Aluminium soap, their nature and gelling properties

Abstract

The chemical nature of aluminium soaps can be studied with great advantage by making use of the reaction between aluminium alkoxides and fatty acids in organic solvents. The heat of the reaction was measured as a function of fatty acid added, and this showed that not more than two fatty-acid molecules react with each Al(OR) 3 molecule. Even with water, not more than two molecules reacted with each A1(OR) 8 molecule. The resulting soaps were analyzed for aluminium and fatty acid, and this confirmed the conclusions drawn from the heat of reaction. The product of aqueous metathesis between an alkali metal soap and an aluminium salt appears to be an adsorption complex of the fatty acid upon hydrated alumina; combination occurs during the drying process. From the above studies a polymeric formula for aluminium soaps has been suggested, based upon sixfold co-ordination aluminium-oxygen octahedra. When these octahedra join through an apex a cli-soap results, when through an edge, a mono-soap. A combination of these results in intermediate values. Some preliminary infra-red measurements of aluminium mono- and di-soaps appear to be consistent with these conclusions. The phenomenon of peptization of aluminium soaps by organic compounds such as alcohols, phenols, amines, etc., is discussed in terms of the above formulation. The physical nature of aluminium soaps in organic solvents has been explored by means of viscosity and streaming birefringence. In very dilute solutions in benzene it appears that the soap units approach molecular dispersion; with increasing concentration the rapid increase in specific viscosity indicates strong intermolecular attraction and the formation of fibrillar aggregates. With the more concentrated solutions (1 to 2 %) simultaneous streaming birefringence and viscosity measurements were made in a coaxial cylinder apparatus, and the effect of peptizers and precipitants determined. It was concluded that the soap molecules are aggregated in solution with the fatty-acid groups extended laterally with respect to the chain. Some solutions were initially birefringent, due to the influence of the walls upon the aggregation.