Insertion of indigo molecules in the sepiolite structure as evidenced by 1H–29Siheteronuclear correlation spectroscopy

Abstract

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