Insight into the roles of water molecules in forming Maya Blue pigments based on structural evolution of palygorskite using oxalic acid

Although the interaction mechanism of Maya Blue components remains under refinement, it has been recognized that the nanochannel structure and water molecules within palygorskite are critical to the formation of Maya Blue. In this study, the structural evolution strategy was developed to thoroughly investigate the interaction mechanisms between indigo and palygorskite in Maya Blue, in which palygorskite was first subjected to gradient acid leaching with oxalic acid followed by grinding with indigo and heat treatment to synthesize series of indigo/palygorskite hybrid pigments. The effect of oxalic acid leaching time on the color and physicochemical properties of hybrid pigments was studied. The results indicated that the progressive losses of water molecule and the changes in the structure might be associated with an obvious degradation in the color, chemical and light stability of hybrid pigments with the gradient structural evolution of palygorskite. The color and stability of hybrid pigments derived from raw palygorskite was superior to the ones prepared with oxalic acid gradient-leached palygorskite, which also was confirmed by the change of L ⁎ value from 29.01 to 37.70, a ⁎ value from −10.74 to −3.54, and b ⁎ value from −24.02 to −19.21. Indigo/raw palygorskite hybrid pigments presented lower ΔE ⁎ values than the ones prepared with acid-leached palygorskite after chemical and UV treatment. It suggested that the displacement of zeolite water in raw palygorskite and the coordinated water bonded with octahedral Mg 2+ at the edge of palygorskite played an important role in the color and environmental stability of Maya Blue, while the structural changes of palygorskite might also contribute to the stability variation during environmental exposure. This study deepened the understanding of the component interactions and environmental-stability mechanism of Maya Blue, contributing to the synthesis of stable organic/inorganic hybrid pigments based on palygorskite and natural or synthetic dyes.