Effect of nitric acid extraction on phase behavior, microstructure and interactions between primary aggregates in the system dimethyldibutyltetradecylmalonamide (DMDBTDMA) n-dodecane water: A phase analysis and small angle X-ray scattering (SAXS) characterisation study

Among the different problems to be solved when designing a liquid-liquid solvent extraction process, third phase formation, i.e. the splitting of the organic phase into two layers when extracting high concentration of solutes, like nitric acid or metallic nitrates (here trivalent f ions), is one of the most important to address. In some conditions the formation of a "third phase" is observed with dimethyldibutyltetradecylmalonamide (DMDBTDMA), a potential extractant used in the DIAMEX process. We have investigated the phase behavior of the system DMDBTDMA/n-dodecane/water/HNO3, in the acceptable concentration limits for the DIAMEX process. The composition of the different phases and the surface properties of the two-phase system were measured. The maximum incorporation of water in the two-phase system corresponds to approximately 0.75 water molecule per DMDBTDMA molecule, whereas at saturation in the three-phase system it is about 1.25 water molecules per extractant molecule. At 0.22 M and 0.46 M DMDBTDMA concentrations, the transitions from the two-phase to the three-phase domain takes place in a region where the [HNO3](extr.)/[DMDBTDMA](init) ratio is around 0.8. We observe that the two-phase to three-phase transition occurs when the water/acid ratio in the organic phase approaches 1. A sharp change of slope of the interfacial tension versus extractant concentration is attributed to aggregate formation in the organic phase. Assuming a neutral form of the molecule in the absence of HNO3, the interfacial area is in this case 112 Angstrom(2). The microstructure of mixtures DMDBTDMA, water and nitric acid in n-dodecane has also been studied using small angle X-ray scattering (SAXS) in order to determine the size and shape of the primary aggregates of DMDBTDMA as well as the interactions between them in the midst of the organic phase. The complexation of nitric acid at constant diamide concentration, strongly favours attractive interactions between the aggregates. On the contrary, the increase of the aggregates volume fraction, at a constant ratio of nitric acid and diamide concentrations in order to control the attractions, force the aggregates to repel each other, and repulsive hard sphere interactions are pointed out. The information obtained in the present work from the SAXS study, and from the interfacial tension measurements, appear to be consistent since they both evidence the onset of an aggregation process at the approach of the organic phase splitting. The simple short range attractive potential defined by Baxter, describing a complex fluid of sticky spheres, is self-consistent to model the experimental data. In the organic phase, the extractant molecules of DMDBTDMA self-assemble into small reversed micelles with a polar cord of similar to 6 - 7 Angstrom radius when the organic phase is contacted with an aqueous phase (acidic or not). Within the organic phase, the aggregates are submitted to three major interactions : (i) the destabilizing van der Waals interaction and (ii) the stabilizing hard sphere repulsion and (iii) a repulsive steric contribution from the remaining aliphatic chains of the extractant molecules. The observable macroscopic effect which is the phase split of the organic phase with third phase formation is the macroscopic translation of the effect of these three interactions acting at the microscopic level.