Molecular dynamics study of the effect of alkyl chain length on melting points of CnMIMPF6 ionic liquids

Y Zhang and EJ Maginn, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 16, 13489-13499 (2014).

DOI: 10.1039/c4cp01048e

Based on molecular dynamics simulations, the melting points T-m of a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids CnMIMPF6 with n = 2, 4, 10, 12, and 14 were studied using the free energy-based pseudosupercritical path (PSCP) method. The experimental trend that the Tm decreases with increasing alkyl chain length for ILs with short alkyl chains and increases for the ones with long alkyl chains was correctly captured. Further analysis revealed that the different trends are the results of the balance between fusion enthalpy and fusion entropy. For the ILs with short alkyl chains (ethyl and butyl groups), fusion entropy plays the dominant role so that C4MIMPF6, which has a larger fusion entropy due to its higher liquid phase entropy has the lower melting temperature. As for the ILs with long alkyl chains, due to the enhanced van der Waals interactions brought about by the long non-polar alkyl chains, enthalpy becomes the deciding factor and the melting points increase when the alkyl chain goes from C10 to C14. While the melting points for C2MIMPF6 and C4MIMPF6 were quantitatively predicted and the trends for the long chain ILs were captured correctly, the absolute melting points for C10MIMPF6, C12MIMPF6 and C14MIMPF6 were systematically overestimated in the simulations. Three possible reasons for the overestimation were studied but all ruled out. Further simulation or experimental studies are needed to explain the difference.

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