A Novel Microemulsion Phase Transition: Toward the Elucidation of Third-Phase Formation in Spent Nuclear Fuel Reprocessing

Abstract

We present evidence that the transition between organic and third phases, which can be observed in the plutonium uranium reduction extraction (PUREX) process at high metal loading, is an unusual transition between two isotropic bicontinuous microemulsion phases. As this system contains so many components, however, we have been seeking first to investigate the properties of a simpler system, namely, the related metal-free, quaternary water/n-dodecane/nitric acid/tributyl phosphate (TBP) system. This quaternary system has been shown to exhibit, under appropriate conditions, three coexisting phases: a light organic phase, an aqueous phase, and the so-called third phase. In the current work, we focused on the coexistence of the light organic phase with the third phase. Using Gibbs ensemble Monte Carlo (GEMC) simulations, we found coexistence of a phase rich in nitric acid and dilute in n-dodecane (the third phase) with a phase more dilute in nitric acid but rich in n-dodecane (the light organic phase). The compositions and densities of these two coexisting phases determined using the simulations were in good agreement with those determined experimentally. Because such systems are generally dense and the molecules involved are not simple, the particle exchange rate in their GEMC simulations can be rather low. To test whether a system having a composition between those of the observed third and organic phases is indeed unstable with respect to phase separation, we used the Bennett acceptance ratio method to calculate the Gibbs energies of the homogeneous phase and the weighted average of the two coexisting phases, where the compositions of these phases were taken both from experimental results and from the results of the GEMC simulations. Both demixed states were determined to have statistically significant lower Gibbs energies than the uniform, mixed phase, providing confirmation that the GEMC simulations correctly predicted the phase separation. Snapshots from the simulations and a cluster analysis of the organic and third phases revealed structures akin to bicontinuous microemulsion phases, with the polar species residing within a mesh and with the surface of the mesh formed by amphiphilic TBP molecules. The nonpolar n-dodecane molecules were observed in these snapshots to be outside this mesh. The only large-scale structural differences observed between the two phases were the dimensions of the mesh. Evidence for the correctness of these structures was provided by the results of small-angle X-ray scattering (SAXS) studies, where the profiles obtained for both the organic and third phases agreed well with those calculated from simulations. Finally, we looked at the microscopic structures of the two phases. In the organic phase, the basic motif was observed to be one nitric acid molecule hydrogen-bonded to a TBP molecule. In the third phase, the most common structure was that of the hydrogen-bonded TBP-HNO-HNO chain. A cluster analysis provided evidence for TBP forming an extended, connected network in both phases. S...

Ion-specific clustering of metal–amphiphile complexes in rare earth separations

“…Although Baxter model has been successful in capturing certain trends and relations between the molecular scale structure and macroscopic observables, it has been criticized for overestimating the interactions between the aggregates. 35,36 According to Baxter model, when aggregates form clusters, they should have long range ordering which should lead to peaks in the high-q region of the SAXS data. However, this is almost never observed in SX systems, even when the increasing slope in the low-q region of the SAXS data suggested clustering.…”

Ion-Specific Clustering of Metal-Amphiphile Complexes in Rare Earth Separations

“…26,27,39 Despite its successes, the Baxter model has been challenged recently. 34,35 In fact, Baxter model is known to overestimate the strength of attraction between the aggregates. At low values of the stickiness parameter (strong attraction), the Baxter model leads to stronger hard sphere peaks due to the increased proximity of hard spheres.…”

“…To overcome these shortcomings, trajectories from molecular dynamics simulations have been used to interpret the SAXS results. 31,35 However, no analytical models have been provided as an alternative to the Baxter model. We use a model based on clustering of hard spheres driven by an anonymous attractive interaction.…”

“…48 Hydrogen bonding between the head groups, sometimes mediated by water, also stabilizes the aggregates. 35,38 The differences in head group interactions, hydration, and hydrogen bonding environments between TOMA-NO3 and TOMA-SCN are the likely causes of their different aggregation structures. For instance, we have recently shown that at the air/aqueous interface, NO3and SCNions show qualitatively different adsorption behavior at quaternary amine Langmuir monolayers.…”

“…Although Baxter model has been successful in capturing certain trends and relations between the molecular scale structure and macroscopic observables, it has been criticized for overestimating the interactions between the aggregates. 34,35 According to Baxter model, when aggregates form clusters, they should have long range ordering which should lead to peaks in the high-q region of the SAXS data. However, this is almost never observed in SX systems, even when the increasing slope in the low-q region of the SAXS data suggested clustering.…”