Aggregation in aluminium hydroxide solutions investigated by small-angle X-ray scattering

Abstract

Aluminium hydroxide particles, created by hydrolysis of aluminium nitrate solutions with sodium carbonate, aggregate rapidly to form mass fractal structures for which the fractal dimension varies from 1.7 to 2.1 depending on the OH/Al-ion ratio of the solution. This system has been studied by small-angle X-ray scattering (SAXS). The OH/Al ratios for the solutions were 1.5, 2.0 and 2.15. The maximum radius of gyration of the aggregates of about 50 Å is found in the solutions having the highest OH/Al ratio. SAXS measurements made at various stages of the aging process show that, in addition to fractal aggregation, there is a second process active in the solutions that leads to the break up of the fractal structure and formation of nonfractal particles, which are probably Al13 ions, [AlO4Al12(OH)24(OH2)12]7+. This process, which is strongly temperature dependent, is governed by an activation energy of approximately 162 kJ mol−1.