Dispersion behavior of oleic acid in aqueous media : from micelles to emulsions

• Published in: Colloid and polymer science 1997-08, Vol.275 (8), p.777-783
• Main Authors: KAIBARA, K, IWATA, E, EGUCHI, Y, SUZUKI, M, MAEDA, H
• Format: Article
• Language: English
• Subjects: Aqueous solutions; Buffers; Chemistry; Colloidal state and disperse state; Emulsions; Exact sciences and technology; Fatty acids; General and physical chemistry; Micelles; Micelles. Thin films; Oleic acid; Phase equilibria; Scattering; Sodium; Sodium oleate; Three phase; Turbidity
• ISSN: 0303-402X; 1435-1536
• DOI: 10.1007/s003960050147

Dispersion behavior of aqueous solutions containing oleic acid (RH), sodium oleate (R-Na +), and NaCl was investigated by turbidity and dynamic light-scattering measurements. Changes of the size of scattering particles in solution composed of 1 mM oleic acid and 100 mM NaCl were traced as a function of the degree of ionization α, in terms of radius of the equivalent hydrodynamic sphere. Large associated micelles with a radius of 30 nm appeared by a slight decline of α and existed at a higher than 0.75. They were responsible for the three-phase equilibrium (solution, micelle and aggregated micelle, and acid-soap, (R-Na+)3RH) characterized by a constant pH of 9.75. The appearance of a new phase, (R-Na+)3RH, contributed to increase both the turbidity and averaged scattering particle size. As the breakdown of the three-phase equilibrium, radius of scattering particles increased significantly. Finally, oleic acid oil droplets were separated from aqueous phase at low α. When the system was buffered by tris (hydroxymethyl)aminomethane (Tris), scattering particles with a weight-averaged hydrodynamic radius of 75 nm existed in a wide range of α from 0.85 to 0.65. In Tris buffered solution, turbidity formation was induced by the increase in the number of aggregated particles.