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

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 <b>M0</b>) uses atom-centered multipole whereas the other two (models <b>M1</b> and <b>M2</b>) 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 indicates that models based on fMDCM are considerably more consistent than those using multipoles. Computed viscosities from models <b>M1</b> and <b>M2</b> are within 30% of measured values and their change with increasing water content is consistent with experiments. This is not the case for model <b>M0</b>.