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

Abstract

The effects of incorporating the ester functional group (COO) into the side chain of the 1-alkyl-3methylimidazolium cation ([C 1 COOC n C 1 im] + , n = 1, 2, 4) paired with [Br] − , [NO 3 ] − , [BF 4 ] − , [PF 6 ] − , [TfO] − , and [Tf 2 N] − 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, [Tf 2 N] − , and the minimum of the corresponding properties correspond to the smallest cation, especially when combined with the smaller anions, including [NO 3 ] − 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 [PF 6 ] − and [BF 4 ] − . 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.

Densities and Derived Volumetric Properties of Ionic Liquids with [Nf2] and [NTf2] Anions at High Pressures

“…The alkyl chains can obstruct the access of anion to cation, especially when they are on the same atom, which could weaken ionic bonds leading to poorer packaging of [PMpyr]­[Nf 2 ] and [BMpyr]­[Nf 2 ] and, therefore, lower densities in comparison to [E 3 Mim]­[Nf 2 ]. In support of that, [PMpyr]­[Nf 2 ] is denser than [BMpyr]­[Nf 2 ] confirming that the density decreases as the length of the alkyl chain rises, due to dispersive interactions, as well as higher steric hindrance. ,,,− Further, the influence of the different anion on the properties of ionic liquids can be observed for [BMpyr]­[Nf 2 ] and [BMpyr]­[NTf 2 ]. It is evident that the ionic liquid containing the [NTf 2 ] − cation is denser than that one with the same cation but [Nf 2 ] − anion (Figure ) due to the greater stability of the [NTf 2 ] − anion which provides stronger ionic bonding. ,,, …”

“…The observation of κ T of the ionic liquids with the same anion and similar cations, [PMpyr]­[Nf 2 ] and [BMpyr]­[Nf 2 ], results in a conclusion that ionic liquids with longer alkyl chains, [BMpyr]­[Nf 2 ], are more compressible due to a bigger cation and more unoccupied space in a molecule. ,,, Considering the length of alkyl chain it could be expected that [E 3 Mim]­[Nf 2 ] has lower compressibility than [PMpyr]­[Nf 2 ] and [BMpyr]­[Nf 2 ], but actually its compressibility is higher than that of [PMpyr]­[Nf 2 ] and very similar to that of [BMpyr]­[Nf 2 ]. The reason for this could be explained by the cation base of [E 3 Mim]­[Nf 2 ].…”

In silico prediction and experimental verification of ionic liquid refractive indices

“…With ILs being seen as designer solvents, the task of screening such a large collection is indeed formidable. Computational approaches based on density functional theory [10][11][12] and molecular dynamics [13,14] have generally been used to provide a mechanistic understanding of the working of ILs but are restricted by the time complexity of the modelling. Faster alternatives have relied on quantitative structure property relationship (QSPR) models wherein physicochemical descriptors derived from the molecular structure are correlated with the property of interest using chemometric and machine learning tools.…”

Toxicity of Selected Imidazolium-based Ionic Liquids on Caenorhabditis elegans: a Quantitative Structure-Activity Relationship Study

“…For the anion of BF 4 − , the lower Dipole may result from the hydrolyzation, which causes less number of LC 50 than cation. And the hydrolysis products may also increase the toxicity [40]. For the anion of dibutyl-phosphate, the lower interaction with cation is owing to the steric effect of the dibutyl chain (FIG.…”

“…The dipole-dipole attraction as one of the nondispersive forces among cation and anion might be responsible for the surface tension of liquids [40]. ILs with low surface tension will easily penetrate through the cell membrane, which might result in the high toxicity to C. elegans.…”