CHARACTERIZATION OF A WATER-IN-OIL MICROEMULSION CONTAINING A CONCENTRATED AMMONIUM FERRIC SULFATE AQUEOUS-PHASE

A water-in-oil (w/o) microemulsion containing high concentrations of a mmonium ferric sulfate in solution was characterized by SAXS, EXAFS, e lectrical conductivity, and viscosity measurements and by its phase be havior. The nanometer-sized aqueous droplets are microemulsified by so dium bis(2-ethylhexyl) sulfosuccinate (AOT) in an isooctane continuous phase. Addition of small amounts of sodium dodecyl sulfate (SDS) as a cosurfactant greatly aids in the solubilization of the inorganic elec trolyte-laden aqueous phase. For this five-component system there is a large region of the composition phase space that exists as a clear, s table w/o microemulsion. A portion of this w/o microemulsion phase spa ce can be characterized as spherically shaped aqueous nanometer-sized droplets. A simple relationship between the total surfactant concentra tion and the amount of water on the droplet size was established. This relationship has the same form as the well-known relationship for the ternary system, AOT/water/isooctane. True thermodynamic equilibrium w as not established in this microemulsion study because the reaction ti mes for the various ferric oxyhydroxide species are prohibitively long . As a result, pseudoequilibria for this ammonium ferric sulfate micro emulsion are reported. Because of the high concentration of inorganic electrolyte that can be dissolved in the microemulsion phase, these sy stems are useful for the synthesis of large quantities of nanophase pa rticles.