Thioindigo Interaction with Palygorskite and Sepiolite

Pigments developed by the Mayan civilization are now known to be significantly `environmentally friendly' a technical skill developed circa 250-900 C.E! [1]. One such pigment called Maya Blue, has been the focus of numerous studies and is believed to be a mixture of palygorskite clay and indigo dye [2,3]. Several derivatives of this pigment have been now developed with intriguing properties. For instance, the dye, textit{thioindigo}, reacts with the \textit{palygorskite} clay to exhibit a broad range of colors from red to blue under UV-Vis excitation. The range of colors produced with \textit{sepiolite} clays is smaller. We present spectroscopic analyses of pigments derived from \textit{thioindigo:palygorskite} and \textit{thioindigo:sepiolite} mixtures. $^{27}$Al MAS-NMR spectra of \textit{sepiolite} mixtures clearly showed changes in the Al coordination upon reacting with \textit{thioindigo}. However, palygorskite-dye mixtures showed only slight changes in Al coordination. Future work will involve $^{27}$Al MAS-NMR analyses of \textit{thioindigo} and clays rich in tetrahedrally coordinated Al to confirm the coordination changes in Al in the presence of \textit{thioindigo}.