%0 Journal Article
%T Computational Study of the Structure of a Sepiolite/Thioindigo Mayan Pigment
%A Manuel Alvarado Jr.
%A Russell C. Chianelli
%A Roy M. Arrowood
%J Bioinorganic Chemistry and Applications
%D 2012
%I Hindawi Publishing Corporation
%R 10.1155/2012/672562
%X The interaction of thioindigo and the phyllosilicate clay sepiolite is investigated using density functional theory (DFT) and molecular orbital theory (MO). The best fit to experimental UV/Vis spectra occurs when a single thioindigo molecule attaches via Van der Waals forces to a tetrahedrally coordinated cation with an additional nearby tetrahedrally coordinated also present. The thioindigo molecule distorts from its planar structure, a behavior consistent with a color change. Due to the weak interaction between thioindigo and sepiolite we conclude that the thioindigo molecule must be trapped in a channel, an observation consistent with previous experimental studies. Future computational studies will look at the interaction of indigo with sepiolite. 1. Introduction Among the ruins of the Mayan civilization are many examples of murals displaying a very vivid and beautiful blue paint known as Maya Blue (Figure 1). For more than 50 years this pigment has been the subject of much interest and debate among the scientific community [1, 2]. In 1931, Merwin published photographs of the ruins of a mural at Chichen Itza, noting that a blue pigment was very distinct among the other colors present [3]. The term Maya Blue was first coined in 1946 by Gettens and Stout [4] as this pigment was believed to exist exclusively on relics of the Mayan civilization in the Yucatan Peninsula region. Since these first investigations this paint has been identified in many other Mesoamerican artifacts found outside this region but is still known by its original designation, Maya Blue. Figure 1: A photograph of a monument in Chichen Itza, YUC (900 C.E.), Mexico, painted with Maya Blue, from [ 6]. This fascinating material, composed of a fibrous clay material (palygorskite) and an organic dye (indigo), has long been renowned for its chemical stability and vivid color. Maya Blue is of great interest due to its resistance to solvents, oxidants, reducing agents, alkalis, extreme humidity, acids, and exposure to ultraviolet radiation. Even more remarkable is the fact that Maya Blue contains no heavy metal content. This material is synthesized using a very simple process involving grinding the clay, mixing with the organic dye, and heating the mixture to a temperature above the boiling point of water (typically 120ˇăC to 190ˇăC), which may have been the method used by Mayan craftsmen [5]. We now know that in addition to indigo a large variety of organic dyes can be used to create similar pigments, with the choice of the dye determining the color of the finished product [6]. Additionally,
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