Linear tricationic ionic liquids: Insights into the structural features using DFT and molecular dynamics simulation

Molecular Insights into Curvature Effects on the Capacitance of Electrical Double Layers in Tricationic Ionic Liquids with Carbon Nanotube Electrodes

Ionic liquid (IL) electrolytes and carbon nanotube (CNT) electrodes have exhibited promising electrochemical performance in supercapacitors. Nevertheless, the adaptability of tricationic ILs (TILs) in CNT-based supercapacitors remains unknown. Herein, the performance of supercapacitors with (6,6), (8,8), (12,12), and (15,15) CNT electrodes in the TIL [C₆(mim)₃](Tf₂N)₃ was assessed via molecular dynamics simulations, paying attention to the electric double-layer (EDL) structures and the relations between the CNT curvature and capacitance. The results disclose that counterion and co-ion number densities near CNT electrodes have a marked reduction, compared with that of the graphene electrode. The capacitance of the EDL in the TIL increases significantly as the CNT curvature increases and the capacitance of the TIL/CNT systems is higher than that of the TIL/graphene system. Moreover, different EDL structures in the TIL and the monocationic IL (MIL) [C₆mim][Tf₂N] near CNT electrodes were revealed, showing higher-concentration anions [Tf₂N]^- at the CNT surfaces in the TIL. It is also verified that the TIL has a greater energy-storage ability under high potentials. Furthermore, the almost flat or weakly camel-like capacitance-voltage (C-V) curve of EDLs in the TIL turns into a bell shape in the MIL, because of the ion accumulation at the CNT surfaces and the associations between ions.

Application of DFT Simulation to the Investigation of Hydrogen Embrittlement Mechanism and Design of High Strength Low Alloy Steel

In this work, first-principles methods were performed to simulate interactions between hydrogen and common alloying elements of high strength low alloy (HSLA) steel. The world has been convinced that hydrogen could be one of the future clean energy sources. HSLA steel with a balance of strength, toughness, and hydrogen embrittlement susceptibility is expected for application in large-scale hydrogen storage and transportation. To evaluate the property deterioration under a hydrogen atmosphere, hydrogen embrittlement (HE) of HSLA steel attracts attention. However, due to the small size of hydrogen atoms, the mechanism of HE is challenging to observe directly by current experimental methods. To understand the HE mechanism at an atomic level, DFT methods were applied to simulate the effects of alloying elements doping in bcc-Fe bulk structure and grain boundary structure. Furthermore, the potential application of DFT to provide theoretical advice for HSLA steel design is discussed.

DFT and COSMO-RS studies on dicationic ionic liquids (DILs) as potential candidates for CO2 capture: the effects of alkyl side chain length and symmetry in cations

The weaker interaction energy between anions and cations, the stronger interaction of a CO2 molecule with the cation. Also, the selectivity of CO2 from H2, CO and CH4 gases decreases slightly with increasing the length of side alkyl chains.

Probing the Effect of Side Alkyl Chain Length on the Structural and Dynamical Micro-heterogeneities in Dicationic Ionic Liquids

The molecular dynamics simulations and Voronoi tessellation analysis of two dicationic ionic liquids (DILs) including [C₅(mim)₂][NTf₂]₂ and [C₅(mim)₂C₄][NTf₂]₂ have been carried out to investigate the effects of side alkyl chain length on the structural and dynamical micro-heterogeneity of these DILs. Radial distribution functions (RDFs), spatial distribution functions (SDFs), and also neighborhood analysis of ions have been calculated to determine the arrangement of the nearest neighboring ions. To better understand the hydrogen-bonding network, microstructures, inter- and intramolecular orientations of ions in the studied DILs, different kinds of combined distribution functions (CDFs) were computed and analyzed. Also, qualitative and quantitative analyses of the structural heterogeneity were explored through total/partial structure factors, heterogeneity order parameters (HOPs), and domain analysis from Voronoi tessellation. The results showed that the side alkyl chains in DILs have significant effects on their micro-organizations in such a way that [C₅(mim)₂C₄][NTf₂]₂ with longer side chains has more microstructural heterogeneity than [C₅(mim)₂][NTf₂]₂ where the linkage alkyl chain is the same in both of them. Furthermore, to shed light on the dynamical heterogeneity, ion pair, ion cage, and hydrogen-bond stabilities and also the reorientation dynamics of ions have been investigated. Results demonstrated that local dynamics differences originate from local structural heterogeneity.

Interfacial and bulk properties of concentrated solutions of ammonium nitrate

We conducted molecular dynamics (MD) simulations to calculate the density and surface tension of concentrated ammonium nitrate (AN) solutions up to the solubility limit of ammonium nitrate in water, by combining the SPC/E, SPCE/F and TIP4P/2005 water models with OPLS model for ammonium and nitrate ions. This is the first time that the properties of concentrated solutions of nitrates, especially AN, have been studied by molecular dynamics. We effectively account for the polarisation effects by the electronic continuum correction (ECC), practically realised via rescaling of the ionic charges. We found that, the full-charge force field MD simulations overestimate the experimental results, as the ions experience repulsion from the interface and prefer to remain in the subsurface layer and the bulk solution. In contrast, reducing the ionic charges results in the behaviour that fits well with the experimental data. The nitrate anions display a greater propensity for the interface than the ammonium cations. We accurately predict both the density and the rise in the surface tension of concentrated solutions of AN, recommending TIP4P/2005 for water and the scaled-charge OPLS model (OPLS/ECC) for the ions in the solutions. We observe that, the adsorption of anions to the interface accompanies their depletion in the subsurface layer, which is preferentially occupied by cations, resulting in the formation of the electric double layer. We demonstrate the ion deficiency for up to 3 Å below the surface and establish the requirement to include the polarisability effects in the OPLS model for AN. While these results confirmed the findings of the previous studies for dilute solutions, they are new in the solubility limit. Concentrated solutions exhibit a strong effect of the abundance of solute on the coordination numbers of ions and on the degree of ion pairing. Surprisingly, ion pairing decreases significantly at the interface compared with the bulk. The present study identifies OPLS/ECC, along with TIP4P/2005, to yield accurate predictions of physical properties of concentrated AN, with precision required for industrial applications, such as a formulation of emulsion and fuel-oil explosives that now predominate the civilian use of AN. An application of this model will allow one to predict the surface properties of supersaturated solutions of AN which fall outside the capability of the present laboratory experiments but are important industrially.

A molecular dynamics study on magnetic imidazolium-based ionic liquids: the effect of an external magnetic field

In this paper, we have reported a molecular dynamics (MD) study on the properties of three different magnetic imidazolium-based ionic liquids in the absence and presence of an external magnetic field. In this regard, the volumetric properties such as density and isobaric expansion coefficient, dynamical properties, namely, viscosity, mean square displacement of ions, diffusion coefficients, transport numbers of cations and anions, and electrical conductivity, and structural properties such as radial distribution function (RDF) and spatial distribution function (SDF) of [emim][FeCl4], [bmim][FeCl4] and [hmim][FeCl4] have been studied at different temperatures using molecular dynamics simulations. After studying the different volumetric, structural, and dynamical properties of the above-mentioned magnetic ILs in the absence of a magnetic field, we investigated the effect of an external magnetic field on the structural properties of one of these systems, i.e., [bmim][FeCl4]. In this regard, we established different contributions in the interactions between the external magnetic field and the studied magnetic ionic liquid (MIL). The number density profiles of the studied MIL before and after imposing an external magnetic field of 1.5 T showed a significant variation in the molecular distribution. The results indicated that the external magnetic field reduced the intensity of RDFs due to the reduction in the interactions between different ion sites as a result of changes in their orientations. After applying the external magnetic field, it was observed that due to the oppositely directed forces on the cations and anions, they moved in opposite directions. The snapshots showed that the static motion of the anion was smaller because of its small size. In the presence of an external magnetic field, the ions distributed more homogeneously compared to that observed in the absence of this field. The results of this study can be used in the rational and accurate design of viscomagnetic fluids and reaction systems in the presence and absence of magnetic fields.

Gas-phase electronic properties of tri-cationic imidazolium-based ionic liquids in comparison with mono- and di-cationic ionic liquids

The optimized geometries, electronic structures, and gas-phase properties of widely applicable non-linear trigeminal tri-cationic ILs (TT-X₃) were investigated using density functional theory (DFT) calculations and compared with mono- (M-X) and di-cationic (D-X₂) ionic liquids. The studied ILs are based on the imidazolium cation containing halide (X^‾) anions, where X^‾ = Cl^‾, Br^‾ and I^‾. Inter-molecular hydrogen bonds were studied by atoms in molecules (AIM) and natural bond orbital (NBO) analyses. Accordingly the most significant cation-anion charge transfer is related to C₁-H₁ … X (X = Cl, Br, I) interaction which strongly occurs in TT-X₃ ILs and especially in TT-Cl₃. Among ILs under investigation, TT-Cl₃ has the strongest cation-anion interaction. Also M - I IL has the largest and D-Cl₂ has the smallest electrical dipole moment.

Computational study of halogen-free Boron based dicationic ionic liquids of [bis-Mim][BMB]2 and [bis-Mim][BScB]2

In this paper, the structures and energetics of 1,3-bis[3-methylimidazolium-yl] pentane ([bis-Mim]²⁺) dication and bis(mandelato) borate [BMB]^- and bis(salicylato) borate [BScB]^-anions in isolated forms, ion pairs (IPs) and dicationic ionic liquids (DILs) were studied by Density Functional Theory (DFT) at the M06-2X/6-31G(d,p) level of theory. According to the IUPAC criteria, the hydrogen bonds between anion and cation were characterized and classified in the optimized geometries of the isolated ions, IPs and DILs. Inspection of the optimized structures revealed that the interionic hydrogen binding has important effect on the ions structures. The interaction energies between a dication and anions have been described in terms of NBO charge distribution, the stabilization energy E⁽²⁾ values, changes of vibrational frequencies and the reduced density gradient (RDG) analysis. Also, the reactivity and interactions between chemical species were interpreted in terms of global electronic properties. Electrostatic potential surfaces (ESP) have been applied for visualizing the charge related properties and characterization of the most energetic sites of isolated ions and ion complexes.