Channel confinement effects in palygorskite and sepiolite: Mechanisms behind color and stability variations in Maya Blue pigments

Maya Blue, a hybrid pigment that can be synthesized by heating a mixture of palygorskite (or sepiolite) and indigo, has unclear relationships between the structural features of clay substrates, heating atmosphere, colors, and stability. This study investigates the color properties, stability, and clay-dye interactions of Maya Blue pigments based on palygorskite and sepiolite, heated under air and vacuum conditions (180 °C for 8 h). Reflectance spectra, CIE color parameters, surface properties, and Fourier-transform infrared spectroscopy were employed to evaluate these pigments. Results indicate that indigo monomers can penetrate the channels of both palygorskite and sepiolite, conferring a greenish-blue hue and high stability. However, the diffusion of indigo is determined by the size match between the channel and indigo monomers. In palygorskite, indigo monomers can diffuse only upon heating because the channel size is only a little bit larger than the indigo molecule, while in sepiolite, lager channels allow easy diffusion, even without heating. However, sepiolite's larger channels make indigo molecules more prone to chemical degradation or migration. Under vacuum conditions, indigo molecules more easily diffuse into the channels of both clay minerals, yielding pigments with higher saturation and enhanced stability. These findings highlight the size-matching between clay minerals' micropores and organic molecules and offer valuable guidance for optimizing the preparation process of clay-dye hybrid pigments.