Sorption of red dyes on palygorskite: Synthesis and stability of red/purple Mayan nanocomposites

Abstract

Palygorskite is a clay mineral whose structure is crossed by nano-channels (0.64 × 0.37 nm) filled by H 2O and exchangeable ions. When mixed and heated with indigo it forms Maya Blue, a well-known pigment in which the guest dye is encapsulated and bound within the hosting matrix tunnels, conferring the compound its astounding stability. Sorption properties of palygorskite were exploited to produce innovative nanostructured materials by grinding the clay with selected red dyes, heating and Soxhlet purifying to remove surplus. Stable palygorskite-based nanocomposites were formed with methyl red and alizarin, whereas Sudan red and murexide adducts lacked required efficiency. Existence of host/guest interactions was investigated by UV–visible and FTIR spectroscopies and X-ray diffraction; chemical stability was tested through acid and alkali attacks. Palygorskite + methyl red (2 wt.%) adduct evidenced stability comparable to Maya Blue: fixation on the hosting matrix stabilizes the dye preventing both deterioration and color changes despite severe pH fluctuations, allowing application as an environmental-friend Mayan pigment. Alizarin solvation (2 wt.%) on palygorskite, alternatively, evidenced pH induced color changes analogous to those of free molecules in solution, implying possible use of the complex as solid pH sensor. Collected evidences for both nanocomposites suggest existence of different supramolecular host/guest interactions formed in separate synthesis steps: methyl red is likely to diffuse inside palygorskite nano-tunnels after heating, forming specific bonds and providing stabilization and shielding from external environment, whereas alizarin forms surface complexes during grinding with the clay superficial silanols, allowing molecule response to pH and related color changes.