Effect of Alkyl-group Flexibility on the Melting Point of Imidazolium-based Ionic Liquids

TL;DR

This study uses molecular dynamics simulations to investigate how the flexibility of alkyl groups in imidazolium-based ionic liquids influences their melting points, revealing that certain dihedral angles significantly affect melting behavior.

Contribution

It explicitly quantifies the impact of dihedral angle flexibility on melting points, highlighting the role of molecular conformations in ionic liquid phase transitions.

Findings

Flexibility of specific dihedral angles can alter melting points by up to 20 K.

Rotational freedom affects anion ordering and melting enthalpy.

Alternating effects linked to conformation defect probabilities.

Abstract

The low melting point of room temperature ionic liquids is usually explained in terms of the presence of bulky, low symmetry and flexible ions, with the first two factors related to the lattice energy while an entropic effect is attributed to the latter. By means of molecular dynamics simulations, the melting points of 1-ethyl-3-methylimidazolium hexafluorophosphate and 1-decyl-3-methyl-imidazolium hexafluorophosphate were determined and the effect of the molecular flexibility over the melting point was explicitly computed by restraining the rotation of dihedral angles in both the solid and the liquid phase. The rotational flexibility over the bond between the ring and the alkyl chain affects the relative ordering of the anions around the cations and results in substantial effects over both the enthalpy and the entropy of melting. For the other dihedral angles of the alkyl group, the contributions are predominantly entropic and an alternating behavior was found. The flexibility of some dihedral angles has negligible effects on the melting point, while others can lead to differences in the melting point as large as 20 K. This alternating behavior is rationalized by the different probabilities of conformation defects in the crystal.