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