Xianggui Ye

Influence of Nitric Acid on Uranyl Nitrate Association in Aqueous Solutions: A Molecular Dynamics Simulation Study

Uranyl ion complexation with water and nitrate is a key aspect of the uranium/plutonium extraction process. We have carried out a molecular dynamics simulation study to investigate this complexation process, including the molecular composition of the various complex species, the corresponding structure, and the equilibrium distribution of the complexes. The observed structures of the complexes suggest that in aqueous solution, uranyls are generally hydrated by 5 water molecules in the equatorial plane. When associating with nitrate ions, a water molecule is replaced by a nitrate ion, preserving the five‐fold coordination and planar symmetry. Analysis of the pair correlation function between uranyl and nitrate suggests that nitrates bind to uranyl in aqueous solution mainly in a monodentate mode, although a small portion of bidentates occur. Dynamic association and dissociation between uranyls and nitrates take place in aqueous solution with a substantial amount of fluctuation in the number of various uranyl nitrate species. The average number of the uranyl mono‐nitrate complexes shows a dependence on acid concentration consistent with equilibrium‐constant analysis, namely, the concentration of [UO2NO3]+ increases with nitric acid concentration.

Molecular Dynamics Simulation of Tri-n-butyl-Phosphate Liquid: A Force Field Comparative Study

Molecular dynamics (MD) simulations were conducted to compare the performance of four force fields in predicting thermophysical properties of tri-n-butyl-phosphate (TBP) in the liquid phase. The intramolecular force parameters used were from the Assisted Model Building with Energy Refinement (AMBER) force field model. The van der Waals parameters were based on either the AMBER or the Optimized Potential for Liquid Simulation (OPLS) force fields. The atomic partial charges were either assigned by performing quantum chemistry calculations or utilized previously published data, and were scaled to approximate the average experimental value of the electric dipole moment. Canonical ensemble computations based on the aforementioned parameters were performed near atmospheric pressure and temperature to obtain the electric dipole moment, mass density, and self-diffusion coefficient. In addition, the microscopic structure of the liquid was characterized via pair correlation functions between selected atoms. It has been demonstrated that the electric dipole moment can be approximated within 1% of the average experimental value by virtue of scaled atomic partial charges. The liquid mass density can be predicted within 0.5-1% of its experimentally determined value when using the corresponding charge scaling. However, in all cases, the predicted self-diffusion coefficient is significantly smaller than a commonly quoted experimental measurement; this result is qualified by the fact that the uncertainty of the experimental value was not available.

Uranyl nitrate complex extraction into TBP/dodecane organic solutions: a molecular dynamics study

Liquid-liquid extraction of uranyl is studied by conducting atomistic molecular dynamics simulation using quantum chemistry calibrated force fields via restrained electrostatic potential fitting of atomic forces. The simulations depict the migration of uranyl nitrate complexes from the aqueous-organic interface into the tri-n-butyl phosphate (TBP)/dodecane organic phase, in the form of UO(2)(NO(3))(2)·H(2)O·2TBP and UO(2)(NO(3))(2)·3TBP. The migration process is characterized by the gradual breaking of all the hydrogen bonds between the complex and the water molecules at the interface. Moreover, our simulation results suggest that the experimentally observed complex UO(2)(NO(3))(2)·2TBP is formed after the migration of the aforementioned complexes into the organic phase by means of a reorganization of the nitrate binding mode from mono to bidentate which removes the excess oxygen atoms bound to uranyl.

Interfacial complex formation in uranyl extraction by tributyl phosphate in dodecane diluent: a molecular dynamics study.

Atomistic simulations have been carried out in a multicomponent two-phase system (aqueous and organic phases in direct contact) to investigate the interfacial molecular mechanisms leading to uranyl extraction from the aqueous to organic phase. The aqueous phase consists of the dissolved ions UO2(2+) and nitrate NO3-, with or without H3O+, in water to describe acidic or neutral condition; the organic phase consists of tributyl phosphate, the extractant, in dodecane as the diluent. We find that the interface facilitates the formation of various uranyl complexes, with a general formula UO2(2+)(NO3-)n *mTBP*kH2O, with n+m+k=5, suggesting a 5-fold coordination. The coordination for all three molecular entities has the common feature that they all bind to the uranyl at the uranium atom with an oxygen atom in the equatorial plane perpendicular to the molecular axis of the uranyl, forming a 5-fold symmetry plane. Nitric acid has a strong effect in enhancing the formation of extractable species, which is consistent with experimental findings.

Template-Free Bottom-Up Method for Fabricating Diblock Copolymer Patchy Particles.

Patchy particles are one of most important building blocks for hierarchical structures because of the discrete patches on their surface. We have demonstrated a convenient, simple, and scalable bottom-up method for fabricating diblock copolymer patchy particles through both experiments and dissipative particle dynamics (DPD) simulations. The experimental method simply involves reducing the solvent quality of the diblock copolymer solution by the slow addition of a nonsolvent. Specifically, the fabrication of diblock copolymer patchy particles begins with a crew-cut soft-core micelle, where the micelle core is significantly swelled by the solvent. With water addition at an extremely slow rate, the crew-cut soft-core micelles first form a larger crew-cut micelle. With further water addition, the corona-forming blocks of the crew-cut micelles begin to aggregate and eventually form well-defined patches. Both experiments and DPD simulations indicate that the number of patches has a very strong dependence on the diblock copolymer composition-the particle has more patches on the surface with a lower volume fraction of patch-forming blocks. Furthermore, particles with more patches have a greater ability to assemble, and particles with fewer patches have a greater ability to merge once assembled.

Morphology Tailoring of Thin Film Block Copolymers on Patterned Substrates

It is well known that chemically patterned substrates can direct the assembly of adsorbed layers or thin films of block copolymers. For a cylinder-forming diblock copolymer on periodically spot-patterned substrates, the morphology of the block copolymer follows the pattern at the substrate; however, with different periodic spacing and spot size of the pattern, novel morphologies can be created. Specifically, we have demonstrated that new morphologies that are absent in the bulk system can be tailored by judiciously varying the mismatch between the width of the pattern and the periodic spacing of the bulk block copolymer, the top surface affinity, and spot size. New morphologies can thus be achieved, such as honeycomb and ring structures, which do not appear in the bulk system. These results demonstrate a promising strategy for fabrication of new nanostructures from chemically patterned substrates.

Molecular simulation of water extraction into a tri-n-butylphosphate/n-dodecane solution.

Molecular dynamics simulations were performed to investigate water extraction into a solution of 30 vol % tri-n-butylphosphate (TBP) in n-dodecane. Our computational results indicate that the TBP electric dipole moment has a significant effect on the predicted water solubility. A larger TBP dipole moment decreases the aqueous-organic interfacial tension, leading to increased roughness of the aqueous-organic interface. Interfacial roughness disrupts the interfacial water hydrogen bonding structure, resulting in a presence of dangling water molecules at the interface. The increased interfacial roughness enhances the probability of water molecules breaking away from the aqueous phase and migrating into the organic bulk phase. By varying the atomic partial charges of the TBP molecules to reproduce a dipole moment close to the experimentally measured value, we were able to predict water solubility in close agreement with experimental measurements. In addition, our simulation results reveal the detailed molecular mechanism of the water extraction process, and the various structural forms of water molecules both at the interface and in the bulk organic phase.

Block Copolymer Morphology Formation on Topographically Complex Surfaces: A Self-Consistent Field Theoretical Study

A self-consistent field theoretic study is performed to study morphological development of lamellae-forming diblock copolymers on substrates with a well-defined roughness, modeled as trenches of varying depth and width engraved into the substrates. There are three possible lamellar orientations observed: horizontal lamellae, vertical lamellae that are parallel to the trench direction, and vertical lamellae that are perpendicular to the trench direction. Which of these three morphologies formed depends upon the trench width and surface affinity; however, trench depth has a relatively insignificant effect on the morphological development. Therefore, tuning trench width, but not trench depth, should allow for a reduction of the morphological defect density in directed self-assembly of lamellar morphology of diblock copolymers.

A self-consistent field study of diblock copolymer/charged particle system morphologies for nanofiltration membranes

A combination of self-consistent field theory and density functional theory was used to examine the stable, 3-dimensional equilibrium morphologies formed by diblock copolymers with a tethered nanoparticle attached either between the two blocks or at the end of one of the blocks. Both neutral and interacting particles were examined, with and without favorable/unfavorable energetic potentials between the particles and the block segments. The phase diagrams of the various systems were constructed, allowing the identification of three types of ordered mesophases composed of lamellae, hexagonally packed cylinders, and spheroids. In particular, we examined the conditions under which the mesophases could be generated wherein the tethered particles were primarily located within the interface between the two blocks of the copolymer. Key factors influencing these properties were determined to be the particle position along the diblock chain, the interaction potentials of the blocks and particles, the block copolymer composition, and molecular weight of the copolymer.

Block copolymer micelle formation in a solvent good for all the blocks

To produce desired aggregate structures of copolymers, the copolymer is usually first dissolved in a common solvent that dissolves all the blocks. However, a solvent having the exact same solubility to all the blocks of a copolymer is rare. Hence, it is extremely important to know whether the block copolymer forms micelle in a common solvent, and if it does, to know the micelle’s structure. In this study, we used polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) dissolved in dimethyl formamide (DMF) as a model system to address block copolymer micelle formation and its structure in a solvent good for all the blocks as DMF dissolves both PS and P4VP. Our atomic force microscopy (AFM) and cryogenic-transmission electron microscopy (cryo-TEM) results clearly demonstrated that PS-b-P4VP with a wide range of molecular weight and P4VP composition in DMF forms a spherical micelle. Furthermore, contact angle measurements and TEM results clearly show that the micelle has a PS core and a P4VP corona. In comparing the dry micelle and the micelle in DMF, we discovered that the micelle core is significantly swelled by DMF. Our findings suggest that soft-core micelles widely exist for block copolymers in solvents good for all the blocks but with significant selectivity between different blocks.