New Insights into the Structure and Degradation of Alizarin Lake Pigments: Input of the Surface Study Approach

Alizarin (Az) lake is a typical example of madder lake pigments that are widely used in cultural heritages. Previous work focused on the complex formation in solution, theoretical models, and crystallized stoichiometric structures (Az/Al3+ = 1:1 or 2:1). However, the classical recipe for preparing madder lake demonstrated that the dosage of the dye was much less than Al3+. Aiming to interpret the structure of a non-stoichiometric Az lake pigment, Az lakes of non-stoichiometric complexes (Az/Al3+ = 1:5 to 1:200) were prepared and characterized. The influences of the dye concentration, pH, and basic reagents on the structures and chromatic properties were studied by attenuated total reflection Fourier transform infrared spectroscopy, nuclear magnetic resonance, and CIE 1976 color space. In order to interpret the interaction between Az molecules and inorganic substrate, Az was deposited on a thin Al2Ox film prepared by the physical vapor deposition method. Polarization modulation infrared reflection absorption spectroscopy was employed to characterize the chemical reactivity and degradation of the Az–Al2Ox film complex. Az derivatives interacted with Al2Ox film like lakes. Al3+ was coordinated with the 1,2-dihydroxyl (catechol) group, instead of the 1-hydroxy-9-keto group. Exposure under UV light resulted in the degradation of the Az–Al2Ox film complex by oxidizing catechol groups to carbonyl. This study presented a new insight into the structure and degradation of Az lake.