Indigo–sepiolite nanohybrids: Temperature-dependent synthesis of two complexes and comparison with indigo–palygorskite systems

Host/Guest Interactions in a Sepiolite-Based Maya Blue Pigment: A Spectroscopic Study

“…The presence of indigo, therefore, causes the structure of dihydrated sepiolite not to collapse, which is consistent with evidence described by previous authors. 23,53 (iii) Maxima at 3600 and 3531 cm À1 are basically left unchanged with respect to pristine sepiolite, implying that at least half of the Mg-coordinated OH 2 is still in the channel in spite of the applied heating. A broad and completely new band, however, appears at lower wavenumbers (3420 cm À1 ), finding no counterpart in pure sepiolite or indigo IR spectra.…”

“…53 (ii) The 3674 cm À1 maximum (hydroxyls of the octahedral sheet: MgÀOH) is still evident, whereas the 3691 cm À1 band decreases and probably shifts at lower frequency (broader 3640 cm À1 feature). Such evidence suggests that indigo fixation is likely to occur inside the hosting clay tunnels, where it allegedly inhibits structural folding of sepiolite in spite of the intervened loss of a significant fraction (nearly half) of structural OH 2 .…”

“…45,48 This very same phenomenon, in fact, is expected to cause transformation of the unstable alleged direct bonds between Mg and indigo into a different and more stable configuration as time proceeds. 53 The Journal of Physical Chemistry C ARTICLE On the basis of this experimental evidence, the existence of direct interactions between the octahedral Mg and the reactive indigo groups at the adopted extreme experimental conditions (150°C at pressure below 5 Â 10 À4 mbar) remains therefore dubious, as all involved shifts and intensity variations related to both the hosting matrix and the guest indigo IR-active modes apparently point to the existence of possible H-bonding interactions with the dye acceptor and donor groups, mediated by sepiolite Mg-coordinated OH 2 .…”

“…23,36 Recent experiences, however, showed that these straight interactions between octahedral ions and carbonyl/ammine groups of indigo tend to disappear in time, as the high-temperature clay/dye composites are cooled in a moisturizing environment. 53 …”

Application of solid-state electrochemistry techniques to polyfunctional organic–inorganic hybrid materials: The Maya Blue problem

“…For our purposes it is pertinent to remark that, although it has been reported the possibility of preparing MB-type materials without heating [14,37], in general, it is accepted that a heating step is required for removing zeolitic and, at least partly, structural water in order to prepare MB [5]. Although a variety of MB-type materials have been recently synthesized [38][39][40], it should be noted that the Mesoamerican people extracted indigo from leaves and twigs of Indigofera tinctoria and other plants by soaking it overnight in a suspension of slaked lime in water, followed by a prolonged mechanical aeration (batido) [27][28][29][30]. This means that the composition of genuine MB could differ significantly from contemporary indigo plus palygorakite (or sepiolite) specimens.…”