Theoretical Study of Renewable Ionic Liquids in the Pure State and with Graphene and Carbon Nanotubes

The Influence of Ionic Liquids Adsorption on the Electronic and Optical Properties of Phosphorene and Arsenene with Different Phases: A Computational Study

Density functional theory (DFT) calculations have been performed to investigate the interfacial interactions of ionic liquids (ILs) on the α- and β-phases of phosphorene (P) and arsenene (As). Nine representative ILs based on the combinations of 1-ethyl-3-methylimidazolium ([EMIM]⁺), N-methylpyridinium ([MPI]⁺), and trimethylamine ([TMA]⁺) cations paired to tetrafluoroborate ([BF₄]⁻), trifluoromethanesulfonate ([TFO]⁻), and chloridion (Cl⁻) anions were used as adsorbates on the 2D P and As nanosheets with different phases to explore the effect of IL adsorption on the electronic and optical properties of 2D materials. The calculated structure, adsorption energy, and charge transfer suggest that the interaction between ILs and P and As nanosheets is dominated by noncovalent forces, and the most stable adsorption structures are characterized by the simultaneous interaction of the cation and anion with the surface, irrespective of the types of ILs and surfaces. Furthermore, the IL adsorption leads to the larger change in the electronic properties of β-phase P and As than those of their α-phase counterparts, which demonstrates that the adsorption properties are not only related to the chemical elements, but also closely related to the phase structures. The present results provide insight into the further applications of ILs and phosphorene (arsenene) hybrid materials.

Imidazolium-fulleride ionic liquids - a DFT prediction

Ionic liquids (ILs) exhibit tunable physicochemical properties due to the flexibility of being able to select their cation-anion combination from a large pool of ions. The size of the ions controls the properties of the ILs in the range from ionic to molecular, and thus large ions play an important role in regulating the melting temperature and viscosity. Here, we show that the exohedral addition of anionic X^- moieties to C₆₀ (X = H, F, OH, CN, NH₂, and NO₂) is a thermodynamically viable process for creating large X-fulleride anions (C₆₀X)^-. The addition of X^- to C₆₀ is modelled by locating the transition state for the reaction between C₆₀ and 1,3-dimethyl-2X-imidazole (IMX) at the M06L/6-311++G(d,p)//M06L/6-31G(d,p) level. The reaction yields the ion-pair complex IM⁺⋯(C₆₀X)^- for X = H, F, OH, CN, NH₂, and NO₂ and the ordered pair of (activation free energy, reaction free energy) is found to be (14.5, 1.1), (6.1, 3.1), (16.7, 2.3), (14.7, -7.9), (27.9, 0.5) and (11.9, 12.4), respectively. The low barrier of the reactions suggests their feasibility. The reaction is slightly endergonic for X = H, F, OH, and NH₂, while X = CN shows a significant exergonic character. The X-fulleride formation is not observed when X = Cl and Br. The ion-pair interactions (E_ion-pair) observed for IM⁺⋯(C₆₀X)^- range from -64.0 to -73.0 kcal mol^-1, which is substantially lower (∼10%) than the typically reported values for imidazolium-based ionic liquids such as [EMIm]⁺[trz]^-, [EMIm]⁺[dc]^-, [EMIm]⁺[dtrz]^-, and [EMIm]⁺[NH₂tz]^-. The quantum theory of atoms in molecules (QTAIM) analysis showed that the C-X bonding in (C₆₀X)^- is covalent, while that in (IM⁺⋯X^-)⋯C₆₀ (for X = Cl and Br) is non-covalent. Furthermore, molecular electrostatic potential (MESP) analysis showed that the X-fulleride could behave as a large spherical anion due to the delocalization of the excess electron in the system over the entire carbon framework. The large anionic character of the X-fulleride is also revealed by the identification of several close lying local energy minima for the IM⁺⋯(C₆₀X)^- ion-pair. The low E_ion-pair value, the significant contribution of dispersion to the E_ion-pair and the spherical nature of the anion predict low-melting point and highly viscous IL formation from X-fullerides and the imidazolium cation.

Molecular interactions and thermal transport in ionic liquids with carbon nanomaterials

We used molecular dynamics simulation to study the effect of suspended carbon nanomaterials, nanotubes and graphene sheets, on the thermal conductivity of ionic liquids, an issue related to understanding the properties of nanofluids. One important aspect that we developed is an atomistic model of the interactions between the organic ions and carbon nanomaterials, so we did not rely on existing force fields for small organic molecules or assume simple combining rules to describe the interactions at the liquid/material interface. Instead, we used quantum calculations with a density functional suitable for non-covalent interactions to parameterize an interaction model, including van der Waals terms and also atomic partial charges on the materials. We fitted a n-m interaction potential function with n values of 9 or 10 and m values between 5 and 8, so a 12-6 Lennard-Jones function would not fit the quantum calculations. For the atoms of ionic liquids and carbon nanomaterials interacting among themselves, we adopted existing models from the literature. We studied the imidazolium ionic liquids [C₄C₁im][SCN], [C₄C₁im][N(CN)₂], [C₄C₁im][C(CN)₃] and [C₄C₁im][(CF₃SO₂)₂N]. Attraction is stronger for cations (than for anions) above and below the π-system of the nanomaterials, whereas anions show stronger attraction for the hydrogenated edges. The ordering of ions around and inside (7,7) and (10,10) single-walled nanotubes, and near a stack of graphene sheets, was analysed in terms of density distribution functions. We verified that anions are found, as well as cations, in the first interfacial layer interacting with the materials, which is surprising given the interaction potential surfaces. The thermal conductivity of the ionic liquids and of composite systems containing one nanotube or one graphene stack in suspension was calculated using non-equilibrium molecular dynamics. Thermal conductivity was calculated along the axis of the nanotube and across the planes of graphene, in order to see the anisotropy. In the composite systems containing the nanotube, there is an enhancement of the overall thermal conductivity, with calculated values comparing well with experiments on nanotube suspensions, namely in terms of the order of the different ionic liquids. In the systems containing the graphene stack, the interfacial region of the ionic liquid near the surface of the material has an enhanced thermal conductivity with respect to the bulk liquid, but no significant discontinuity in the temperature profiles were observed. This is important information for models of thermal conduction in nanofluids.

Exfoliation of graphene and fluorographene in molecular and ionic liquids

We use molecular dynamics simulations to study the exfoliation of graphene and fluorographene in molecular and ionic liquids using computer experiments in which one layer of the 2D nanomaterial is peeled, in vacuum or with solvent present.