Optical and vibrational spectroscopic studies of synthetic Maya pigments as a function of concentration of indigoid dyes

Pigments developed by the Mayan civilization around 8th century, represent some of the most versatile pigments known to date. Several derivatives of these pigments are popular subjects of current research interest. This is due to the characteristic stability which is provided by a bonding mechanism between the dye and the clay. One such pigment “Maya Blue”, a mixture of Indigo and Palygorskite, provides a dramatic background for murals and ceramics throughout Mesoamerica. Several research groups have devoted time and interest in unlocking its particular features.1-3 The work embodied in this thesis is focused on the synthesis and characterization of three pigments: Maya Blue, Maya Purple and Royal Blue with varying concentrations (1-25%) of the organic dyes. Samples were prepared by heating the corresponding dye with Palygorskite (Inorganic clay) at 170°C for 9 hours. Various factors which account for the stability of these complexes are discussed by a critical analysis of the results obtained. Ultra Violet-Visible (UV-Vis) spectra of Maya Blue, Maya Purple and Royal Blue samples provide an evidence for variations in the electronic structure of the dyes after they have incorporated into the Palygorskite matrix. This is suggested by a bathochromic shift of π→π* transition associated with dyes [λmax(Indigo) = 584 nm, λmax(Maya Blue 6%) = 656 nm; λmax(Thioindigo) = 507 nm, λ max(Maya Purple 6%) = 590 nm]. In contrast, upon increasing the concentrations of the dye in the pigment, the absorption maxima shift to a lower wavelength which is suggestive of partial contribution of the dye at higher concentrations. Analysis of Fourier Transform Infrared (FTIR) spectra provides a qualitative bonding description of the C=O, N-H, C=C, O-H and Si-O-X (where X = H, Al, Si) groups. The stretching band due to C=O group shifts to at lower wavenumber after the pigments formation [(vC=O, cm-1) = 1626 (Indigo), 1622 (Maya Blue), 1655 (Thioindigo), 1627 (Maya Purple)]. The vN-H band disappeared at lower concentrations of the dye in the Maya Blue samples. These data support the involvement of such groups in bonding during the pigment formation. On the contrary, the bands due to C=O, N-H groups become more sharp at higher concentrations of the dye. In a linear argument, the appearance of sharp bands of the C=O group suggests an excess of Indigo and Thioindigo dyes. Fourier Transform Raman (FT Raman) spectroscopic, Powder X-ray diffraction (XRD), and Differential Scanning Calorimetric (DSC) studies further provide evidences to develop the binding mechanism of the dye and the clay. Based on all the results, it is envisaged that, at lower concentrations of the dye (\u3c6%), the dye molecules may penetrate into the channels of clay while on increasing the concentrations (\u3e6% - \u3c16%), the dye molecules bind with the exposed surface involving Si-O-Mn+ (M = Al, Fe) sites. At much higher concentrations (\u3e16%) of the dye, the surfacial activity predominates and the dye accumulates in the form of layers on the outer surface of the clay