18 research outputs found
Impact of variation in multicomponent diffusion coefficients and salinity in CO<sub>2</sub>-EOR: A numerical study using molecular dynamics simulation
Engineering porous two-dimensional lattices via self-assembly of non-convex hexagonal platelets
A Novel Micro-Emulsion Phase Transition: Towards the Elucidation of Third Phase Formation in Spent Nuclear Fuel Reprocessing
Light triggered pore size tuning in photoswitching covalent triazine frameworks for low energy co2 capture
Comparative Molecular Dynamics Study on Tri-n-butyl Phosphate in Organic and Aqueous Environments and Its Relevance to Nuclear Extraction Processes
A steric hindrance alleviation strategy to enhance the photo-switching efficiency of azobenzene functionalized metal–organic frameworks toward tailorable carbon dioxide capture
Fluid–Fluid Interfacial Effects in Multiphase Flow during Carbonated Waterflooding in Sandstone: Application of X-ray Microcomputed Tomography and Molecular Dynamics
A Novel Microemulsion Phase Transition: Toward the Elucidation of Third-Phase Formation in Spent Nuclear Fuel Reprocessing
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/<i>n</i>-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 <i>n</i>-dodecane (the third phase) with a phase more dilute in
nitric acid but rich in <i>n</i>-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 <i>n</i>-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<sub>3</sub>–HNO<sub>3</sub> chain. A cluster analysis provided evidence for TBP forming an extended,
connected network in both phases. Studies of the effects of metal
ions on these systems will be presented elsewhere
Hierarchical Aggregation in a Complex Fluid─The Role of Isomeric Interconversion
There is an ever-increasing
body of evidence that metallic
complexes
involving amphiliphic ligands do not form normal solutions in organic
solvents. Instead, they form complex fluids with intricate structures.
For example, the metallic complexes may aggregate into clusters, and
these clusters themselves may aggregate into superclusters. To gain
a deeper insight into the mechanisms at play, we have used an improved
force field to conduct extensive molecular dynamics simulations of
a system composed of zirconium nitrate, water, nitric acid, tri-n-butyl phosphate, and n-octane. The important
new finding is that a dynamic equilibrium between the cis and trans
isomers of the metal complex is likely to play a key role in the aggregation
behavior. The isolated cis and trans isomers have similar energies,
but simulation indicates that the clusters consist predominantly of
cis isomers. With increasing metal concentration, we hypothesize that
more clustering occurs and the chemical equilibrium shifts toward
the cis isomer. It is possible that such isomeric effects play a role
in the liquid–liquid extraction of other species and the inclusion
of such effects in flow sheet modeling may lead to a better description
of the process