Spectroscopic insights into the formation of alizarin-aluminium lake pigments under variable synthesis conditions

Alizarin (1,2-dihydroxyanthraquinone) forms coloured coordination complexes with metal ions; however, the molecular structure and formation mechanisms of aluminium-based lake pigments remain incompletely defined. In this work, Fourier Transform Infrared (FTIR) spectroscopy was employed to investigate the coordination environment and phase composition of alizarin-aluminium pigments synthesised via two distinct approaches: an alkaline complexation route and an acid-mediated precipitation route. Within each method, pH was systematically varied to evaluate its influence on complex formation and precipitation processes. FTIR spectra display significant modifications in the carbonyl and phenolic stretching regions, reflecting changes in alizarin deprotonation and metal-ligand coordination modes as a function of synthesis pathway. Shifts in ν(CO) and ν(CO) bands, together with variations in band profiles, indicate differences in binding geometry and degree of complexation. The synthetic route markedly affects nucleation and precipitation behaviour, altering the relative contributions of coordinated alizarin species and amorphous Al(OH)₃ substrate. Furthermore, pH governs ligand deprotonation equilibria and co-precipitation processes, ultimately determining pigment composition and structural homogeneity. These findings highlight the critical importance of each synthesis stage in directing coordination structure, phase development, and ultimately the properties of the resulting lake pigments.