Sans study of reverse micelles formed upon the extraction of inorganic acids by TBP in n-octane

Small-angle neutron scattering (SANS) data for n-octane solutions of TBP loaded with progressively larger amounts of HNO3, HClO4, H2SO4, and H3PO4 up to and beyond the LOC (limiting organic concentration of acid) condition, were interpreted using the Baxter model for hard spheres with surface adhesion. The coherent picture of the behavior of the TBP solutions derived from the SANS investigation discussed in this paper confirmed our recently developed model for third phase formation. This model analyses the features of the scattering data in the low Q region as arising from van der Waals interactions between the polar cores of reverse micelles. Our SANS data indicated that the TBP micelles swell when acid and water are extracted into their polar core. The swollen micelles have critical diameters ranging from 15 to 22 angstrom, and polar core diameters between 10 and 15 angstrom, depending on the specific system. At the respective LOC conditions, the TBP weight-average aggregation numbers are similar to 4 for HClO4, similar to 6 for H2SO4, similar to 7 for HCl, and similar to 10 for H3PO4. The comparison between the behavior of HNO3, a non-third phase forming acid, and the other acids provided an explanation of the effect of the water molecules present in the polar core of the micelles on third phase formation. The thickness of the lipophilic shell of the micelles indicated that the butyl groups of TBP lie at an angle of similar to 25 degrees relative to a plane tangent to the micellar core. The critical energy of intermicellar attraction, U(r), was about -2k(B)T for all the acids investigated. This value is the same as that reported in our previous publications on the extraction of metal nitrates by TBP, confirming that the same mechanism and energetics are operative in the formation of a third phase, independent of whether the chemical species extracted are metal nitrate salts or inorganic acids.