Atomistic Insights into Structure and Dynamics of Neodymium(III) Complexation with a Bis-lactam Phenanthroline Ligand in the Organic Phase.

Rare-earth elements (REEs) such as neodymium are critical materials needed in many important technologies, and rigid neutral bis-lactam-1,10-phenanthroline (BLPhen) ligands show one of the highest extraction performance for complexing Nd(III) in REE uptake and separation processes. However, the local structure of the complexes formed between BLPhen and Nd(III) in a typical organic solvent such as dichloroethane (DCE) is unclear. Here, we perform first-principles molecular dynamics (FPMD) simulations to unveil the structure of complexes formed by BLPhen with Nd(NO3)3 in the DCE solvent. BLPhen can bind to Nd(III) in either 1:1 or 2:1 fashion. In the 1:1 complex, three nitrates bind to Nd(III) via the bidentate mode in the first solvation shell, leading to the formation of a neutral complex, [Nd(BLPhen)(NO3)3]0, in the organic phase. In contrast, there are two nitrates in the first solvation shell in the 2:1 complex, creating a charged complex, [Nd(BLPhen)2(NO3)2]+. The third nitrate was found to be far away from the metal center, migrating to the outer solvation shell. Our simulations show that the binding pocket formed by the two rigid BLPhen ligands allows ample space for two nitrates to bind to the Nd(III) center from opposite sides. Our findings of two nitrates in the first solvation shell of the 2:1 complex and the corresponding bond distances agree well with the available crystal structure. This study represents the first accurate FPMD modeling of the BLPhen-Nd(III) complexes in an explicit organic solvent and opens the door to more atomistic understanding of REE separations from first principles.

Solvent-Assisted Li-Ion Transport and Structural Heterogeneity in Fluorinated Battery Electrolytes.

The electrolytes diluted with fluorinated solvents show promising properties toward better battery technology than existing ones. The transport of Li ions in these fluorinated electrolytes is essential to access the performance of a battery. It is believed that the transport of the Li ion in these electrolytes occurs through polar solvents in the matrix of nonpolar solvent molecules. The atomistic details of this mechanism are yet to be proved using the dynamics of these mixtures. In this study, we performed classical molecular dynamics simulations at various temperatures to probe this mechanism through the structure and dynamics of electrolytes at the atomic level. Here, we have shown that the polar fluorinated solvents assist the Li-ion transport in a region of nonpolar solvent. Highly polar molecules also solvate the Li ion at a lower temperature. The nonpolar solvent solvates the Li ion weakly as compared to others. The calculated values of the ionic conductivity from the Green-Kubo relation provide a better match than that from an experimental conductivity meter. Furthermore, we probed the heterogeneity in the dynamics of the electrolytes by calculating the non-Gaussian parameter. We also show that the transport mechanism of the Li ion in diluted concentrated electrolytes is different than a few of the other reported electrolytes. We have also calculated the ion-pair and ion-cage lifetimes to see the most and least lived ion/ion-solvent pairs. The mechanism given from the present study may help to design the fluorinated electrolytes for Li-ion batteries.

Hydrogen Bond Kinetics, Ionic Dynamics, and Voids in the Binary Mixtures of Protic Ionic Liquids with Alkanolamines.

Classical molecular dynamics simulations were used to investigate the structural and dynamical properties of the mixtures of ionic liquids (ILs) with the conjugate forms of the cation in a 1:1 molar ratio. The experimental studies suggested the combination of ethanolamines and ILs as novel absorbents for acidic gases such as CO2 and H2S, which provide the advantage of efficient absorption of gases at low pressures. However, the microscopic properties of the ionic mixtures are not studied. From our computational investigations, the densities of mixtures are reported and compared with the experimental results. The structural evolution of mixtures is reported by radial distribution functions, coordination numbers, void analysis, and spatial distribution functions. The mixtures' dynamic properties were studied by analyzing the hydrogen bond, ion-pair, and ion-cage lifetimes of the system. Monoethanolammonium and triethanolammonium ILs show different types of spatial distribution functions. The cations have lesser effect on dynamics compared with anions. The charge on the anion greatly affects the dynamics of mixtures. The dianion mixtures show slower dynamics than the monoanionic mixtures. The hydrogen bonding between cations and anions is stronger than that between cations and neutral molecules due to strong coulombic attractive forces. The cations spend more time around the dianions as compared to monoanions. The distributions of voids show that the void sizes are smaller in triethanolamine-based mixtures. The sulfobenzoate-based mixtures show voids smaller than those of pyridine-3-carboxylate-based mixtures due to more available free space between the entities, which facilitates the overall dynamics.

Connecting Correlated and Uncorrelated Transport to Dynamics of Ionic Interactions in Cyclic Ammonium-Based Ionic Liquids.

The correlations present in the ionic liquids (ILs) are essential to interpret the ion association and dynamics processes. The correlated and uncorrelated ionic conductivities reported from two different types of experiments were calculated employing molecular dynamics methods by exercising appropriate care to obtain the diffusive regions. The ionic conductivity is found to be correlated with lifetimes of the ion-pair and ion-cage formation. In this study, the structure and dynamic properties of five cyclic ammonium-based ILs were investigated by comparing the experimental results with the calculated transport properties: (1) 1-allyl-1-methylpyrrolidinium, (2) 1-propyl-1-methylpyrrolidinum, (3) 1-methyl-1-allylpiperidinium, (4) 4-allyl-4-methylmorpholin-4-ium, and (5) 4-allyl-4-ethylmorpholin-4-ium with bis(trifluoromethanesulfonyl)imide anion as the common anion. We observed the linear relationship between the inverse of ion-pair lifetimes and ionic conductivities. The diffusion coefficients obtained from velocity autocorrelation functions follow a trend similar to that of the experiment as compared to mean square displacements (MSDs). The ionic conductivities from correlated MSD and current autocorrelation functions are compared to the ionic conductivities from the conductometer experiment. The time correlation functions of the ion-pair and ion-cage dynamics were calculated. The correlation functions were used to obtain the lifetimes. Pyrrolidinium-based ILs show lower lifetimes than other ILs, which correlates with the conductivity. Morpholinium-based ILs show higher interaction between ions than other ILs. This result supports the slower dynamics present in morpholinium-based ILs than in other ILs. In this work, our objective is to give atomic insight into the dynamics of IL, which could not be extracted from the experiment, and relate microscopic properties with macroscopic properties.

Ionic Dynamics of Hydroxylammonium Ionic Liquids: A Classical Molecular Dynamics Simulation Study.

We report the structure and dynamics of four ionic liquids (ILs), 2-hydroxyethylammonium formate, bis-(2-hydroxyethyl) ammonium formate, tris-(2-hydroxyethyl) ammonium formate (THEF), and 2-hydroxyethylammonium lactate, employing classical molecular dynamics simulations. The dynamics of ILs are represented by studying mean squared displacements (MSDs), velocity autocorrelation functions (VACFs), and current auto-correlation functions (CACFs). Diffusion coefficients calculated from the VACFs are higher than those obtained from MSDs. The diffusion coefficients calculated from both the methods (MSDs and VACFs) were averaged to calculate the uncorrelated ionic conductivities (ICs). ICs from these two methods agree with the experimental trend. The correlated and uncorrelated ICs were calculated by four methods and compared with experiments. The difference between CACF and center of mass VACF accounts for the correlated motion present in the ILs. The addition of hydroxyalkyl chains on cations causes the dynamics to become slow. The number of hydroxyl groups present on the cations affects the dynamics of ILs studied. A tris-(2-hydroxyethyl) ammonium cation has lower diffusion than any other ions because of the higher molecular weight and number of hydroxyl groups on the cation. We explored the dynamics of hydrogen bonding by calculating the continuous and intermittent hydrogen bond autocorrelation functions. Radial distribution functions between the functional groups of cations and anions reveal the structural arrangement in ILs. The coordination numbers decrease with the increase in the bulkiness of cations due to steric hindrance. Spatial distribution functions of anions around cations show that anions occupy the space around the ammonium hydrogen atoms of the cations. Ion-pair and ion-cage dynamics show that THEF has slower dynamics than the other three ILs and is consistent with MSDs. The inverse of ion-pair and ion-cage lifetimes shows a linear relationship with ICs.

Structure and Conformational Response of Pure and Lithium-Doped Ionic Liquids to Pressure Alterations from Molecular Dynamics Simulations.

The response of ions in ionic liquids (ILs) to elevated external pressure facilitates the induced structural changes that give a chance to understand the change in chemical and physical properties because of perturbation. Employing classical molecular dynamics simulations, we report various structural properties of IL mixtures with Li-salt under varying pressure. Here, we aim to explore the effect of pressure on three ILs N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr1A-TFSI, A = 3, 6, 9) and their mixtures with 30 mol % lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI). The microheterogeneity in these systems was explored by investigating the total and partial structure factors. Intramolecular distribution functions were calculated to study the conformational changes. The distribution of clusters of alkyl chains provides information about aggregation behavior. The calculated total structure factors compliment well with the main features of the experimental results. The prepeak, charge-ordering peak, and main peaks from simulations are in good agreement with the experimental results. The three pure ILs are structurally similar, which show both polarity and charge-ordering peaks. The addition of Li-salt makes the charge-ordering peak disappear. We find that three ILs respond quite differently to the addition of Li-salt. The mesoscopic structure of these ILs is affected by high pressure. The height of the prepeak is diminished significantly with the application of high pressure. This decrease in the height of the prepeak is due to the reduction of alkyl chain aggregation. This motivated us to calculate the aggregation of the alkyl chain quantitatively. The number of alkyl chains present in a given cluster fades with the rise in pressure. The addition of Li-salt decreases the tendency of alkyl chain aggregation. The presence of lithium-ions causes the absence of the charge-ordering peak, which occurs at around 8.0 nm-1 in pure ILs. From the partial structure factors, the charge-ordering is observed to be present in Li-containing mixtures. The conformation change in the ionic entity is also observed; the distance between the nitrogen atom and terminal carbon of the alkyl chain in cation decreases with increasing the pressure. When the Li-ion is present in the mixture, the cis configuration of anions slightly dominates the trans configuration. The conformational change of anions from trans to cis occurs when pressure changes from a low to a high value.

Heterogeneity in the microstructure and dynamics of tetraalkylammonium hydroxide ionic liquids: insight from classical molecular dynamics simulations and Voronoi tessellation analysis.

Microscopic structural and dynamic heterogeneities were investigated for three ionic liquids (ILs), namely tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), and tetrabutylammonium hydroxide (TBAH), by employing classical molecular dynamics (MD) simulations. Structural heterogeneity was explored through radial distribution functions, domain analysis from Voronoi tessellation, spatial distribution functions, combined distribution functions, and structure factors. Radial distribution functions reveal that TEAH has a slightly different structure than TPAH and TBAH. The domain analysis shows that the three ILs have structural heterogeneity. The cations form continuous domains and the anions form discreet domains. The anions are trapped in the voids formed by the cationic domains. When the cation has sufficiently longer alkyl chains, the polar domains become discontinuous. The total structure factor reveals that a pre-peak is absent in TEAH due to a short alkyl chain. The cation head-anion partial structure factor further confirms this phenomenon. Polar and non-polar domains can be better distinguished in the ILs with longer alkyl chains. This distinctive structural behaviour leads to dynamic heterogeneity, which plays an important role in the applications related to ion transportation. To shed light on dynamic heterogeneity, we have calculated the mean square displacements, vibrational density of states, van Hove correlation, and non-Gaussian parameter. The diffusion of ions is lowest in TPAH among all the ILs. The dynamics are highly sluggish in these ILs. The peaks in the low frequency vibrational density of states undergo redshift with an increase in the alkyl chain length. The van Hove correlation functions and non-Gaussian parameter suggest the presence of dynamic heterogeneity in all three ILs. Dynamic heterogeneity is discussed through van Hove correlation functions and the non-Gaussian (NG) parameter was calculated to account for the deviation from Gaussian behaviour. The NG parameter suggests that TEAH has different dynamics than TPAH and TBAH.

Nanostructure domains, voids, and low-frequency spectra in binary mixtures of N,N-dimethylacetamide and ionic liquids with varying cationic size.

Classical molecular dynamics (MD) simulations were carried out on binary mixtures of N,N-dimethylacetamide (DMA) with hydroxide based ammonium ionic liquids (ILs), tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), tetrabutylammonium hydroxide (TBAH), at three different mole fractions of IL (X IL). The solvation of DMA molecules by ions of ILs was studied by the combined distribution function (CDF). CDFs show that anions have strong correlations with the DMA due to the hydrogen bonding. Increasing the DMA disrupts the nanosegregated domains and causes changes in correlations of cation-DMA and anion-DMA. Also, increased translational motion of ions, as well as the fluidity of IL and a significant improvement in self-diffusion coefficients, are observed with the presence of more DMA. The structural microheterogeneity was investigated using the Voronoi tessellation method. Domain analysis confirms the formation of discreet domains by anions at all the mole fractions. The results also complement the experimental observations, which suggest that two types of aggregations are possible in given mixtures: below and above 0.5 X IL. When the alkyl chain length on the cation increases, a notable decrease in ion translational motion was observed in the IL rich region. In the concentrated IL mixture, the self-diffusion coefficient of the cation is higher than that of the corresponding anion; further addition of IL (X IL < 0.5) results in weaker interactions between DMA and anion when compared to DMA-cation. The mean collision time of each species is found to have an inverse relation with X IL. The analysis of the vibrational density of states provides the low-frequency spectral feature of the mixtures.

Protic ammonium carboxylate ionic liquids: insight into structure, dynamics and thermophysical properties by alkyl group functionalization.

This study is aimed at characterising the structure, dynamics and thermophysical properties of five alkylammonium carboxylate ionic liquids (ILs) from classical molecular dynamics simulations. The structural features of these ILs were characterised by calculating the site-site radial distribution functions, g(r), spatial distribution functions and structure factors. The structural properties demonstrate that ILs show greater interaction between cations and anions when alkyl chain length increases on the cation or anion. In all ILs, spatial distribution functions show that the anion is close to the acidic hydrogen atoms of the ammonium cation. We determined the role of alkyl group functionalization of the charged entities, cations and anions, in the dynamical behavior and the transport coefficients of this family of ionic liquids. The dynamics of ILs are described by studying the mean square displacement (MSD) of the centres of mass of the ions, diffusion coefficients, ionic conductivities and hydrogen bonds as well as residence dynamics. The diffusion coefficients and ionic conductivity decrease with an increase in the size of the cation or anion. The effect of alkyl chain length on ionic conductivity calculated in this article is consistent with the findings of other experimental studies. Hydrogen bond lifetimes and residence times along with structure factors were also calculated, and are related to alkyl chain length.