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 ammonium ferric sulfate in solution was characterized by SAXS, EXAFS, electrical conductivity, and viscosity measurements and by its phase behavior. The nanometer-sized aqueous droplets are microemulsified by sodium 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 electrolyte-laden aqueous phase. For this five-component system there is a large region of the composition phase space that exists as a clear, stable w/o microemulsion. A portion of this w/o microemulsion phase space can be characterized as spherically shaped aqueous nanometer-sized droplets. A simple relationship between the total surfactant concentration 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 was not established in this microemulsion study because the reaction times for the various ferric oxyhydroxide species are prohibitively long. As a result, pseudoequilibria for this ammonium ferric sulfate microemulsion are reported. 31 refs., 7 figs., 1 tab.