Published: July 22, 2011 r2011 American Chemical Society 16764 dx.doi.org/10.1021/jp203270c | J. Phys. Chem. C 2011, 115, 16764–16776 ARTICLE pubs.acs.org/JPCC Host/Guest Interactions in a Sepiolite-Based Maya Blue Pigment: A Spectroscopic Study Roberto Giustetto,* ,†,§ Kalaivani Seenivasan, ‡,§ Francesca Bonino, ‡,§ Gabriele Ricchiardi, ‡,§ Silvia Bordiga, ‡,§ Michele R. Chierotti, ‡,§ and Roberto Gobetto ‡ † Department of Mineralogical and Petrologic Sciences, Universit  a di Torino, Via Valperga Caluso 35, 10125 Torino, Italy ‡ Department of Inorganic, Physical and Materials Chemistry, Universit  a di Torino, via Giuria 7, 10125 Torino, Italy § NIS Centre of Excellence, Via Quarello 11, 10135 Torino, Italy 1. INTRODUCTION Maya Blue is a well-known blue pigment used by ancient Mayas mainly in the Yucatan peninsula (Mexico) from VI to XVI century A.D., to decorate statues, pottery, and mural paintings. Rediscovered in modern times, 1 Maya Blue immediately cata- lyzed the interest of the scientific community due to its astound- ing stability being basically unaffected by both acids and alkali attacks and immune to light exposure. It is nowadays reckoned that Maya Blue has to be considered a precursor of modern nanocomposite materials, in which the guest indigo dye is adsorbed on a hosting microporous clay (palygorskite and/or sepiolite). Procedure for the pigment preparation is well- known, both using raw materials and ancient Mayan technologies (clays extracted from local outcrops and indigo from the leaves of Indigofera suffruticosa 2 ) or pristine precursors and laboratory techniques. 3,4 The simplest way to obtain a stable pigment consists in dry grinding the clay (palygorskite or sepiolite) with proper indigo amounts (e2 wt %) and moderately heating (120À 190 °C) for variable times (30 min to several hours). Everlasting stability is achieved during heating as a ground but unheated clay/ indigo mixture, although, similar in aspect to Maya Blue, is discoloured when attacked by acids. Sepiolite is a fibrous colorless clay mineral and an end-member in the palygorskite-sepiolite group. Its structure can be described as a chessboard-like disposition of TOT units, 5 with octahedral (O) sheets broken in ribbons elongated in the [001] direction and tetrahedral (T) sheets, which maintain their continuity thanks to a periodic inversion in the orientation of the apical oxygen atoms, pointing alternatively up and down and bonding to the upper or lower discontinuous O strip (Figure 1a). Microtunnels, filled by weakly bound zeolitic H 2 O and exchangeable cations, cross the structure along [001]. Tightly bound, structural OH 2 6 completes the coordination of Mg ions at the edges of the O ribbons. Differences between sepiolite and palygorskite stand in the tunnel dimensions (10.6 Â 3.7 Å in the former and 6.4 Â 3.7 Å in the latter), TOT ribbons width (eight O sites in the former and Figure 1. (a) The structure of sepiolite (Post et al. 50 ); (b) the indigo molecule. Received: April 8, 2011 Revised: July 4, 2011 ABSTRACT: Maya Blue pigment forms through heating-induced encapsula- tion and bonding of indigo in microporous clays, namely, palygorskite or sepiolite. Stabilizing host/guest interactions in a sepiolite-based Maya Blue were investigated by means of vibrational and NMR spectroscopies and proved to be H-bonds formed between indigo reactive groups and the clay structural OH 2 . Indigo sorption inhibits sepiolite structural folding induced by partial OH 2 loss with temperature rise, but despite this, no direct Mg/indigo bonding was detected. Encapsulation is favored by breaking of intermolecular bonds, allow- ing dye diffusion inside the clay microtunnels. H-bond formation involves both CdO/NÀH indigo groups, its molecule undergoing distortion and partial oxidation to dehydroindigo. Both the strength and number of sepiolite/indigo interactions are less marked than those occurring in a palygorskite-based composite. In the wider sepiolite channels, H-bonds can form only on one side of the guest molecule, whereas both sides are involved in palygorskite. Halving in bond number undermines the stability of the resulting adduct.