. The observed red-shifts of the species present in these systems confirm that the interaction of the dyes in the internal domains of the tactoids are stronger than on the external surfaces. This fact, together with the time evolution behavior permits one to rule out the presence of dimers in the internal regions of the tactoids. (C) 1996 Academic Press, Inc.