A combined synchrotron powder diffraction and vibrational study of the thermal treatment of palygorskite–indigo to produce Maya blue

Abstract

The heating process (30-200 QC) of a paly gorskite-indigo mixture has been monitored in situ and simultaneously by synchrotron powder diffraction and Raman spectroscopy. During this process, the dye and the clay interact to form Maya blue (MB), a pigment highly resistant to degradation . It is shown that the formation of a very stable pigment occurs in the 70-130 QC interval; i.e., when palygorskite starts to loose zeolitic water, and is accompanied by a reduction of the crystallographic a parameter, as well as by alterations in the C=C and C=O bonds of indigo. Mid-and near-infrared spectroscopic work and rnicroporosity measurements, employed to study the rehydration process after the complex formation, pro vide evidence for the inhibition of the rehydration of l\1B as compared with palygorskite. These results are consistent with the blocking of the palygorskite tunnel entrance by indigo molecules with a possible partial penetration inside the tunnels. The surface silanols of palygorskite are not perturbed by indigo, suggesting that MB is not a surface complex.

On the interpretation of the Raman spectra of Maya Blue: a review on the literature data

“…The above bands fall in the region of indigo lattice vibrations, which, following the explicit statement of Tomkinson et al, [48] is not usable for characterising the indigo-clay interaction in MB. In fact, the peak at 606 cm −1 has been also observed in the indigo spectrum and attributed to δ(C-Cring) and δ(C-N) vibrations, [22,29,38,48] while the 425 cm −1 peak could correspond to a Raman inactive vibrational mode of symmetry A u [48] eventually activated by planarity perturbation (videinfra). 2.…”

“…[29] Ref. [38] This work In all cases, chemical quality commercial indigo was used for sample preparation. ns, non-specified data.…”

“…In particular, the shift of the C O symmetric stretching mode of indigo from 1703 to 1688 cm −1 in MB can be interpreted in terms of the formation of a hydrogen bond with the nearest clay structural water molecules, whereas the hydrogen bonding through the N-H unit should be weaker. [24] Sánchez del Río et al, [38] combining Raman and near infrared (NIR) data, conclude that the changes observed in C O and C C features are essential in MB formation, while the interactions involving the N-H moiety and aromatic rings cannot be excluded but should be much less important.…”

“…[38] Analogously, 27 Al magic angle spinning (MAS) NMR and 1 H- 27 Al cross polarization-magic angle spinning (CP/MAS) spectra for palygorskite-indigo specimens suggest that there are no significant modifications in the environment of the Al 3+ centres upon attachment of indigo to the palygorskite matrix. [44] As previously described, it is also possible to conjecture that the variations in the Raman spectra when comparing indigo and MB are due to the alteration in the planarity of the indigo molecule as a result of the interactions with the palygorskite matrix framework.…”

“…[37] These authors consider the possibility of a head-to-tail interaction of indigo molecules in the channels of the clay. Finally, Sánchez del Río et al [38] have provided evidence of the inhibition of rehydration of MB when compared with palygorskite, suggesting a significant blocking of the tunnel entry by indigo molecules with possibly only partial penetration occurring inside the tunnels. In turn, Chianelli et al [29 -31] interpreted the infrared spectra of indigo-palygorskite specimens in terms of the direct interaction of dye molecules with Al 3+ ions acting as the essential factor determining dye attachment to the clay.…”