Dicationic versus monocationic ionic liquids: distinctive ionic dynamics and dynamical heterogeneity

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Ionic liquids (ILs) are salts with low melting points, widely studied for their unique properties.
• Understanding the relationship between structure and dynamics is crucial for designing novel ILs.
• Dicationic ionic liquids (DILs) offer different properties compared to conventional monocationic ILs.

Purpose of the Study:

• To compare the dynamical properties of a dicationic ionic liquid, [C(6)(MIm)(2)][NTf(2)](2), with its monocationic counterpart, [C(3)MIm][NTf(2)].
• To investigate relaxation processes and collective dynamics using molecular dynamics simulations.
• To elucidate the influence of cation structure on IL dynamics and structural heterogeneity.

Main Methods:

• Molecular dynamics (MD) simulations were employed to study two ionic liquids: [C(6)(MIm)(2)][NTf(2)](2) and [C(3)MIm][NTf(2)].
• Analysis included relaxation of polarizability anisotropy, mean-squared displacement (MSD), non-Gaussian parameter, and intermediate scattering functions.
• Kerr spectra and X-ray structure factors were calculated to probe librational dynamics and structural correlations.

Main Results:

• Differences in Kerr spectra were attributed to distinct angular momentum and relaxation behaviors of the dicationic ([C(6)(MIm)(2)](2+)) and monocationic ([C(3)MIm](+)) species.
• The anion ([NTf(2)](-)) exhibited a common resonance-type peak in librational dynamics, indicating coupled intermolecular and vibrational motion.
• A low-k peak (0.20 Å(-1)) in X-ray structure factors was observed for the DIL, [C(6)(MIm)(2)][NTf(2)](2), but not for the monocationic IL, suggesting structural heterogeneity.

Conclusions:

• The dicationic ionic liquid [C(6)(MIm)(2)][NTf(2)](2) displays dynamical heterogeneity.
• This dynamical heterogeneity is strongly correlated with structural variations or heterogeneity within the dicationic ionic liquid.
• Anion-cation and anion-anion correlations significantly contribute to the observed structural features in the dicationic ionic liquid.