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Nonpolarizable Force Fields through the Self-Consistent Modeling Scheme with MD and DFT Methods: From Ionic Liquids
Yoshiki Ishii1,2, Nobuyuki Matubayasi2,3, Hitoshi Washizu1,2
1Graduate School of Information Science, University of Hyogo, 7-1-28 minatojima-Minamimachi, Chuo-ku, Kobe, Hyogo 650-0047, Japan.
The Journal of Physical Chemistry. B
|June 13, 2022
Summary
This study refines force fields for ionic liquids and crystals using density functional theory, improving molecular dynamics simulations. The new force field (GAFF-DFT) accurately predicts ionic conductivity and reveals restricted ion mobility in self-assembled structures.
Area of Science:
- Computational Chemistry
- Materials Science
- Soft Matter Physics
Background:
- Accurate molecular dynamics (MD) simulations rely on precise force fields, especially for electrostatic interactions in ionic soft materials.
- Nonpolarizable force fields for ionic liquids (ILs) and ionic liquid crystals (ILCs) require refinement due to significant charge transfer and polarization effects.
- Simulating ionic liquids and their self-assembled structures necessitates improved models for electrostatic interactions.
Purpose of the Study:
- To develop and validate an accurate nonpolarizable force field for ionic liquids and self-assembled ionic liquid crystals.
- To investigate the impact of hydrophobic moieties on charge localization and electrostatic interactions in amphiphilic ionic molecules.
- To analyze ion transport mechanisms and conductivity within the self-assembled nanostructures of ionic liquid crystals.
Main Methods:
- Utilized a self-consistent modeling scheme combining density functional theory (DFT) with periodic boundary conditions to refine atomic charges.
- Updated the Generalized Amber Force Field (GAFF) with DFT-derived charges, creating the GAFF-DFT force field for condensed-phase simulations.
- Performed molecular dynamics (MD) simulations to calculate ionic conductivity and analyze ion-conductive pathways in ILs and ILCs.
Main Results:
- The GAFF-DFT force field significantly improved the prediction accuracy of ionic conductivity compared to standard force fields.
- DFT-derived charges indicated that hydrophobic substitutions enhance charge localization in ionic groups, amplifying electrostatic interactions.
- MD simulations revealed strong cation-anion interactions in ILCs, maintaining nanosegregation but restricting ion mobility, leading to lower conductivity than expected based on ionic domain volume.
Conclusions:
- The GAFF-DFT force field provides a more accurate representation of electrostatic interactions in ionic liquids and their self-assembled systems.
- Hydrophobic moieties play a crucial role in modulating charge distribution and inter-ionic forces within amphiphilic ionic molecules.
- Despite ordered nanostructures, ion mobility in self-assembled ionic liquid crystals is significantly hindered by strong inter-ionic attractions, impacting overall conductivity.
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