Molecular dynamics simulation study of sodium ion structure & dynamics in water in ionic liquids electrolytes using 1-butyl-3-methyl imidazolium tetrafluoroborate and 1-butyl-3-methyl imidazolium hexafluorophosphate

In this paper, we have performed an all-atom molecular dynamics simulation to understand the structure and dynamics of Na+ ions in water mixed Ionic liquids (Water in Ionic liquid). Two ionic liquid (IL) systems consist of (1) 1-butyl-3-methylimidazolium [BMIM] tetrafluoroborate [BF4] and (2) 1-butyl-3-methylimidazolium [BMIM] hexafluorophosphate [PF6] were considered in this work. We understand various inter-molecular structures and dynamic and thermodynamic behaviours of Na+ ions in the water-mixed IL systems. The water (H2O) mole fractions (x) varied from 0.33 to 0.71. The neat ILs [BMIM][BF4] and [BMIM][PF6] pairwise radial distribution functions show a decrease with an increase in x. The [BMIM][PF6] exhibits a strong coordination structure with Na+ ions across the entire range of x values. The rdf between the pairs of Na+-[PF6] presents a significant interaction compared to Na+ and [BF4]. The Na + ions manifested greater coordination with H2O In H2O-[BMIM][PF6] compared to H2O-[BMIM][BF4]. The self-diffusion coefficient (D) values of Na + ions increase with the rise in x in both ILs. The D values of Na + ions are 10-fold higher in [BMIM][BF4] than [BMIM][PF6]. The ionic conductivity values are higher for [BMIM][BF4]. Overall, this paper unveils molecular-level insights for understanding the behavior of Na+ ions in the water in ionic liquid systems.

Molecular simulations of understanding the Na+ ion structure, dynamic and thermodynamic behavior in ionic liquids: Butyl ammonium hydrogen bisulfate and tri-butyl ammonium hydrogen bisulfate

This manuscript presents the all-atom molecular dynamics simulations to investigate intermolecular structure and solvation thermodynamics of Na+ ion in two different ammonium-based protic ionic liquids (1) Butyl Ammonium hydrogen bisulfate [BA+][HSO4-], (2) Tri-butyl ammonium hydrogen bisulfate [TBA+][HSO4-]. The ionic liquid [BA+][HSO4-] show a more coordinated behavior when compared to [TBA+][HSO4-], which is observed over the temperature range from 278 K to 348 K. Hydrogens of the cations show a hydrogen bonding interaction with oxygens of anions. The cationic [TBA+] molecules show more solvation behavior with anions when compared to the [BA+]. The Na+ ion show a strong coordination structure with [HSO4-] in [TBA+][HSO4-] when compared to the [BA+][HSO4-]. We further calculate the detailed solvation free energy (ΔG) calculations using thermodynamic integration. We found that the ΔG of Na+ is more favorable in [TBA+][HSO4-] when compared to [BA+][HSO4-] in the temperature range varying from 278 K to 348 K. With the temperature rise, we observe the more favorable solvation of Na+ in both ionic liquids. On the other hand, the solvation of Cl- becomes less favorable. Overall, this manuscript provides detailed molecular level structural and thermodynamic origins of Na+ in protic ionic liquids useful for designing and developing sustainable electrolytes for Na+ battery applications.

Structure and Conformational Properties of a Short Polyaniline Chain in a Mixture of Water and Ionic Liquid [1-Ethyl-3-methyl-imidazolium][bistriflimide] Investigated by All-Atom Molecular Dynamics Simulations

Development of antifouling membranes for water treatment using conducting polymers and their composites is a fundamental strategy to mitigate the fouling. This manuscript presents an all-atom molecular dynamics simulations of a conducting polymer, polyaniline (PANI), immersed in an ionic liquids (ILs)-water mixtures. We have considered the ionic liquid 1-ethyl-3-methyl imidazolium bistriflimide, [EMIM]+[BIS]-. The two forms of polyaniline, emeraldine base (EB) and emeraldine salt (ES), were considered. Various intra- and intermolecular structural properties of PANI were analyzed, such as polymer chain radius of gyration Rg, radial distribution functions, and torsional angle distributions. The Rg of EB shows an increase, while the Rg of ES shows a decrease with an increase in the IL concentration. The backbone torsional angle probability distributions show a significant trans state for EB, while a combination of trans and gauche states was observed for ES. Similar supportive distributions were seen in the backbone angular distributions. Radial distribution functions between the carbon atoms at ortho and meta positions of the benzene ring on both ES and EB, as well as the amine group attached between two benzene rings, show an enhanced interaction with the ionic liquid compared to water. Anions have a dominant interaction with the polymer chain when compared to cations. The solvent accessible surface area (SASA) calculations were in accordance with the EB and ES structural properties. The SASA values are more favorable for ES than for EB. H-bond analysis shows a decrease in the number of H-bonds with water as the IL concentration increases.

Extraction of Gd3+ and UO22+ Ions Using Polystyrene Grafted Dibenzo Crown Ether (DB18C6) with Octanol and Nitrobenzene: A Molecular Dynamics Study

Atomistic molecular dynamics (MD) simulations are performed in order to derive thermodynamic properties important to understand the extraction of gadolinium (Gd³⁺) and uranium dioxide (UO₂) with dibenzo crown ether (DBCE) in nitrobenzene (NB) and octanol (OCT) solvents. The effect of polystyrene graft length, on DBCE, on the binding behavior of Gd³⁺ and UO₂²⁺ is investigated for the first time. Our simulation results demonstrate that the binding of Gd³⁺ and UO₂²⁺ onto the oxygens of crown ethers is favorable for polystyrene grafted crown ether in the organic solvents OCT and NB. The metal ion binding free energy (ΔG_Binding) in different solvent environments is calculated using the thermodynamic integration (TI) method. ΔG_Binding becomes more favorable in both solvents, NB and OCT, with an increase in the polystyrene monomer length. The metal ion transferability from an aqueous phase to an organic phase is estimated by calculating transfer free-energy calculations (ΔG_Transfer). ΔG_Transfer is significantly favorable for both Gd³⁺ and UO₂²⁺ for the transfer from the aqueous phase to the organic phase (i.e., NB and OCT) via ion-complexation to DBCE with an increase in polystyrene length. The partition coefficient (log P) values for Gd³⁺ and UO₂²⁺ show a 5-fold increase in separation capacity with polystyrene grafted DBCE. We corroborate the observed behavior by further analyzing the structural and dynamical properties of the ions in different phases.

Factors responsible for the aggregation behavior of hydrophobic polyelectrolyte PEA in aqueous solution studied by molecular dynamics simulations

Self-association (i.e. interchain aggregation) behavior of atactic poly(ethacrylic acid) PEA in dilute aqueous solution as function of degree-of-neutralization by Na⁺ counter-ions (i.e. charge fraction f) was investigated by molecular dynamics simulations. Aggregation is found to occur in the range 0≤f≤0.7 in agreement with experimental results compared at specified polymer concentration C_p=0.36mol/l in dilute solution. The macromolecular solution was characterized and analysed for radius-of-gyration, torsion angle distribution, inter and intra-molecular hydrogen bonds, radial distribution functions of intermolecular and inter-atomic pairs, inter-chain contacts and solvation enthalpy. The PEA chains form aggregate through attractive inter-chain interaction via hydrogen bonding, in the range f<0.7, in agreement with experimental observation. The numbers of inter-chain contacts decreases with f. A critical structural transition occurs at f=0.7, observed via simulations for the first time, in R_g as well as inter-chain H-bonds. The inter-chain distance increases with f due to repulsive interactions between COO- groups on the chains. PEA-PEA electrostatic interactions dominant solvation enthalpy. The PEA solvation enthalpy becomes increasingly favorable with increase in f. The transition enthalpy change, in going from uncharged (acid) state to fully charged state (f=1) is unfavorable towards aggregate formation.

Effect of salt valency and concentration on structure and thermodynamic behavior of anionic polyelectrolyte Na+-polyethacrylate aqueous solution

The intermolecular structure and solvation enthalpy of anionic polyelectrolyte atactic Na⁺-polyethacrylate (PEA) in aqueous solution, as a function of added salt concentration C _s (dilute to concentrated) and valency (NaCl versus CaCl₂), were investigated via molecular dynamics simulations with explicit-ion-solvent and atomistic polymer description. An increase in C _s leads to a decrease in α, which stabilizes to a constant value beyond critical C _s. A significant reduction in R _g in the presence of CaCl₂ salt was observed, due to ion bridging of PEA by Ca²⁺ ions, in agreement with results available in literature on other similar polycarboxylates. An increase in salt valency reduces the value of critical C _s for the onset of stabilization of the overall size and shape of the polymer chain. The critical C _s ratio for the divalent to monovalent salt case is in excellent agreement with results of Langevin dynamics studies on model systems available in the literature. PEA-water H-bond half-life increases with C _s for CaCl₂, but no appreciable effect is seen for NaCl. The hydration of PEA becomes stronger in the presence of divalent salt. The strength of H-bond interaction energy is greater for cations as compared to anions of the salt. The salt cation effect in displacing water molecules from the vicinity of PEA, with increase in C _s, is greater for NaCl solution. The decrease in water coordination to PEA carboxylate groups, due to increased C _s, is more pronounced in NaCl solution. The nature of the behavior of the solvation enthalpy of PEA and the type of intermolecular interactions contributing to it, is in agreement with experimental observations from the literature. The hydration enthalpy of PEA in divalent CaCl₂ aqueous salt solution is more exothermic compared to monovalent NaCl salt solution, in agreement with experimental data. The solvation of PEA is thermodynamically more favorable in the case of CaCl₂ solution. The exothermic solvation enthalpy, H-bond lifetime, number of H-bonds and H-bond interaction energy are greater in magnitude in CaCl₂ aqueous solution.

Polyelectrolyte conformational transition in aqueous solvent mixture influenced by hydrophobic interactions and hydrogen bonding effects: PAA-water-ethanol

Molecular dynamics simulations of poly(acrylic acid) PAA chain in water-ethanol mixture were performed for un-ionized and ionized cases at different degree-of-ionization 0%, 80% and 100% of PAA chain by Na(+) counter-ions and co-solvent (ethanol) concentration in the range 0-90vol% ethanol. Aspects of structure and dynamics were investigated via atom pair correlation functions, number and relaxation of hydrogen bonds, nearest-neighbor coordination numbers, and dihedral angle distribution function for back-bone and side-groups of the chain. With increase in ethanol concentration, chain swelling is observed for un-ionized chain (f=0) and on the contrary chain shrinkage is observed for partially and fully ionized cases (i.e., f=0.8 and 1). For un-ionized PAA, with increase in ethanol fraction ϕeth the number of PAA-ethanol hydrogen bonds increases while PAA-water decreases. Increase in ϕeth leads to PAA chain expansion for un-ionized case and chain shrinkage for ionized case, in agreement with experimental observations on this system. For ionized-PAA case, chain shrinkage is found to be influenced by intermolecular hydrogen bonding with water as well as ethanol. The localization of ethanol molecules near the un-ionized PAA backbone at higher levels of ethanol is facilitated by a displacement of water molecules indicating presence of specific ethanol hydration shell, as confirmed by results of the RDF curves and coordination number calculations. This behavior, controlled by hydrogen bonding provides a significant contribution to such a conformational transition behavior of the polyelectrolyte chain. The interactions between counter-ions and charges on the PAA chain also influence chain collapse. The underlying origins of polyelectrolyte chain collapse in water-alcohol mixtures are brought out for the first time via explicit MD simulations by this study.