Small Angle X-Ray Scattering from Aluminum Hydroxide Gels. II

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Abstract
In a continuation of previously reported work [H. D. Bale and P. W. Schmidt, J. Phys. Chem. 62, 1179 (1958)] a further study of the physical structure of aluminum hydroxide gels prepared by different methods has been carried out, using small angle x‐ray scattering techniques. The effects of collimation corrections on some of the earlier data have been calculated, and further experiments have been conducted. Previously, the scattering from the fresh gels was found to be independent of the pH at which precipitation was carried out, and was shown to depend on the concentration and charge of the anion of the aluminum salt from which the gel was precipitated. The present study shows that the anion concentration during precipitation has a much greater effect on the scattering pattern than does the aluminum ion concentration. The scattering from gels prepared under conditions in which no ions were present has also been studied. Scattering data from sols which are made from the gels indicate that the gels can be treated as assemblies of nearly independent particles. All fresh gels are amorphous. No aging effects are observed in gels precipitated at pH 7 from aluminum sulfate. When gels are precipitated at pH 7 from aluminum nitrate, thin boehmite platelets are first formed. After further aging, platelets of gibbsite appear. Values of platelet thickness have been determined. When gels are precipitated from aluminum nitrate and aged at high pH, large bayerite crystals are formed. When the gels are dried, a marked effect is observed on the scattering patterns for the fresh gels, while little change is observed in the scattering from the aged gels. According to the usual interpretation of scattering data, the fresh gels contain some particles or aggregates at least as large as a few hundred Angstroms. Also, a considerable number of the particles must have some dimensions of the order of 25 A or less. The relative importance of the large and small structures in the fresh gels varies with the conditions of preparation. The fresh gel results can be interpreted in terms of a distribution of particles or in terms of an aggregation of small particles.