Solvation structure and dynamics of choline chloride-based deep eutectic solvents with isomeric butanediol hydrogen bond donors.

Choline chloride-based deep eutectic solvents (DESs) are being investigated as redox flow battery electrolytes, where solvent viscosity plays a central role in governing ionic mobility, mass transport, and pumping efficiency. In this combined experimental and computational study, a series of DESs formed from choline chloride and butanediol isomer hydrogen bond donors (HBDs) is examined to probe how HBD molecular geometry affects liquid structure and dynamics while holding the molecular formula and hydroxyl group count constant. Density, viscosity, ionic conductivity, and ET(30) polarity are measured experimentally, while molecular dynamics simulations are used to quantify coordination numbers, hydrogen-bond lifetimes, self-diffusivities, dynamic heterogeneity, and local packing. Although the dominant first-shell interaction motifs are preserved across all isomers, systematic differences in hydrogen-bond dynamics, diol-diol connectivity, and spatial heterogeneity give rise to distinct transport behaviors, including substantial variations in viscosity and conductivity, that cannot be attributed to changes in local coordination structure alone. These results establish hydroxyl group arrangement as a key structural parameter governing DES transport properties and provide a controlled molecular framework for the rational design of choline chloride-based electrolytes.