Aggregation in solvent extraction systems containing a malonamide, a dialkylphosphoric acid and their mixtures

Aggregation phenomena in n-alkane solutions of di-n-hexylphosphoric acid (HDHP), N,N '-dimethyl-N,N '-dioctylhexylethoxymalonamide (DMDOHEMA), and their mixtures, were investigated by electrospray ionization-mass spectrometry (ESI-MS), vapor pressure osmometry (VPO), and small-angle X-ray and neutron scattering (SAXS and SANS). The objective of the study was to probe the formation of mixed HDHP-DMDOHEMA species before and after extraction of trivalent lanthanide and actinide (M3+) nitrates. The most important species formed by HDHP upon metal extraction has the composition M(DHP)(3)(HDHP)(3)(H2O). These species exist as spherical aggregates of the reverse micelle type with a polar core diameter of similar to 7 angstrom and total diameter of similar to 11 to similar to 15 angstrom. The aggregation of DMDOHEMA is a progressive phenomenon, with an average aggregation number of similar to 2 in the 0.2 to 0.6M range and larger aggregates forming at higher concentrations. The metal loaded DMDOHEMA aggregates can be considered as interacting spheres with a polar core diameter between similar to 11 and similar to 16 angstrom, depending on composition, a total diameter of up to similar to 25 angstrom, and a weight-average aggregation number of similar to 9. The results obtained in this work provide strong evidence for the formation of mixed aggregates when mixtures of HDHP and DMDOHEMA are used for the extraction of trivalent Ln and An cations. These mixed reverse micelles have a diameter between 19 and 24 angstrom with a polar core diameter of 10 and to 14 angstrom. The most recurrent micellar composition is 2 HDHP and either 4 or 5 DMDOHEMA molecules.