Structural and dynamical properties of ionic liquids: a molecular dynamics study employing DL_POLY 4

ABSTRACT

Molecular dynamics simulations are often used to study the structures and dynamics of ionic liquids. Here, we have simulated three ionic liquids, trihexyl(tetradecyl)-phosphonium chloride [P₆₆₆₁₄][Cl], 1-butyl-3-methylimidazolium acecate [BMIm][Oac] and 1-ethyl-3-methylimidazolium dicyanamide, [EMIm][DCA] in a comparison of two force fields, GAFF and CL&PFF. In most cases, the resulting theoretical densities agree with experimental values within a 2% error. Diffusive properties were characterised by mean squared displacements to show the significant effect of the alkyl chain on the movement of the [P₆₆₆₁₄] cation. Activation energies of diffusion were calculated from linear Arrhenius plots which agree with previous studies. Simulations of the dynamical behaviour show retention of short and medium-range structure of the ionic liquids with temperature. However, although with increasing temperature more high energy local configurations become accessible, they are observed less frequently as energy barriers are more easily overcome, resulting in more ordered time-averaged structures.

KEYWORDS:

• Ionic liquids
• molecular dynamics simulations
• phosphonium-based cations

Acknowledgements

Finally, the authors also acknowledge the use of the UCL@Legion and UCL@Grace High Performance Computing Facility, and associated support services, in the completion of this work.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was carried out as part of the ‘4CU’ programme grant, aimed at the sustainable conversion of carbon dioxide into fuels, led by the University of Sheffield and carried out in collaboration with the University of Manchester, Queen's University Belfast, University College London and Cardiff University. The authors therefore gratefully acknowledge the Engineering and Physical Sciences Research Council (EPSRC) for supporting this work financially [grant number EP/K001329/1]. Via our membership of the UK’s HPC Materials Chemistry Consortium, which is funded by EPSRC [EP/L000202], this work made use of the facilities of ARCHER, the UK’s national high-performance computing service, which is funded by the Office of Science and Technology through EPSRC’s High End Computing Programme.