Insertion of indigo molecules in the sepiolite structure as evidenced by 1H-29Si heteronuclear correlation spectroscopy

Despite the numerous studies of the famous indigo-based pigment Maya Blue, there are still many questions regarding the elucidation of its structure. Here, two-dimensional (2D) (1)H-(29)Si heteronuclear correlation (HETCOR) spectroscopy with frequency-switched Lee-Goldburg (FSLG) homonuclear decoupling is applied to sepiolite and sepiolite-indigo complexes. Owing to the high resolution in the (1)H dimension of the 2D (1)H-(29)Si HETCOR spectrum obtained by FSLG homonuclear decoupling, the assignment of the (29)Si cross-polarization magic-angle spinning (CPMAS) spectrum of sepiolite is clearly confirmed. Moreover, 2D (1)H-(29)Si FSLG-HETCOR spectroscopy gives the first direct evidence that some indigo molecules are inserted in the sepiolite structure whereas no interaction between indigo and the external side surface (silanol groups) is observed in the (29)Si CPMAS spectra. These results are consistent with the fact that indigo molecules interact with water coordinated to magnesium and suggest that Maya Blue made from sepiolite is not a surface complex.     

Interaction of pyridine derivatives with sepiolite

In this study, the adsorption behavior of pyridine derivatives, i.e., 2-aminopyridine and 2,2'-bipyridyl, onto sepiolite, a natural clay mineral, has been investigated by bottle adsorption and IR spectroscopic techniques. The results indicate that 2-aminopyridine and 2,2'-bipyridyl molecules adsorb onto sepiolite through hydrogen bonding of the amino groups to the water molecules in the octahedral sheet and to the surface hydroxyls (Si-OH) in the tetrahedral sheet. These findings reveal that pyridine molecules not only adsorb onto the external surface of sepiolite but are also incorporated in its channels and tunnels with adsorption taking place at corners and/or edges, depending on the position of water molecules. A structural model is proposed to account for the orientation of pyridine derivatives in the sepiolite matrix.     

Dichroism in dye-doped colloidal liquid crystals

Nematic liquid crystals were obtained in sterically stabilized suspensions of rodlike particles of sepiolite clay, with an average length up to 900 nm and aspect ratio up to 40. In agreement with computer simulations for hard spherocylinders, the isotropic-nematic transition shifted to lower volume fractions with increasing aspect ratio. However, the coexistence gap was broadened noticeably due to particle polydispersity. The sepiolite crystal structure includes channels filled with zeolitic water, which can be replaced by indigo dye molecules. The indigo molecules are constrained inside the zeolitic channels to be aligned along the long axes of the rods. As a result, the colloidal nematic phase showed a marked dichroism, with an order parameter up to 0.5 for magnetically aligned samples, similar to typical values for dye-doped thermotropic liquid crystals.     

Multinuclear magnetic resonance spectroscopy studies of the structuration of the tunnels of sepiolite in the presence of intracrystalline pyridine molecules

Sepiolite, a naturally occurring magnesiosilicate nanoporous clay mineral with a tunnel structure, contains two types of water molecules in the structure: zeolitic water trapped inside the tunnels and structural water molecules which interact strongly with magnesium cations. The zeolitic water was removed by heating the sepiolite to 120 degrees C. The partially dehydrated sepiolite absorbed pyridine vapor to produce an intercalated material where the pyridine takes the place of the zeolitic water. 1H solid-state MAS NMR spectroscopy showed that there is isotopic H/D exchange between pyridine-d5 and the remaining structural water molecules of the sepiolite framework. The exchange takes place at room temperature over several days. Wide line solid state 2H NMR of the sequestered pyridine-d5 showed that two populations of pyridine molecules coexist in the material: one very mobile physisorbed population, which can be removed by heating at 90 degrees C, and a population due to pyridine trapped in the tunnels. Except for small in-plane librations, the trapped pyridine was shown to be held rigidly by the sepiolite. When the pyridine intercalated material is heated at 400 degrees C the structural water and some of the pyridine is lost. The remaining pyridine takes the place of the structural water to produce a new inorganic-organic nanohybrid material with the pyridine bound to the terminal Mg(II) in the structure. The pyridine in this material as well as the intercalated material was characterized by slow-spinning 15N and 13C CP/MAS NMR spectroscopy. The 15N NMR was shown to be a very sensitive probe to characterize the various types of pyridine. The data indicate that pyridine molecules in the inorganic-organic nanohybrid material are directly bound to magnesium cations exposed in the tunnels of sepiolite.     

The Nature of the Colour Centres in ‘Maya Blue’ — the Incorporation of Organic Pigment Molecules into the Palygorskite Lattice

It is shown by optical spectroscopy that indigo molecules, most likely incorporated into the channels of the palygorskite host structure, are the solely colour centres in the ancient Maya Blue pigment — in use during the Maya cultural period in Yucatan and Guatemala. This study also shows that the blue colour of solid indigo shifts toward turquoise and greenish hues when fastened to the host; it is suggested that the corresponding spectral modifications originate from the energy when single indigo molecules undergo hydrogen bonding with the hydroxy‐groups in the channels (or in the open channels on the surface). Indeed artefacts from the Maya culture exhibit rather greenish than pure blue colours. Similarly a tetrachloro‐derivative of thioindigo forms hydrogen bonds with palygorskite, as evidenced by an again distinct colour change from red‐violet to blueish‐violet. Bulkier pigment molecules bearing hydroxy‐groups, such as anthrachinon derivatives, seem to directly react with the host surface, but without producing very significant colour shifts.     

Comparative study of different indigo-clay Maya Blue-like systems using the voltammetry of microparticles approach

Using the voltammetry of microparticles approach, the electrochemical response of complexes prepared with indigo plus different clays in contact with aqueous electrolytes is described. Indigo presents a strong attachment with palygorskite and sepiolite in contrast to a weak attachment to planar clays (montomorillonite and kaolinite). Cyclic voltammetric and chronoamperometric data provide estimates of the variation of the concentration of indigo and dehydroindigo with the depth on clay crystals. The indigoids (indigo and dehydroindigo) penetrate more in palygorskite than in sepiolite, and this penetration is favoured by thermal treatments (very efficient up to 130 °C). The indigo concentration decreases monotonically versus depth, while the dehydroindigo one increases from zero in the external region of the crystals to a maximum at a depth between 40 and 80 nm and then decreasing rapidly. These facts are directly linked to the much higher resistance to acid attack of palygorskite–indigo pigments (Maya Blue) than sepiolite–indigo ones.     

FT-IR Spectroscopic Investigation of Adsorption of Pyrimidine on Sepiolite and Montmorillonite from Anatolia

The adsorption of pyrimidine (PM) on natural montmorillonite and sepiolite from Turkey was investigated by FT-IR spectroscopy. The intercalation of PM within montmorillonite has been shown by X-ray diffraction to increase the interlayer spacing. The spectroscopic results indicate that PM molecules adsorbed on sepiolite are coordinated to Lewis acidic centers or surface hydroxyls by H-bonding interaction through one of the pyrimidine ring nitrogen lone pairs. Moreover, some of the adsorbed PM molecules may enter the interior channels of the sepiolite structure and replace zeolitic water. The intercalated PM molecules within montmorillonite are coordinated to exchangeable cations directly or indirectly through water bridges.     

Decolorization of vegetable oils: chlorophyll-a adsorption by acid-activated sepiolite

The adsorption characteristics of acid activated sepiolite (AAS) for the removal of chlorophyll-a (C(55)H(72)MgN(4)O(5)) from rapeseed oil was studied as a function of different sepiolite dosages and bleaching temperatures. A correlation has been shown between the adsorption capacity and a combination of AAS amount, bleaching temperature and oxidative reactions of chlorophyll-a. The adsorption equilibrium was found to follow the Langmuir isotherm model with a maximum adsorption capacity of 0.36 mg/g on AAS and K(L) value ranging from 6.11 to 19.51 kg/mg at 80 and 100 degrees C. It was found that AAS is an effective sorbent for the removal of chlorophyll-a molecule which is believed to adsorb as a protonated species onto SiOH groups at the edge in the tetrahedral sheet of sepiolite. These findings reveal that chlorophyll-a molecules not only adsorb onto the external surface of AAS but replace the released Mg(2+) ions in the octahedral sheet; they, depending on the pore size of AAS, are also incorporated in the channels and tunnels of sepiolite. A structural model is proposed to account for the orientation of chlorophyll-a in the sepiolite matrix.     

Gelatin renaturation and the interfacial role of fillers in bionanocomposites

This work describes a systematic study of gelatin-sepiolite structural bionanocomposites to show how the renaturation level of the biopolymer is highly dependent on the type of mineral particle used. The aim of the work is to prove that chemical interactions between both components (hydrogen and covalent bonding) determine the organization level of the biopolymer which in turn results in drastic differences in the elastic properties of the prepared bionanocomposites. To assess this, several systematic modifications were introduced into the silicate structure and surface, generating four derivatives. Two derivatives prepared by thermal treatments, monohydrated sepiolite and protoenstatite, and two chemically modified sepiolites, amino and epoxy terminated, were prepared and used as the inorganic (or hybrid) phase in the bionanocomposites. The thermal and chemical modifications performed on the sepiolite surface induced a dramatic decrease in the renaturation level as determined by DSC and FTIR techniques. On the other hand, untreated sepiolite induced a higher renaturation level in the polypeptide, probably due to the alignment of the collagen-like triple helix along sepiolite external surface channels. The measured mechanical properties of the studied compositions confirm that the renaturation level of gelatin is a key factor in understanding the elastic properties of bionanocomposites. These results suggest that mineral particles introduced in the polypeptide matrix provide an effective control over the matrix crystallinity giving rise to tunable mechanical properties of the final bionanocomposite.     

Adsorption of isoniazid onto sepiolite–palygorskite group of clays: An IR study

The adsorption of isoniazid (INH) on sepiolite, loughlinite (natural Na-sepiolite) and palygorskite from Anatolia was investigated by FT-IR spectroscopy. Experimental results indicated that INH molecules, adsorbed on sepiolite–palygorskite group of clays, are coordinated to surface hydroxyls by H-bonding interaction through the pyridine ring nitrogen lone pairs. Moreover, some of the adsorbed INH molecules may enter the interior channels of the sepiolite–palygorskite structure and involve H-bonding interaction with zeolitic water. Some intensity and frequency changes in the OH stretching band of surface hydroxyls (Si–OH) of the INH-treated sepiolite and loughlinite were observed. However, this band is found to be less affected by the adsorption of isoniazid in the case of palygorskite, probably because the surface Si–OH groups in palygorskite appear to be less abundant than in sepiolite or loughlinite.     

Fabrication of Microporous Metal–Organic Frameworks in Uninterrupted Mesoporous Tunnels: Hierarchical Structure for Efficient Trypsin Immobilization and Stabilization

Hierarchically porous metal–organic frameworks (HP-MOFs) are promising in various applications. Most reported HP-MOFs are prepared based on the generation of mesopores in microporous frameworks, and the formed mesopores are connected by microporous channels, limiting the accessibility of mesopores for bulky molecules. A hierarchical structure is formed by constructing microporous MOFs in uninterrupted mesoporous tunnels. Using the confined space in as-prepared mesoporous silica, highly dispersed metal precursors for MOFs are coated on the internal surface of mesoporous tunnels. Ligand vapor-induced crystallization is employed to enable quantitative formation of MOFs in situ, in which sublimated ligands diffuse into mesoporous tunnels and react with metal precursors. The obtained hierarchically porous composites exhibit record-high adsorption capacity for the bulky molecule trypsin. The thermal and storage stability of trypsin is improved upon immobilization on the composites.     

Fabrication of Microporous Metal–Organic Frameworks in Uninterrupted Mesoporous Tunnels: Hierarchical Structure for Efficient Trypsin Immobilization and Stabilization

Hierarchically porous metal–organic frameworks (HP‐MOFs) are promising in various applications. Most reported HP‐MOFs are prepared based on the generation of mesopores in microporous frameworks, and the formed mesopores are connected by microporous channels, limiting the accessibility of mesopores for bulky molecules. A hierarchical structure is formed by constructing microporous MOFs in uninterrupted mesoporous tunnels. Using the confined space in as‐prepared mesoporous silica, highly dispersed metal precursors for MOFs are coated on the internal surface of mesoporous tunnels. Ligand vapor‐induced crystallization is employed to enable quantitative formation of MOFs in situ, in which sublimated ligands diffuse into mesoporous tunnels and react with metal precursors. The obtained hierarchically porous composites exhibit record‐high adsorption capacity for the bulky molecule trypsin. The thermal and storage stability of trypsin is improved upon immobilization on the composites.     

Physico-chemical study of selected surfactant-clay mineral systems

A physicochemical study of the systems formed by the clay minerals, montmorillonite and kaolinite (layered) and sepiolite (non-layered) and the surfactants Triton X-100 (TX100, non-ionic), dodecyl sodium sulfate (SDS, anionic) and trimethyloctadecyl-ammonium bromide (ODTMA, cationic), with different chemical structure, was carried out by X-ray diffraction (XRD), infrared spectroscopy (FTIR) and thermogravimetric and differential thermal analysis (TG/DTA). TG/DTA results indicated an increase in the thermal stabilization of non-ionic (TX100) and cationic (ODTMA) surfactants adsorbed by all clay minerals in relation to pure compounds. This effect was greater in montmorillonite and sepiolite than in kaolinite owing to these minerals must allow the establishment of a stronger bond with the surfactants as indicated by XRD and FTIR results. Differences in decomposition of anionic surfactant SDS are not emphasized due to the low adsorbed amount of this surfactant by all systems. The results obtained indicate the interest of taking into account the structure of surfactant and the clay mineral type when preparing customized surfactant-clay mineral systems which contribute to establish more efficient soil and water remediation strategies based in the use of these systems.     

Maya blue: a computational and spectroscopic study

Maya Blue pigment, used in pre-Colombian America by the ancient Mayas, is a complex between the clay palygorskite and the indigo dye. The pigment can be manufactured by mixing palygorskite and indigo and heating to T > 120 degrees C. The most quoted hypothesis states that the dye molecules enter the microchannels which permeate the clay structure, thus creating a stable complex. Maya Blue shows a remarkable chemical stability, presumably caused by interactions formed between indigo and clay surfaces. This work aims at studying the nature of these interactions by means of computational and spectroscopic techniques. The encapsulation of indigo inside the clay framework was tested by means of molecular modeling techniques. The calculation of the reaction energies confirmed that the formation of the clay-organic complex can occur only if palygorskite is heated at temperatures well above the water desorption step, when the release of water is entropically favored. H-bonds between the clay framework and the indigo were detected by means of spectroscopic methods. FTIR spectroscopy on outgassed palygorskite and freshly synthesized Maya Blue samples showed that the presence of indigo modifies the spectroscopic features of both structural and zeolitic water, although no clear bands of the dye groups could be observed, presumably due to its very low concentration. The positions and intensities of delta(H2O) and nu(H2O) modes showed that part of the structural water molecules interact via a hydrogen bond with the C=O or N-H groups of indigo. Micro-Raman spectra clearly evidenced the presence of indigo both in original and in freshly synthesized Maya Blue. The nu(C=O) symmetric mode of Maya Blue red-shifts with respect to pure indigo, as the result of the formation of H-bonds with the nearest clay structural water. Ab initio quantum methods were applied on the indigo molecule, both isolated and linked through H-bonds with water, to calculate the magnitude of the expected vibrational shifts. Calculated and experimental vibrational shifts appeared to be in good agreement. The presence of a peak at 17.8 ppm and the shift of the N-H signal in the 1H MAS NMR spectrum of Maya Blue provide evidence of hydrogen bond interactions between indigo and palygorskite in agreement with IR and ab initio methods.     

Study of raw and thermally treated sepiolite from the Mantoudi area,Euboea, Greece

Raw and thermally treated sepiolites from the Mantoudi area, Euboea, Greece, were investigated by means of X-ray diffraction (XRD) in combination with thermo-gravimetric analysis (TG/DTG) and differential thermal analysis (DTA), as well as Fourier transform (FTIR) spectroscopy, in order to study the collapse of the sepiolite structure with increasing temperature. The main mineral constituent (>95%) is a well crystallized sepiolite. Quartz and dolomite occur in minor amounts. Calcination of the samples was carried out up to 350, 720 and 820°C, for 2 h, and ‘sepiolite dihydrite’, ‘sepiolite anhydrite’ and ‘enstatite’ were formed, respectively, as magnesium co-ordinated water and octahedrically co-ordinated hydroxyl groups, are removed and the dehydroxylated phase recrystallize to enstatite (MgSiO₃). These structural and textural changes play an important role to the properties and uses of the studied sepiolites.     

A new series of three-dimensional metal-organic framework, [M2(H2O)][C5N1H3(COO)2]3.2H2O, M = La, Pr, and Nd: synthesis, structure, and properties

A new series of lanthanide pyridine dicarboxylates of the general formula, [M2(H2O)][C5N1H3(COO)2]3.2H2O, M = La (1), Pr (2), and Nd (3), has been prepared by the reaction of trivalent lanthanide salts and pyridine dicarboxylic acids employing a mild condition hydrothermal reaction. The structures are built up from MO8N and MO7N2 (M = lanthanide) polyhedra connected to the dicarboxylate anions forming the three-dimensional structure with one-dimensional channels. A striking feature of this structure is the presence of an unusual Z-shaped tetramer of the formula M4O24N6. Extraframework water molecules, located within the open channels, are reversibly adsorbed. Detailed in situ and ex situ investigations using FTIR and PXRD studies clearly show that the removal of the water molecules is reversible and accompanied by changes in the size of the channel. Partial substitution at the La sites by Eu gives rise to characteristic red-pink luminescence, indicating a ligand-sensitized metal-centered emission.     

New Hints on the Maya Blue Formation Process by PCA-Assisted In Situ XRPD/PDF and Optical Spectroscopy

The exact recipe to prepare the ancient Maya Blue (MB), an incredibly resistant and brilliant pigment prepared from indigo (dye) and Palygorskite (clay), is lost to the ages. To unravel the key features of the MB formation process, several inorganic-dye couples were heated to 200 °C and cooled to RT, to investigate their reactivity and the diffusion and degree of sequestration of the dye into the inorganic host. In situ XRPD/PDF and fiber optic reflectance spectroscopy (FORS) data, along with TGA, provided a comprehensive overview on MB formation mechanism. XRPD/PDF gave information on long/short range behaviors of water desorption/adsorption and indigo sequestration, while TGA and in situ FORS gave information on mass and optical changes within temperature. Ex situ dye removal was used to understand the sample stability after the thermal treatment. A statistical approach based on principal component analysis was exploited to efficiently and jointly analyze the ≈3000 collected patterns. MB formation starts below 110 °C with disordered distribution of indigo within the channels, reaching maximum reaction speed and higher ordering at 150 °C. Above 175 °C, color changes and a stronger sequestration of indigo into framework channels are observed, whereas the affinity for water is dramatically reduced. The origin of different colors, hues, and stability in historical MB samples can then be explained in terms of different thermal histories of the starting mechanical indigo/palygorskite mixtures.     

Plasma Treated Sepiolite: A New Adsorbent for Removal of Malachite Green from Contaminated Water

Surface modification of clay materials has become an important issue to improve the efficiency of the adsorbent. The adsorption capacity of the clay material can be increased by thermal or chemical modifications. In this study, plasma technology was applied for the surface modification of sepiolite to improve the removal of malachite green from contaminated water. This study is novel in preparing and examining the effectiveness of sepiolite in adsorption of malachite green from contaminated water. To achieve the aim, plasma application time, CO₂, N₂, or Ar plasma gases effect and pH were investigated with respect to the adsorption capacity of MG. The surface properties of raw and plasma treated sepiolite were investigated with SEM, FTIR, BET surface area and XRD measurements. The monolayer adsorption capacity was found to be 143 mg/g.     

Photophysics of an indigo derivative (keto and leuco structures) with singular properties

Spectral and photophysical properties of the indigo derivative Cibalackrot in keto and reduced (leuco) forms were studied by absorption spectra, fluorescence and pulse radiolysis and compared with the structurally similar indigo. With the keto form of this dye, fluorescence (phiF = 0.76) and intersystem crossing (phiT = 0.11) are dominant, whereas with indigo, efficient internal conversion (phiIC = 0.99) is observed, probably involving proton transfer through intramolecular hydrogen bonds. With Cibalackrot, this pathway is blocked, supporting the above model for indigo. With the reduced form of Cibalackrot, more than 98% of the absorbed quanta are dissipated through S1 approximately --> S0 internal conversion, which contrasts with leuco-indigo, where fluorescence (phiF = 0.35), internal conversion (phiIC = 0.53) and intersystem crossing (phiT = 0.125) are found to be competitive. In addition, a synthetic precursor of Cibalackrot (preCiba) was also investigated. This has a rigid molecular structure (with a moiety identical to Cibalackrot and the other to indigo), but intra- or intermolecular proton transfer is allowed between adjacent carbonyl and N-H groups. With this precursor in its keto structure the photophysical parameters are generally very close to those of the keto form of indigo, and different from those of Cibalackrot. A more detailed investigation of the time-decay profiles of preCiba in dioxane (and with added water and D2O) has shown that these follow biexponential laws with a shorter component of 14-25 ps, which appears associated with a risetime at longer wavelength emissions (and to a positive preexponential at shorter emission wavelengths) and a longer lived (decay) component of 104-130 ps. In the steady-state spectra of preCiba, the variation with temperature reveals a blue shift of the emission maxima, which is interpreted as the presence (simultaneous emission) of two species (keto and enol) in the excited state. Indigo and deuterated indigo are also found to present a similar behavior. The overall data are interpreted as to be due to an excited-state process involving the proton transfer between keto and enol forms. Rate constants with values of 7 x 10(10) s-1 for preCiba and 1.6 x 10(11) s-1 for deuterated indigo were obtained. This inverse isotope effect is justified on the basis of the proposed model for proton-transfer excited-state deactivation.     

Structural effects of drying and rehydration for enzymes in soils: a kinetics-FTIR analysis of alpha-chymotrypsin adsorbed on montmorillonite

The effects of desiccation and rehydration cycles encountered by extracellular enzymes in soils are studied on -chymotrypsin adsorbed on montmorillonite. The controlled hygrometric FTIR cell used in this study enables to monitor drying and rehydration steps undergone by the -chymotrypsin-montmorillonite suspension or by the enzyme alone. Relative humidity (RH) determines the amount of deuterated water in the FTIR cell atmosphere. The molar water/protein ratio (W/P) as well as the conformational and solvation states of the enzyme have been determined using H/D exchange monitored by FTIR-transmission spectroscopy. When the W/P ratio decreases from 3500 to approximately 400, unfolding of beta-secondary structure in three different domains involves about 8% of the polypeptide backbone with respect to the most solvated states. Desiccation induces beta-unfolding, which opens channels allowing free vapor water molecules to diffuse into the enzyme at 15% RH. On drying to 0% RH, displacements of internal water (H2O) in the enzyme are demonstrated by reverse peptide isotopic exchanges (COND ==> CONH). Specific beta-structures, only formed in highly solvated states, sequester around 20 internal H2O molecules. Indeed, most of the unfolded secondary structures during the drying step are refolded at W/P approximately 1000 during rehydration. However, self-association hinders the recovery of the initial closed tertiary structure. The pD-dependent structural changes controlling inward and outward water diffusion are suppressed, whether the protein is initially in an adsorbed state or in solution. Changes in secondary structures encountered during desiccation/rehydration cycle are similar for the protein either free or in the adsorbed state. Thus domains that are unfolded by adsorption are not concerned by the desiccation/rehydration cycle.