A study of aluminum speciation in aluminum chloride solutions by small angle x-ray scattering and ²⁷Al NMR

Aluminum ions may exist in one of several different types of polynuclear species depending upon the degree to which it is hydrolyzed. The hydrolysis of aluminum can be explained by a metastable equilibrium among Al(H₂O)³⁺₆, Al(OH)(H₂O)²⁺₅, Al₁₃O₄(OH)₂₄(H₂O)⁷⁺₁₂, and an oligomer. We have made small angle x-ray scattering and ²⁷Al NMR measurements on 0.3 M aluminum chloride solutions over a range of OH/Al ratios. ²⁷Al NMR shows two sharp peaks corresponding to the monomer Al(H₂O)³⁺₆ and the tetrahedrally coordinated aluminum in Al₁₃O₄(OH)₂₄(H₂O)⁷⁺₁₂, and a broad peak due to the octahedrally coordinated aluminum in the oligomer. The observed radius of gyration (R_g) of the species in solution increases from 3.0 to 3.6 A, and the intensity at zero angle (which is proportional to the weight average molecular weight of the species) also increases as the OH/Al ratio increases from 1.0 to 2.2. The oligomer has an R_g of approximately 3.3 A, a hydrolysis ratio close to 2.0, and may be a six-membered ring with a structure similar to that of a gibbsite layer.