PHASE EQUILIBRIA IN THE SYSTEM WATER-OCTYL SULFONATE-DIACID® (WESTVACO) MAGNESIUM SOAP

ABSTRACT

Partial phase equilibria were determined for the magnesium soap of a dicarboxylic acid (5-(and6-)carboxy-4-hexyl-2-cyclohexene-l-yl octoanic acid) in combination with a C₈ sulfonate surfactant and water. At high water contents the isotropic sodium octyl sulfonate micellar phase was stable to relatively high magnesium soap contents. However, above a certain magnesium soap/sodium sulfonate ratio, two isotropic liquids were in equilibrium. For systems containing no octyl sulfonate, one phase was a dilute aqueous solution of magnesium soap; the second phase was a concentrated liquid soap with water content near 45 weight percent.

At high surfactant concentrations, the isotropic solution was in equilibrium with a hexagonal liquid crystalline phase. Changes in the liquid crystal lattice parameter, obtained by low-angle X-ray diffraction measurements at varied magnesium soap contents, indicated that the dicarboxylate soap assumed a folded conformation where both carboxylate groups were in contact with the aggregate-water interface. The folded conformation found for the magnesium soap suggested a relationship between the conformation of the dianion and the hydrotropic action of soaps of the dicarboxylic acid.

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Tony Flaim

Permanent address: Dow Chemicals, Midland, Michigan 48640