Evidence of Topological Indigo/Dehydroindigo Isomers in Maya Blue-Like Complexes Prepared from Palygorskite and Sepiolite

Abstract

Association of indigo to palygorskite and sepiolite phyllosilicates, forming Maya Blue-like systems, is studied by means of electron microscopy, solid-state multinuclear magnetic resonance ( 1 H-13 C CP MAS, 27 Al, 29 Si NMR), visible and infrared spectroscopies (ATR-FTIR, VIS), and solid state electrochemistry. Combination of such techniques suggest that Maya Blue must be viewed as a complex polyfunctional organic-inorganic hybrid material in which different topological isomers of indigo and dehydroindigo molecules, distributed in the surface of the palygorskite framework and in the clay channels, as suggested by size-excluding electrochemical experiments, are involved. Such isomers can tentatively be assigned to different dye-clay interactions involving hydrogen bonding with interaction with Mg 2+ and Al 3+ ions mediated by structural water and interaction of dye molecules with silanol units of the clay.

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

“…[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. Thus, Coupry et al [49] attributed the new bands at 1610 and 1299 cm −1 appearing in the Raman spectra of indigodyed textiles to the activation of vibrational modes of indigo of B u symmetry due to perturbations in the planarity of the molecule.…”

“…[24,27,29] No Raman spectra have been reported for MB specimens prepared using vat-dyeing-type procedures. [25,27,28,30,37] It should be noted, however, that these methods allow only a fraction of indigo to penetrate into the clay network so that (1) treatments for removing the excess of indigo with HNO 3 , [24] organic solvents [44,45] or Soxhlet extraction [37] are probably convenient for obtaining 'clean' MB spectra and (2) the maximum amount of indigo effectively entering into the palygorskite (or sepiolite) channel system is probably below 4.5% (w/w). [37] This means that in a significant part of the literature data, one can expect that the spectrum of MB can be more or less contaminated by the spectrum of free indigo crystals adhered to the clay.…”

“…The vibrational spectrum of indigo was first analysed by Tatsch and Schrader [47] and was recently refined by Tomkinson et al [48] With regard to the variety of MB-type samples and experimental conditions, it should be noted that the preparation of synthetic analogues to MB follow the three main methods initially proposed by Van Olphen: [7] embedding palygorskite with a solution of indoxylacetate, vat dyeing and 'dry' crushing indigo with powdered clay, all followed by heating at temperatures between 130 and 200 • C. This last method has been widely used in recent literature for preparing MB pigments from indigo with palygorakite and sepiolite which is a similar aluminosilicate of composition Si 6 Mg 4 O 15 (OH) 2 · 6H 2 O but with larger channels of dimension 10.6×3.7 Å and frequently used for preparing MB-type specimens. [17,25,27,28,37,43,44] In this report, we present a review of the literature concerning the Raman spectra of indigo, synthetic MB specimens and genuine MB samples. Most of this literature is more or less explicitly influenced by three 'uniqueness hypotheses': (1) there is a unique organic component in MB; (2) there is a unique type of attachment between indigo and palygorskite: and (3) there was a unique preparation recipe used by the ancient Maya people.…”

“…Most of this literature is more or less explicitly influenced by three 'uniqueness hypotheses': (1) there is a unique organic component in MB; (2) there is a unique type of attachment between indigo and palygorskite: and (3) there was a unique preparation recipe used by the ancient Maya people. In previous works, [44,45] we have presented a series of reasons for questioning all three hypotheses based on vis-UV, attenuated total reflection Fourier transform infrared (ATR-FTIR), NMR and electrochemical data. The current report provides a survey of Raman data acting in support of the idea that (1) dehydroindigo [33] and eventually other dyes [42,56] accompany indigo in MB, (2) different topological dye isomers are probably responsible for the observed Raman spectra and (3) different types of MB samples probably exist.…”

Electroanalytical chemistry for the analysis of solids: Characterization and classification (IUPAC Technical Report)

“…"Local" analysis, performed on restricted regions of solid systems, can be made using the pencil electrode methodologies [95], of interest in the fields of archaeometry, conservation, and restoration [55]; in particular, this technique can be applied to the determination, layer-by-layer, of the composition of stratified corrosion layers in metals [198,199]. The variation of the time scale of voltammetric and chronoamperometric experiments can be applied to trace the variation of the concentration of electro active species entrapped in the porous host deep in the particles [56,200,201]. This is based on the consideration that the progress of the electrochemical reaction through the solid involves charge transfer across the same so that the electrochemical response at different times should be representative of the composition in the different regions of the particles.…”