Spectroscopic characterization and photo/thermal resistance of a hybrid palygorskite/methyl red Mayan pigment

The structural features and host/guest interactions existing in a stable, red-purplish hybrid material obtained by grinding and heating palygorskite clay with 2 wt.% acid red 2 (methyl red) were investigated by means of vibrational and UV–vis spectroscopies in controlled experimental conditions and TGA analyses. Experimental evidences suggest that modified methyl red molecules, in the form of specific zwitterions and/or altered trans-isomers, can diffuse inside the palygorskite tunnels or superficial grooves once zeolitic H2O is removed due to heating or vacuum. The dye is likely to interact with the palygorskite framework by means of several bond types, which include H-bonding to Mg-coordinated OH2 and electrostatic forces, responsible for the compound stability. Sorption in the hosting matrix enhances both the photo- and thermal stability of methyl red and increases steadiness of the palygorskite structure by preventing folding with progressive heating. Such a hybrid material, which shows the same stability and structural features of the famed Maya Blue used in Pre-Columbian America, could therefore be used as an innovative Maya Red pigment.

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► Methyl red can diffuse in palygorskite tunnels/grooves forming a stable composite.
► H-bond/electrostatic interactions form between structural OH2 and the dye carboxyl.
► Encapsulation and bonding stabilize methyl red and strengthen the clay framework.
► Palygorskite structural folding with heating is prevented by methyl red diffusion.
► Sheltering of methyl red enhances resistance to solar/UV-A irradiation and T rise.