Spectroscopic characterization of a sepiolite-based Maya Blue pigment

Maya blue, an ancient pigment used by Mayas in Mesoamerica, can be considered an ancestor of modern nano-composites as it forms by grinding and heating the guest indigo molecule with a hosting clay framework (palygorskite or sepiolite). specific bonds form between the guest dye and the hosting matrix, conferring the pigment an exceptional stability to both acid and alkali attacks. While most works have been focused on the study of the palygorskite/indigo complex, this project is aimed to unveil the structural features of Maya blue when sepiolite is the hosting framework. a freshly-synthesized sepiolite-based Maya blue was prepared accordingly to ancient Mayas recipes and investigated with various spectroscopic techniques (uv-visible, FtiR, Raman). all evidences show that by strongly crushing and heating (190°C in air or 150°C in vacuum) the sepiolite + indigo (2 wt%) composite, the dye aggregates dissociate to monomers favouring diffusion inside the clay tunnels emptied from loosely bound zeolitic H2o. once encapsulated indigo partly decomposes to dehydroindigo, its oxidized form, whose flexible molecule can easily diffuse inside the tunnels slightly changing the pigment colour (from deep blue to blue/green). applied heating, however, does not cause loss of Mg-coordinated oH2 which forms H-bonds with the indigo reactive groups (C=o and n–H) after encapsulation, thus stabilizing the composite. spectroscopic evidences supporting presence of H-bonds are less evident in sepiolite-based rather than in palygorskite-based composites with indigo. in fact while indigo is perfectly juxtaposed in the narrower tunnels of palygorskite (6.4 x 3.7 Å), receiving H-bonds on both sides of the molecule, in the wider tunnels of sepiolite (10.6 x 3.7 Å) its molecule has to get near to one border in order to form a single bond. in a sepiolite-based Maya blue pigment the global number of possible host/guest interactions is therefore dramatically halved with respect to a palygorskite-based one, inevitably reducing resistance to chemical attacks. Consequently, the extent of Maya blue renowned stability is directly related to the composition of its main clay constituents.