Understanding the sepiolite–indigo interaction and its effect on Maya blue pigment by alkali pre-treatment of sepiolite

Sepiolite is considered a suitable substrate for Maya blue pigment. However, the interaction between sepiolite and indigo dye has not been fully understood. Previous studies have demonstrated that pre-treatment of sepiolite by heating or acid was useful in identifying the sepiolite–indigo interaction. The purpose of the present study was to prepare a series of hybrid sepiolite–indigo pigments after modifying the sepiolite using various alkali treatments (NaOH), then to evaluate their properties with respect to color, chemical resistance, and photostability. Samples were characterized using reflectance spectroscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, transmission electron microscopy, and N2 adsorption-desorption. Under alkaline conditions, Si4+ and Mg2+ ions in sepiolite partially dissolved, disrupting the coordinated water associated with them. Mg2+ ions precipitated and blocked the structural channels of the sepiolite. The impact of the alkali treatment on the microporous structure and coordinated water of sepiolite significantly influenced the color properties and stability of the hybrid pigments. Proper alkaline treatment enhanced the greenish hue and chemical stability of the pigment, while severe treatments apparently compromised the structural integrity of the sepiolite, thus diminishing the quality of the hybrid pigment. Results from this study provide new insights into the color-causing and stabilizing mechanisms of sepiolite-based Maya blue pigment and also provide guidance for developing hybrid pigments based on clay minerals and organic dyes.