Origin of low melting point of ionic liquids: dominant role of entropy

Ionic liquids (ILs) are salts with an extremely low melting point. Substantial efforts have been made to address their low melting point from the enthalpic standpoint (i.e. interionic interactions). However, this question is still open. In this study, we report our findings that entropic (large fusion entropy), rather than enthalpic, contributions are primarily responsible for lowering the melting point in many cases, based on a large thermodynamic dataset. We have established a computational protocol using molecular dynamics simulations to decompose fusion entropy into kinetic (translational, rotational, and intramolecular vibrational) and structural (conformational and configurational) terms and successfully applied this approach for two representatives of ILs and NaCl. It is revealed that large structural contribution, particularly configurational entropy in the liquid state, plays a deterministic role in the large fusion entropy and consequently the low melting point of the ILs.

Automated summary

In this study, researchers found that the primary reason for the lower melting point in ionic liquids is the entropic (large fusion entropy) rather than enthalpic contributions. They established a computational protocol using molecular dynamics simulations to analyze the kinetic and structural aspects of fusion entropy and successfully applied it to different systems. The study revealed that the large structural contribution, particularly the configurational entropy in the liquid state, plays a deterministic role in the low melting point of ionic liquids.