Force Fields for Deep Eutectic Mixtures: Application to Structure and 2D-Infrared Spectroscopy

TL;DR

This study develops and compares electrostatic models for simulating eutectic mixtures, demonstrating that fluctuating minimal distributed charges provide more accurate structural and spectroscopic predictions than atom-centered multipoles.

Contribution

The paper introduces and evaluates new electrostatic models for ionic eutectic systems, showing improved agreement with experimental data over traditional multipole models.

Findings

fMDCM models outperform multipole models in reproducing experimental spectra

Viscosity predictions from fMDCM models are within 30% of experimental values

Adjusted Lennard-Jones parameters match hydration properties within a few percent

Abstract

Parametrizing energy functions for ionic systems can be challenging. Here, the total energy function for an eutectic system consisting of water, SCN$^-$, K$^+$ and acetamide is improved vis-a-vis experimentally measured properties. Given the importance of electrostatic interactions, two different types of models are considered: the first (model M0) uses atom-centered multipole whereas the other two (models M1 and M2) are based on fluctuating minimal distributed charges (fMDCM) that respond to geometrical changes of SCN$^-$. The Lennard-Jones parameters of the anion are adjusted to best reproduce experimentally known hydration free energies and densities which are matched to within a few percent for the final models irrespective of the electrostatic model. Molecular dynamics simulations of the eutectic mixtures with varying water content (between 0% and 100%) yield radial distribution functions and frequency correlation functions for the CN-stretch vibration. Comparison with experiments indicate that models based on fMDCM are considerably more consistent that those using multipoles. Computed viscosities from models M1 and M2 are within 30% of measured values and their change with increasing water content is consistent with experiments. This is not the case for model M0.