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 ([C<sub>2</sub>OHmim]<sup>+</sup>) cation and the six
different conventional anions, including [Cl]<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> 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 [C<sub>2</sub>OHmim][Cl] followed by [C<sub>2</sub>OHmim][NO<sub>3</sub>], and [C<sub>2</sub>OHmim][Tf<sub>2</sub>N] 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 [C<sub>2</sub>OHmim]<sup>+</sup> 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]<sup>−</sup> and [NO<sub>3</sub>]<sup>−</sup> anions are mainly involved in cation–anion interactions.
The formation of the intramolecular hydrogen bonding in the [C<sub>2</sub>OHmim]<sup>+</sup> cation is another interesting result of
the present study