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 (Rg) 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.