Trapping and diffusion of organic dyes inside of palygorskite clay: The ancient Maya Blue pigment

Abstract

The Maya Blue is an artificial colorant with a remarkable stability to chemical attacks and a characteristic pale blue color, with tonalities going from greenish to turquoise. A systematic study of the energetics, structural, bonding and electronic properties of the unusual stable pigment known as Maya Blue is presented. Total energy calculations based on density functional theory are used. Particular attention is paid to the high stability of the Maya blue pigment. We present a comparative study of the energetic barriers for indigo and dehydroindigo in the tunnels taking into account the presence of aluminum in the palygorskite. A clay-framework dependence for the indigo diffusion and a blocking due to dehydroindigo is shown by the binding energy as the molecules take several positions into the tunnels. In the hybrid dye-clay system, the high binding energy is due to the strong hydrogen bonds of carbonyl and amine functional groups of the dyes with the structural water molecules at the edge of the dioctahedral layer in the clay. The electronic and optical properties near the Fermi level in the nanostructured Maya blue are exclusively due to the slightly modified electronic states from the molecules with no contribution from the clay. Indigo and dehydroindigo show a red- and blue-shifted, respectively.