OPLS Force Field for Choline Chloride-Based Deep Eutectic Solvents

Deep
eutectic solvents (DES) are a class of solvents frequently
composed of choline chloride and a neutral hydrogen bond donor (HBD)
at ratios of 1:1, 1:2, or 1:3, respectively. As cost-effective and
eco-friendly solvents, DESs have gained considerable popularity in
multiple fields, including materials, separations, and nanotechnology.
In the present work, a comprehensive set of transferable parameters
have been fine-tuned to accurately reproduce bulk-phase physical properties
and local intermolecular interactions for 8 different choline chloride-based
DESs. This nonpolarizable force field, OPLS-DES, gave near quantitative
agreement at multiple temperatures for experimental densities, viscosities,
heat capacities, and surface tensions yielding overall mean absolute
errors (MAEs) of ca. 1.1%, 1.6%, 5.5%, and 1.5%, respectively. Local
interactions and solvent structuring between the ions and HBDs, including
urea, glycerol, phenol, ethylene glycol, levulinic acid, oxalic acid,
and malonic acid, were accurately reproduced when compared to radial
distribution functions and coordination numbers derived from experimental
liquid-phase neutron diffraction data and from first-principles molecular
dynamics simulations. The reproduction of transport properties presented
a considerable challenge and behaved more like a supercooled liquid
near room temperature; higher-temperature simulations, e.g., 400–500
K, or an alternative polarizable force field is recommended when computing
self-diffusion coefficients.