Biodegradable Ionic Liquids: Effects of Temperature, Alkyl Side-Chain Length, and Anion on the Thermodynamic Properties and Interaction Energies As Determined by Molecular Dynamics Simulations Coupled with ab Initio Calculations

The effects of incorporating the ester functional group (COO) into the side chain of the 1-alkyl-3-methylimidazolium cation ([C₁COOC_<i>n</i>C₁im]⁺, <i>n</i> = 1, 2, 4) paired with [Br]⁻, [NO₃]⁻, [BF₄]⁻, [PF₆]⁻, [TfO]⁻, and [Tf₂N]⁻ anions on the various thermodynamic properties and interaction energies of these biodegradable ionic liquids (ILs) were investigated by means of molecular dynamics (MD) simulations combined with ab initio calculations in the temperature range of 298–550 K. Excluding the simulated density, the highest values of the volumetric properties such as molar volume, isobaric expansion coefficient, and isothermal compressibility coefficient can be attributed to the largest cation incorporated with the weakest coordinating anion, [Tf₂N]⁻, and the minimum of the corresponding properties correspond to the smallest cation, especially when combined with the smaller anions, including [NO₃]⁻ and [Br]⁻. In addition, ion-pair, cationic, and anionic volumes were computed using MD simulations as well as ab initio calculations. The results revelaed an increasing trend in the molar enthalpy of vaporization. The reverse trends of the volumetric properties were observed for the cohesive energy density, Hildebrand solubility parameter, surface tension, surface excess enthalpy, lattice energy, thermal pressure, internal pressure, binding energy, and interaction energy. On the basis of the optimized structures, we believed that a reduction in the strength of the hydrogen bonds due to the larger charge distribution and steric hindrance of bulkier ions is responsible for the observed trends. These results were also confirmed by calculating the critical and boiling temperatures (by two different empirical equation), surface excess enthalpies, parachors, and standard molar entropies. The other derivatives of the thermodynamic properties such as the isobaric and isochoric heat capacities, isothermal bulk moduli, and speeds of sound in the ILs were computed as functions of temperature. Interestingly, a direct relationship was found between the simulated results for the surface tension and the computed values of the bulk modulus. Furthermore, it was found that sound waves are transmitted faster in a compact IL than in a compressible IL. In addition, for each IL, the molar refraction, refractive index, dielectric constant, and mean static polarizability were approximated at room temperature. The smallest values of these properties were observed for ILs composed of the spherically symmetric anions [PF₆]⁻ and [BF₄]⁻. In addition, the formation of multiple intramolecular hydrogen bonds between the O atoms of the ester functional group and the hydrogen atoms of the cation was also observed for all optimized conformations. Finally, the obtained results demonstrate that the introduction of an ester group significantly increases the interionic interactions and, subsequently, the packing efficiency of these ILs in comparison with those of conventional imidazolium-based ILs

Hydroxyl-Functionalized 1‑(2-Hydroxyethyl)-3-methyl Imidazolium Ionic Liquids: Thermodynamic and Structural Properties using Molecular Dynamics Simulations and ab Initio Calculations

The influences of hydroxyl functional group (−OH) on the thermodynamic and structural properties of ionic liquids (ILs) composed of 1-(2-Hydroxyethyl)-3-methyl imidazolium ([C₂OHmim]⁺) cation and the six different conventional anions, including [Cl]⁻, [NO₃]⁻, [BF₄]⁻, [PF₆]⁻, [TfO]⁻, and [Tf₂N]⁻ have been extensively investigated using classical molecular dynamics (MD) simulations combined with ab initio calculations over a wide range of temperature (298–550 K). The volumetric thermodynamic properties, enthalpy of vaporization, cohesive energy density, Hildebrand solubility parameter, and heat capacity at constant pressure were estimated at desired temperature. The simulated densities were in good agreement with the experimental data with a slight overestimation. The interionic interaction of selected ILs was also computed using both the MD simulations and ab initio calculations. It was found that the highest association of cation and anion is attributed to [C₂OHmim][Cl] followed by [C₂OHmim][NO₃], and [C₂OHmim][Tf₂N] with the bulkiest anion has the weakest interionic interaction among chosen ILs. The similar trend of interactions energies was nearly observed from cohesive energy density results. Additional structural details were comprehensively yielded by calculating radial distribution functions (RDFs) and spatial distribution function (SDFs) at 358 K. The most stable configurations of isolated and dimer ion pairs of these ILs were in excellent consistency with RDFs and SDFs results. Significant changes in arrangement of anions around the [C₂OHmim]⁺ cation in comparison with conventional imidazolium-based ILs can be inferred from the MD simulations and ab initio results. Also, microscopic structural properties disclosed that the most strong cation–cation interaction is ascribed to the hydroxyl-functionalized ILs composed of bulkier anions, whereas ILs incorporating [Cl]⁻ and [NO₃]⁻ anions are mainly involved in cation–anion interactions. The formation of the intramolecular hydrogen bonding in the [C₂OHmim]⁺ cation is another interesting result of the present study

Molecular Dynamics and <i>ab Initio</i> Studies of the Effects of Substituent Groups on the Thermodynamic Properties and Structure of Four Selected Imidazolium-Based [Tf₂N^–] Ionic Liquids

All-atom molecular dynamics simulations combined with <i>ab initio</i> calculations are used to study the thermodynamic properties and microscopic structure of four ionic liquids (ILs) based on the imidazolium cation with different alkyl side branches, ([bmmim]⁺, 1-butyl-2,3-dimethylimidazolium; [bmim]⁺, 1-butyl-3-methylimidazolium; [apmim]⁺, 1-(3-aminopropyl)-3-methylimidazolium; [mim]⁺, 1-methylimidazolium), paired with the [(CF₃SO₂)₂N]⁻, bis­(trifluoromethanesulfonyl)­imide anion, in the temperature range of (298 to 600) K. We observed the highest value of the molar internal energy, enthalpy of vaporization, and cohesive energy density for the amine-functionalized [apmim]­[Tf₂N] ionic liquid. Structural analysis shows that the amine functionalization of the end of the alkyl side chain of imidazolium cation does not significantly affect the organization of [Tf₂N]⁻ around [apmim]⁺, but additional NH₂ groups lead to short-range cation–cation structural correlations between neighboring [apmim]⁺. The C2 methylation extensively affects preferential out-of-plane face-to-face locations of [Tf₂N]⁻ around [bmmim]⁺ and also the cation–cation distributions. [mim]­[Tf₂N] has the highest simulated density and better packing efficiency of liquid phase in comparison with other studied ILs. The strongest first shell probability density region of [mim]⁺ neighbors above and below the imidazolium ring of the reference cation represents better π–π stacking in the liquid phase of this ionic liquid. The presented results determine the role of the cation structure on the properties of this family of ILs. Good agreement was achieved between simulation results of the bulk phase and quantum calculations which are performed to determine the optimized structure of isolated ion pairs in chosen configurations and the strength of cation–anion interactions