SANS Study of HCl Extraction by Selected Neutral Organophosphorus Compounds in n-Octane

The extraction of HCl by tri(2-ethylhexyl) phosphate (TEHP), tri-n-octyl phosphate (TOP), and tri-n-octylphosphine oxide (TOPO) in n-octane was investigated by liquid-liquid distribution of acid and water and small-angle neutron scattering (SANS) measurements. No formation of a heavy organic phase (third phase) was observed with TEHP and TOP under the experimental conditions used, whereas for 0.4M TOPO the HCl limiting organic concentration (LOC) at 23 degrees C was 0.32M (with 5.1M HCl in the equilibrium aqueous phase). For higher HCl concentrations in the aqueous phase, the organic phase splits into a light and a heavy layer. For TEHP and TOP, the SANS results, interpreted using the Baxter model for hard spheres with surface adhesion, indicated the formation of only small reverse micelles with little intermicellar attraction. For TOPO, the scattering signals suggested the formation of much larger and strongly interacting micelles. The critical values of the stickiness parameter, -1, and the interaction potential energy, U(r), for the LOC sample in the TOPO system were consistent with the model for third-phase formation previously developed for tri-n-butyl phosphate (TBP). According to this model, organic phase splitting is due to van der Waals interactions between the polar cores of reverse micelles formed by the extractants in the organic phase.