Polarizability versus mobility: atomistic force field for ionic liquids

Area of Science:

• Computational Chemistry
• Materials Science
• Physical Chemistry

Background:

• Room temperature ionic liquids (RTILs) are salts with low melting points, widely studied for their unique properties.
• Classical molecular dynamics simulations are crucial for understanding RTIL behavior, but accuracy depends heavily on force fields (FFs).
• Non-polarizable FFs often neglect electronic polarization, leading to inaccuracies in simulating condensed-phase ionic liquids.

Purpose of the Study:

• To investigate the impact of Coulombic interactions on RTIL properties using classical molecular dynamics.
• To propose a method for improving the accuracy of non-polarizable FFs for RTILs by adjusting electrostatic charges.
• To enable more realistic simulations of ionic transport and thermodynamic properties in RTILs.

Main Methods:

• Classical molecular dynamics simulations were performed on RTILs containing 1,3-dimethylimidazolium (MMIM(+)), N-butylpyridinium (BPY(+)), and bis(trifluoromethane sulfonyl)imide (TFSI(-)) ions.
• A uniform scaling of electrostatic charges on all interaction sites was applied to existing FFs.
• Density and heat of vaporization were calculated to determine the appropriate charge scaling factor, with ionic transport properties also analyzed.

Main Results:

• Excessive Coulombic energy, due to the neglect of electronic polarization, was found to hinder ionic transport in the original models.
• The original Lopes and Padua models systematically overestimated RTIL density and heat of vaporization.
• A linear relationship between density and charge scaling allowed for the derivation of a correct scaling factor.

Conclusions:

• Uniformly scaling electrostatic charges in non-polarizable FFs significantly improves the simulation of RTIL properties.
• The proposed charge scaling method corrects overestimations of density and heat of vaporization, leading to more accurate ionic dynamics.
• This technique offers a feasible approach to enhance the quality of existing non-polarizable FFs for simulating various RTILs, including [BPY][TFSI] and [MMIM][TFSI].