A STUDY OF ALUMINUM SPECIATION IN ALUMINUM-CHLORIDE SOLUTIONS BY SMALL-ANGLE X-RAY-SCATTERING AND AL-27 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)63+, Al(OH)(H2O)52+, Al13O4(OH)24(H2O)127+, and an oligomer. We have made small angle x-ray scattering and Al-27 NMR measurements on 0.3 M aluminum chloride solutions over a range of OH/Al ratios. Al-27 NMR shows two sharp peaks corresponding to the monomer Al(H2O)63+ and the tetrahedrally coordinated aluminum in Al13O4(OH)24(H2O)127+, 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.