5 research outputs found
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
Comparative Molecular Dynamics Study on Tri‑<i>n</i>‑butyl Phosphate in Organic and Aqueous Environments and Its Relevance to Nuclear Extraction Processes
A refined model for tri-<i>n</i>-butyl phosphate (TBP),
which uses a new set of partial charges generated from our ab initio
density functional theory calculations, has been proposed in this
study. Molecular dynamics simulations are conducted to determine the
thermodynamic properties, transport properties, and the microscopic
structures of liquid TBP, TBP/water mixtures, and TBP/<i>n</i>-alkane mixtures. These results are compared with those obtained
from four other TBP models, previously described in the literature.
We conclude that our refined TBP model appears to be the only TBP
model from this set that, with reasonable accuracy, can simultaneously
predict the properties of TBP in bulk TBP, in organic diluents, and
in aqueous solution. The other models only work well for two of the
three systems mentioned above. This new TBP model is thus appropriate
for the simulation of liquid–liquid extraction systems in the
nuclear extraction process, where one needs to simultaneously model
TBP in both aqueous and organic phases. It is also promising for the
investigation of the microscopic structure of the organic phase in
these processes and for the characterization of third-phase formation,
where TBP again interacts simultaneously with both polar and nonpolar
molecules. Because the proposed TBP model uses OPLS-2005 Lennard-Jones
parameters, it may be used with confidence to model mixtures of TBP
with other species whose parameters are given by the OPLS-2005 force
field
Photonic Crystals Fabricated by Block Copolymerization-Induced Microphase Separation
We
present a method for fabricating photonic crystals (PCs) by
polymerization-induced microphase separation of block copolymers (BCPs).
Molecular weight of BCP for PCs is so large that it has been difficult
for conventional solution casting and annealing methods to complete
the microphase separation to form periodically ordered submicron structures.
Our method overcomes the difficulty by inducing the microphase separation
and transitions during the polymerization, when the molecular weight
of the BCPs is small enough for the microphase separation and transitions.
The microphase-separated structure is then enlarged while maintaining
the self-similarity. We succeeded in fabricating PCs with reflection
wavelength λ<sub>m</sub> ≈ 1000 nm and a full width at
half-maximum Δλ = 0.05λ<sub>m</sub> by living-radical
bulk block copolymerization of polyÂ(methyl methacrylate)-<i>block</i>-polystyrene
Small-Angle Neutron Scattering Study on Specific Polymerization Loci Induced by Copolymerization of Polymerizable Surfactant and Styrene during Miniemulsion Polymerization
We
have investigated the origin of a specific polymerization loci
that yielded copolymer particles with a bimodal gel permeation chromatographic
profile in the early stage of miniemulsion polymerization of styrene
(St) with the polymerizable surfactant <i>N</i>-<i>n</i>-dodecyl-<i>N</i>-2-methacryloyloxyethyl-<i>N</i>,<i>N</i>-dimethylammonium bromide (C<sub>12</sub>DMAEMA). The formation of particles by miniemulsion polymerization
using C<sub>12</sub>DMAEMA was investigated as a function of the polymerization
time (<i>t</i><sub>p</sub>) and was compared with that using
the nonpolymerizable surfactant cetyltrimethylammonium bromide (CTAB).
St was miniemulsified with C<sub>12</sub>DMAEMA or CTAB and was polymerized
with either a water-soluble initiator, 2,2′-azobisÂ(2-amidinopropane)
dihydrochloride (V50), or an oil-soluble initiator, 2,2′-azobisÂ(isobutyronitrile)
(AIBN), at 60 °C in the presence of polystyrene (PSt) as a hydrophobe.
Gel permeation chromatography (GPC) and elemental analysis indirectly
predicted two different polymerization loci in the St/C<sub>12</sub>DMAEMA/V50 polymerization system. Time-resolved small-angle neutron
scattering (SANS) was used to directly observe and examine the specific
polymerization loci in the miniemulsion polymerization solutions.
The droplets formed in the St/C<sub>12</sub>DMAEMA/V50/PSt at the
initial stage of the polymerization were not fully stabilized by C<sub>12</sub>DMAEMA and polyÂ(C<sub>12</sub>DMAEMA-<i>ran</i>-St) and were found to build up the large aggregates. The spherical
droplets were stabilized later in the polymerization (<i>t</i><sub>p</sub> > 20 min) by forming a homogeneous dispersion in
the
water phase. In contrast, the droplets stabilized by CTAB in St/CTAB/V50/PSt
maintained their size and shape throughout the polymerization. As
a result, in both polymerization systems of St/C<sub>12</sub>DMAEMA/V50/PSt
and St/CTAB/V50/PSt, the particle size was found to depend strongly
on the size of the droplets formed in the early stage of polymerization.
Porod analysis of the power law scattering observed by SANS provided
direct evidence for the specific polymerization loci, which appeared
in the early stage of the polymerization system of St/C<sub>12</sub>DMAEMA/V50/PSt on the surface of the droplet, and was the origin
of the bimodal peaks in the GPC chromatogram
Small-angle neutron scattering study of specific interaction and coordination structure formed by mono-acetyl-substituted dibenzo-20-crown-6-ether and cesium ions
<p>This study uses small-angle neutron scattering (SANS) to elucidate the coordination structure of the complex of mono-acetyl-substituted dibenzo-20-crown-6-ether (ace-DB20C6) with cesium ions (Cs<sup>+</sup>). SANS profiles obtained for the complex of ace-DB20C6 and Cs<sup>+</sup> (ace-DB20C6/Cs) in deuterated dimethyl sulfoxide indicated that Cs<sup>+</sup> coordination resulted in a more compact structure than the free ace-DB20C6. The data were fitted well with SANS profiles calculated using Debye function for scattering on an absolute scattering intensity scale. For this theoretical calculation of the scattering profiles, the coordination structure proposed based on density functional theory calculation was used. Consequently, we conclude that the SANS analysis experimentally supports the proposed coordination structure of ace-DB20C6/Cs and suggests the following: (1) the complex of ace-DB20C6 and Cs<sup>+</sup> is formed with an ace-DB20C6/Cs molar ratio of 1/1 and (2) the two benzene rings of ace-DB20C6 fold around Cs<sup>+</sup> above the center of the crown ether ring of ace-DB20C6.</p