Influence of indigo-hydroxyl interactions on the properties of sepiolite-based Maya blue pigment

Abstract

Maya blue is a greenish-blue and stable pigment composed of indigo and palygorskite or sepiolite. Previous studies implied that the properties of Maya blue were influenced by different hydroxyl groups in the clay structure, e.g., zeolitic water, coordinated water, and structural hydroxyls. Therefore, an in-depth investigation was planned to analyze the structures and properties of indigo-sepiolite hybrid pigments in this work. Sepiolite materials were heated at 160, 360, 610, and 880 °C to remove zeolitic water, the first coordinated water, the second coordinated water, and structural hydroxyls. A series of Maya blue simulants were prepared by heating the mixture of indigo and sepiolite materials at 180 °C. Reflectance spectroscopy and CIE 1976 color space were employed to characterize color properties and color differences of hybrid pigments. Chemical resistance to concentrated HNO3 and dimethyl sulfoxide (DMSO), and photostability under visible light were evaluated. Microporous analysis and Fourier transform infrared spectroscopy were applied to study the indigo-sepiolite interaction. Coordinated water was essential for the greenish-blue color and outstanding stability of Maya blue, while zeolitic water and structural hydroxyls did not affect pigments’ properties. Indigo molecules were confirmed to be inserted into sepiolite channels and formed hydrogen bonds with coordinated water at one side. Structural hydroxyls were insignificant for the formation of Maya blue. This work provides a guideline for preparing stable clay-dye hybrid pigments.