Quantifying Dimer and Trimer Formation by Tri- n -butyl Phosphates in n -Dodecane: Molecular Dynamics Simulations

2021

Chem. Sci.

Section: Force Fields and Benchmarkingmentioning

confidence: 80%

Unexpected inverse correlations and cooperativity in ion-pair phase transfer

Kumar

2021

Chem. Sci.

“…The Force Field Implementation. The interactions of n-dodecane was taken from Vo et al 27 , while the force field of TODGA was generated by the Generalized AMBER Force Field (GAFF2) 28 parametrization using the geometry-optimized structure of TODGA using density functional theory (DFT) with the B3LYP 29,30 functional and 6-31g* basis set. 31 The restrained electrostatic potential (RESP) approach was employed to derive the partial charges.…”

Section: Simulation Protocolmentioning

confidence: 99%

The Contrasting Role of Water and Acid Within Organic Phase Amphiphile Aggregation

Sadhu

2021

Preprint

The self-assembly of amphiphiles is often modified by the presence of co-solutes and significant study has examined this behavior in aqueous systems. Much less is known about the role of polar co-solutes upon amphiphile aggregation within non-polar media, however such conditions are relevant to a variety of industrial processes - not the least of which are separations systems like those found in liquid-liquid extraction (LLE). Therein, surface active amphiphiles extract water, acid, and other solutes of interest. Intriguing increases to amphiphile aggregates have been experimentally observed upon water and acid extraction, however a myriad of competitive intermolecular interactions have thus far prevented a fundamental understanding of the individual and dual role of these solutes upon amphiphile self-assembly. Toward this end, this work employs classical molecular dynamics and graph theory analyses to deconstruct the individual affects of water and nitric acid upon the self-assembly of N,N,N',N'-tetraoctyl-3-oxapentanediamide (TODGA), a prevalent amphiphile extractant used in metal ion separations. In the absence of acid, and at low water concentration, H2O is found to promote local dimer and trimer formation of TODGA, however as [H2O]org increases, the preferential solvation of water with itself causes the formation of large water clusters that serve to link large TODGA clusters on the periphery (causing extended aggregation). Addition of HNO3 to the humid solutions disrupts the water hydrogen bond network and inhibits the formation of large water clusters - thus preventing extended aggregation behavior. We rationalize the prior experimental observations as being attributed primarily to the role of water in the self-assembly of TODGA rather than co-extracted HNO3, thus providing valuable new insight into the means by which extractant aggregation can be tuned within LLE processes. In addition, this work differentiates the role of polar solutes upon amphiphile self-assembly via their individual hydrogen bonding capabilities and competitive interactions that disrupt preferred solvation environments.

“…The UO 2+ 2 force field reproduces the experimentally observed hydration of 5 in the first solvation shell in bulk water, 8,38,42 whereas the Li + potential was parameterized to reproduce the experimental hydration free energies and ion hydration in the aqueous phase. 40,43 The all-atom General Amber Force Field (GAFF) 44 were implemented for n-hexane, with modified Lennard-Jones parameters to reproduce the experimental density and enthalpy of vaporization as developed by Vo et al [45][46][47] The UO 2+ 2 , Li + , and NO − 3 atom charges were then scaled from 100% to 75% in 5% increments using ECC, 48 which is an indirect correction to account for solvent driven polarization effects on hydrated ions. 49 In this manner, the coulombic interaction were tuned to reproduce the experimentally determined equilibrium constants 18,36 for different uranyl nitrate species and ensure coordination numbers and nitrate denticity that are consistent with experimental studies and prior ab-initio studies.…”

Section: Introductionmentioning

confidence: 99%

Uranyl Speciation in the Presence of Specific Ion Gradients at the Electrolyte/Organic Interface

Kumar

Servis

Solvent Extraction and Ion Exchange

2021

Uranyl (UO2+ 2 ) speciation at the liquid/liquid interface is an essential aspect of the mech?anism that underlies its extraction as part of spent nuclear fuel reprocessing schemes and environmental remediation of contaminated legacy waste sites. Of particular importance is a detailed perspective of how changing ion concentrations at the liquid interface alter the distribu?tion of hydrated uranyl ion and its interactions with complexing electrolyte counterions relative to the bulk aqueous solution. In this work, classical molecular dynamics simulations have ex?amined uranyl in bulk LiNO3(aq) and in the presence of a hexane interface. UO2+ 2 is observed to have both direct coordination with NO− 3 and outer-sphere interactions via solvent-separated ion-pairing (SSIP), whereas the interaction of Li+ with NO− 3 (if it occurs) is predominantly as a contact ion-pair (CIP). The variability of uranyl interactions with nitrate is hypothesized to prevent dehydration of uranyl at the interface, and as such the cation concentration is un?perturbed in the interfacial region. However, Li+ loses waters of solvation when it is present in the interfacial region, an unfavorable process that causes a Li+ depletion region. Although significant perturbations to ion-ion interactions, solvation, and solvation dynamics are observed in the interfacial region, importantly, this does not change the association constants of uranyl with nitrate. Thus, the experimental association constants, in combination with knowledge of the interfacial ion concentrations, can be used to predict the distribution of interfacial uranyl nitrate complexes. The enhanced concentration of uranyl dinitrate at the interface, caused by excess adsorbed NO− 3 , is highly relevant to extractant ligand design principles as such nitrate complexes are the reactants in ligand complexation and extraction events. File list (2) download file view on ChemRxiv Uranyl_complexation_and_solvation_in_nitrate_f.pdf (1.59 MiB) download file view on ChemRxiv Uranyl_complexation_and_solvation_in_nitrate_SI.pdf (4.00 MiB)