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

International audienceDespite 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) H-1-Si-29 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 H-1 dimension of the 2D H-1-Si-29 HETCOR spectrum obtained by FSLG homonuclear decoupling, the assignment of the Si-29 cross-polarization magic-angle spinning (CPMAS) spectrum of sepiolite is clearly confirmed. Moreover, 2D H-1-Si-29 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 Si-29 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

The incorporation of indigo molecules in sepiolite tunnels

International audienceEvidence for access of molecules the size of acetone or pyridine to the intracrystalline tunnels of nanofibre clay (sepiolite) has indicated formation of a new type of organic-inorganic nanocomposites. However, the introduction of larger molecules has been a recurring problem. It is now agreed that for indigo, the molecules are located on the external surface and at the ends of the fibres, thus blocking access to internal tunnels. We claim, however, that it is possible for indigo molecules to access the internal channels of sepiolite. FTIR and XRD analyses have provided evidence for folding of the sepiolite structure preheated at high temperature (above 350°C). By comparison, we have shown that for indigo/sepiolite mixtures treated in the same conditions, no change in the crystalline structure of the sepiolite is observed, and that blue samples, related to Maya blue, with indigo molecules incorporated deeply enough into sepiolite to prevent folding of the tunnels, can be obtained. NMR, FTIR and thermal analysis confirm the interaction of indigo with the water coordinated to magnesium(II) and located inside the internal and external channels of sepiolite. Two other hypotheses are excluded; we show both that zeolitic water is not blocked in the tunnels by indigo, and that if thermal decomposition products of indigo can be formed, they are in a minority

Structural studies of adsorbed protein (betalactoglobulin) on natural clay (montmorillonite)

International audienceIn this work, the adsorption of a small globular protein (betalactoglobulin, BLG), on a natural montmorillonite clay (Mt) was investigated in acidic buffer (pH = 3). The combination of different characterization techniques such as zetametry, X-ray diffraction, transmission electronic microscopy, fluorescence and solid state nuclear magnetic resonance spectroscopies shed light on the interaction mechanism between the clay mineral and the proteins. For low BLG concentration, a slight increase of the interlayer spacing of the clay mineral was noticed as well as structural changes of the protein. In contrast, as the concentration of BLG increased, the adsorption led to a partial exfoliation of the clay mineral, accompanied with significant secondary structural changes of the protein characterized by a loss of β-sheet organization. Altogether, our results revealed an unexpected adsorption scheme where the increase of the BLG/Mt weight ratio of the hybrid material leads to a partial exfoliation of the Mt, but at the expense of the protein native structure