Comparing two tetraalkylammonium ionic liquids. I. Liquid phase structure

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Chemistry

Background:

• Ionic liquids (ILs) are salts that are liquid at room temperature, with applications in various fields.
• Tetraalkylammonium-based ILs exhibit complex liquid structures influenced by cation and anion interactions.
• Understanding the nanoscale structure of ILs is crucial for predicting and optimizing their properties.

Purpose of the Study:

• To investigate the structural differences between n-butyl-trimethylammonium bis(trifluoromethanesulfonyl)imide ([N1114][NTf2]) and methyl-tributylammonium bis(trifluoromethanesulfonyl)imide ([N1444][NTf2]) ionic liquids.
• To elucidate the impact of cation asymmetry on the liquid structure of ILs.
• To resolve existing controversies regarding the liquid structure of tetraalkylammonium-based ILs.

Main Methods:

• X-ray scattering experiments were conducted at room temperature.
• Molecular dynamics (MD) simulations were performed using established force fields with novel partial charges.
• Analysis of static structure factors (S(k)) and partial structure factors was carried out.

Main Results:

• Diffraction data showed shifts in charge ordering and short-range correlation peaks between [N1114][NTf2] and [N1444][NTf2].
• MD simulations confirmed opposite shifts in charge and adjacency peaks, consistent with experimental observations.
• Simulations revealed that anions can approach both cations closely due to their positioning between cation alkyl chains.
• Partial structure factors highlighted differences in polar and non-polar components for the asymmetric [N1114][NTf2] cation, unlike the symmetric [N1444][NTf2].

Conclusions:

• The study provides detailed insights into the liquid structure of [N1114][NTf2] and [N1444][NTf2] ionic liquids.
• Cation molecular asymmetry significantly influences the partial structure factors and overall liquid organization.
• The findings contribute to a better understanding of structure-property relationships in tetraalkylammonium-based ionic liquids.