Investigation of structural ordering in network forming ionic liquids: A molecular dynamics study

Area of Science:

• Materials Science
• Computational Chemistry
• Physical Chemistry

Background:

• Network-forming ionic liquids (NIL) are crucial in various applications.
• Understanding their structural ordering with temperature is key to optimizing their performance.
• Previous studies have explored ordering in ionic liquids, but temperature-dependent anomalous ordering in NILs requires further investigation.

Purpose of the Study:

• To investigate the temperature-dependent structural ordering in network-forming ionic liquids (NIL) with varying alkyl-diammonium cation side chain lengths.
• To elucidate the molecular mechanisms driving short- and medium-range ordering in these systems.
• To identify the role of cation-anion interactions and molecular configurations in the observed phenomena.

Main Methods:

• Utilizing molecular dynamics (MD) simulations to model NIL 5-6 (6 carbon side chains) and NIL 5-3 (3 carbon side chains) systems.
• Analyzing short- and medium-range ordering through two-particle correlation functions.
• Examining molecular shape distributions, including radii of gyration, at different temperatures.

Main Results:

• Anomalous short- and medium-range ordering was observed with increasing temperature in NIL 5-6, while NIL 5-3 showed weaker ordering.
• Short-range ordering is linked to thermal volume expansion, whereas medium-range ordering requires both volume expansion and elevated temperatures.
• Cation-anion interactions were identified as the primary drivers of the observed ordering and complex 3D correlations.
• Narrowing of the molecular radii of gyration distribution at higher temperatures facilitates increased ordering.

Conclusions:

• The study reveals a unique temperature-induced ordering phenomenon in network-forming ionic liquids with specific cation structures.
• Cation-anion interactions and temperature-dependent changes in molecular flexibility (manifested in radii of gyration) are critical factors governing this ordering.
• Findings provide insights into the structure-property relationships of NILs, guiding the design of materials with tailored thermal responses.