Indigo chemisorption in sepiolite. Application to Maya blue formation

Abstract

Having first studied the loss of water in sepiolite via TGA, we then observed through FTIR the evolution of the indigo/sepiolite chemical bonds during the Maya blue formation process. Three stages, corresponding to three different temperature domains, were observed. They explain the peculiar properties of this pigment.

Asbestiform sepiolite coated by aliphatic hydrocarbons from Perletoa, Aosta Valley Region (Western Alps, Italy): characterization, genesis and possible hazards

“…6) are consistent with previous works (Nagy and Bradley, 1955;Brauner and Preisinger, 1956;Martin Vivaldi and Cano Ruiz, 1956;Caillère and Hénin, 1957;Preisinger, 1959;Hayashi et al, 1969;Nagata et al, 1974;Rautureau and Mifsud, 1977;Ruiz et al, 1996;Weir et al, 2002;Hubbard et al, 2003;Ovarlez et al, 2006Ovarlez et al, , 2009. Although in sepiolite correlations between TGA features and structural changes are not straightforward nor partitioning among different kinds of water so strict (Mifsud et al, 1987), reasonable interpretations can be sketched.…”

The Incorporation of Indigo Molecules in Sepiolite Tunnels

“…The bands at 1488, 1462, 1410, 1320 and 1300 cm À1 that correspond to indigo/sepiolite complexes are present in the indigo/sepiolite mixtures preheated at 300 8C (Figure 9ae) but not in the mixture preheated for 72 h at 120 8C (Figure 9f). [29,38] However, a new and very weak band is detected at 1736 cm À1 in indigo/sepiolite samples preheated at 300 8C but is not observed in mixtures preheated for 72 h at 120 8C (Figure 9a-e versus Figure 9f). This weak band could be assigned to indigo decomposition during heating.…”

“…The crystalline indigo produces two bands at 1410 and 1394 cm À1 but for 4, 5, 6 and 7 % indigo/sepiolite mixtures only the band at 1410 cm À1 is observed. [29,38] For the 11.5 % indigo/sepiolite sample the band is broader; this is due to the existence of the band at 1394 cm À1 indicating the presence of indigo crystals. As already shown by NMR, pure and complexed indigo coexist.…”

“…This behaviour of the mixture suggests the presence of indigo inside the sepiolite tunnels and interactions between indigo and coordinated water situated along the tunnels. [3,29,30] However, for sepiolite the combination of an NMR and a thermal analysis leads some authors to propose that indigo molecules cover the channel openings, so that the principal interactions are between their carbonyl and amino groups and external silanol groups of the sepiolite and ( Figure 1a). [31] More details have been given concerning the stable blue indigo/attapulgite mixtures; on the basis of high-resolution TEM (HRTEM), FTIR, Raman, thermogravimetric analysis (TGA) and theoretical studies, organic molecules forming surface complexes with organic molecules filling the first part of the tunnels and blocking water inside these tunnels were proposed.…”

“…We have shown previously that it is possible to determine changes in the crystalline structure of indigo in a preheated indigo/sepiolite mixture. [29] Here, we propose an elaboration process favouring access of organic molecules to the external and internal channels. By using FTIR spectroscopy and XRD, we determine unambiguously the location of the indigo molecules inside the clay.…”