Beyond Al₃₀: Small-Angle X-ray Scattering of Large Equiaxed Macromolecular Aluminum-Oxo-Hydroxy Clusters in Solution

Aluminum-oxo-hydroxy (AOH) clusters are useful coagulants that can destabilize waterborne contaminants, including clay and natural organic matter, by coagulation/flocculation processes. The structures of the most studied clusters, Al₁₃ and Al₃₀, are known. Here, the structural study of AOH clusters is extended to a reported but unidentified large polynuclear species using small-angle X-ray scattering (SAXS). First, the overall size and shape of the cluster in solution were deduced from model-independent and indirect Fourier transformation (IFT) analysis of SAXS data and compared with Al₁₃ and Al₃₀. The radii of gyration (R_g) of the clusters from smallest to largest were: Al₁₃ (0.41 nm) < Al₃₀ (0.44 nm) < large AOH (0.82 nm). Second, low-resolution 3D shapes of Al₁₃, Al₃₀, and large AOH clusters were reconstructed from the experimental SAXS data using an ab initio shape restoration method. Third, the internal structure of the large cluster was probed by a SAXS/model-fitting approach with a simple tetrahedron built from closely packed spheres. SAXS, ²⁷Al NMR, and XRD data suggest that the large AOH cluster, which is the dominant species in concentrated aluminum chlorohydrate solutions containing rare earths (La → Nd) after heating (130 °C; 3 days), is a tetramer comprising Al₁₃-like subunits.