Scale-dependent transport in deep eutectic solvents: nanoscale dynamics of cholinium ions in reline

Transport in deep eutectic solvents (DESs) exhibits a pronounced dependence on length scale, with macroscopic hydrodynamic descriptions failing to capture the complex dynamics that emerge at the molecular level. This study seeks to elucidate the molecular origins of these transport anomalies by examining the nanoscale diffusion of cholinium ions within reline using quasielastic neutron scattering (QENS). Our findings reveal that nanoscale transport within DESs arises from a combination of two distinct dynamical processes: rapid localized motion and slower long-range translational diffusion. The localized dynamics occur within a deformable hydrogen-bonded network, exhibiting a high apparent activation energy (44.1 kJ mol^-1) that reflects the thermodynamic softening and expansion of the confinement cage rather than a true microscopic kinetic barrier. In contrast, long-range diffusion requires structural rearrangements to escape this transient confinement, overcoming a true energetic barrier of 17.6 kJ mol^-1. Comparison with pulse field gradient NMR reveals a pronounced decoupling between nanoscale and macroscopic transport, reflecting the fundamentally different spatiotemporal regimes probed by the two techniques. Collectively, these results demonstrate that transport in DESs arises from a hierarchy of dynamical processes controlled by structural heterogeneity within transient hydrogen-bonded networks.