Evidence of Topological Indigo/Dehydroindigo Isomers in Maya Blue-Like Complexes Prepared from Palygorskite and Sepiolite

Abstract

Association of indigo to palygorskite and sepiolite phyllosilicates, forming Maya Blue-like systems, is studied by means of electron microscopy, solid-state multinuclear magnetic resonance (1H−13C CP MAS, 27Al, 29Si NMR), visible and infrared spectroscopies (ATR-FTIR, VIS), and solid state electrochemistry. Combination of such techniques suggest that Maya Blue must be viewed as a complex polyfunctional organic−inorganic hybrid material in which different topological isomers of indigo and dehydroindigo molecules, distributed in the surface of the palygorskite framework and in the clay channels, as suggested by size-excluding electrochemical experiments, are involved. Such isomers can tentatively be assigned to different dye−clay interactions involving hydrogen bonding with interaction with Mg2+ and Al3+ ions mediated by structural water and interaction of dye molecules with silanol units of the clay.