Molecular dynamics investigation of biomimetic ionic liquids

Ionic liquids having long alkyl chains containing 18-carbon atoms have recently been synthesized and found to have low melting points close to −50 °°C. The inspiration for this work arose from the concept of homeoviscous adaption (HVA), whereby the fluidity of phospholipid bilayers is increased by the addition of unsaturated lipids in the bilayer. In the present work, results of molecular dynamics simulations are reported for one of the low melting point ionic liquids examined in this earlier study: 1-oleyl-3-methylimidazolium (9,10-cis-octadecenyl) bis(trifluoromethanesulfonyl)imide ([C18(1[9])mim] [Tf2N]). This ionic liquid contains a single unsaturated bond in the cis configuration. For comparison, calculations are also performed on the saturated all-trans analogue: 1-stearyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C18mim] [Tf2N]). Since the melting point behavior of these ILs is analgous to HVA of phospholipid bilayers, these ILs are referred to as “biomimetic” ILs. Simulations are performed at temperatures 1.2 times the corresponding experimental melting points of the ionic liquids as well as at a common temperature to interrogate the structure and dynamics of the two liquids. Additional molecular dynamics simulations are carried out at high pressure. Results of thermophysical properties such as densities and isothermal compressibilities are reported. The local structure of the ionic liquids is examined via various orientational and radial distribution functions. Consistent with other simulation and experimental reports for ionic liquids having long alkyl groups on the cations, these ionic liquids exhibit nanoscale heterogeneity where the polar and hydrophobic regions are segregated. Subtle differences between the orientation and packing of the saturated and unsaturated alkyl chains are observed. The mean square displacement and cation rotational dynamics are also computed for both ILs. It is observed that at low pressure, the translational dynamics of the [C18(1[9])mim] cation is greater than that of the [C18mim] cation, consistent with experimental viscosity measurements. The simulations reveal that the motion of the ions is severely retarded at low temperatures and high pressures.