Alkyl chain length and temperature effects on structural properties of pyrrolidinium-based ionic liquids: A combined atomistic simulation and small-angle x-ray scattering study

Molecular dynamics (MD) simulations of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([C _nMPy][Tf ₂N], n = 3, 4, 6, 8, 10) were conducted using an all-atom model. Radial distribution functions (RDF) were computed and structure functions were generated to compare with new X-ray scattering experimental results, reported herein. The scattering peaks in the structure functions generally shift to lower Q values with increased temperature for all the liquids in this series. However, the first sharp diffraction peak (FSDP) in the longer alkyl chain liquids displays a marked shift to higher Q values with increasing temperature. Alkyl chain-dependent ordering of the polar groups and increased tail aggregation with increasing alkyl chain length were observed in the partial pair correlation functions and the structure functions. The reasons for the observed alkyl chain-dependent phenomena and temperature effects were explored.