Cohesive Properties of Ionic Liquids Calculated from First Principles

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Ionic liquids (ILs) possess low volatility, a desirable property that complicates direct vapor pressure measurements.
• Accurate determination of vaporization and sublimation enthalpies is crucial for understanding IL behavior but experimentally challenging.
• First-principles calculations offer a potential route to compute these thermodynamic properties.

Purpose of the Study:

• To assess the performance of first-principles calculations for determining sublimation properties of ionic liquids.
• To adapt computational methodologies developed for molecular crystals to ionic liquid systems.
• To develop a reliable computational scheme for temperature-dependent sublimation and vaporization data of ILs.

Main Methods:

• Utilized periodic density functional theory (DFT) for unit-cell geometries and quasi-harmonic phonon calculations.
• Employed many-body expansion schemes for ab initio refinement of lattice energies in crystalline ILs.
• Modeled the vapor phase using rigid rotor-harmonic oscillator models with corrections from hindered rotors and molecular dynamics.

Main Results:

• The computational approach achieved chemical accuracy for simple molecular crystals but faced challenges with ILs due to their significantly larger cohesive energies.
• Ionic liquid crystals exhibit cohesive energies up to an order of magnitude greater than common molecular crystals.
• The study successfully combined computational and experimental frameworks, yielding a promising new method.

Conclusions:

• Directly applying methods for molecular crystals to ionic liquids is difficult due to stronger interionic forces.
• The developed hybrid computational-experimental approach shows promise for generating accurate, temperature-dependent sublimation and vaporization data for ILs.
• This work paves the way for more reliable thermodynamic characterization of ionic liquids.