Role of conformational entropy in low melting point of ionic liquids: a molecular dynamics simulation study

Ionic liquids (ILs) are liquid salts which have very low melting points among the salts. To understand the contribution of conformational entropy (Sconf) to the low melting points of ILs, molecular dynamics (MD) simulations of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([Cnmim][NTf2] where n = 2, 4, 6, 8, and 10) were performed in liquid and gas states. The increase in Sconf with increasing alkyl chain length corresponded to an increase in fusion entropy (ΔfusS), suggesting that the other entropic contributions such as kinetic and configurational entropies depend negligibly on the alkyl chain length. Comparing the same cation, Sconf in the liquid state with a long alkyl chain was slightly smaller than that in the gas state because the trans conformers of the cation were favored in the liquid state. The trans conformers of the cations in the liquid state were stabilized by the van der Waals and Coulomb interactions. Meanwhile, populations of the trans conformer for the anion in the gas and liquid state were almost the same.