Heterogeneous Orientational Relaxations and Translation-Rotation Decoupling in (Choline Chloride plus Urea) Deep Eutectic Solvents: Investigation through Molecular Dynamics Simulations and Dielectric Relaxation Measurements

Molecular dynamics simulations and dielectric relaxation (DR) measurements in the frequency window, 0.2 <= nu/GHz <= 50, have been performed to explore the heterogeneous reorientation dynamics in [f choline chloride + (1 - f) urea] deep eutectic solvents (DESs) at f = 0.33 and 0.40 in the temperature range 293 <= T/K <= 333. The solution viscosity varies by more than an order of magnitude. DR measurements in these DESs reveal multiple relaxation timescales-tau(1) similar to 500 ps, tau(2) similar to 100 ps, tau(3) similar to 30 ps, and tau(4) similar to 5 ps. Simulated rank-dependent collective single-particle reorientational (C-l(t), l being the rank) and structural H-bond [C-HB(t)] relaxations can explain the microscopic origin of all these DR timescales. The average DR times, , exhibit a pronounced fractional viscosity dependence, proportional to (eta/T)(p), with p = 0.1. This experimental evidence of pronounced heterogeneous reorientation dynamics in these DESs is supported by a strong translation-rotation decoupling and a significant deviation of the average reorientational correlation times () from Debye's l(l + 1) law. The simulated ratios between the average rotation and translation timescales for both urea and choline correctly reduce to the appropriate hydrodynamic limit at high temperatures. The stretched exponential relaxations of the simulated self-dynamic structure factors and the non-Gaussian single-particle displacement distributions further support strong temporal heterogeneity in these DESs. Dynamic susceptibilities from the simulated four-point correlations exhibit long correlated timescales. Moreover, simulated activation energies estimated from the temperature-dependent C-1(t) decays and the translational diffusion coefficients from the velocity autocorrelation functions agree favorably with those from the corresponding DR and the pulsed field gradient nuclear magnetic resonance measurements.

Automated summary

Molecular dynamics simulations and dielectric relaxation measurements were used to study the heterogeneous reorientation dynamics in [f choline chloride + (1 - f) urea] deep eutectic solvents. The results revealed pronounced reorientation dynamics with multiple relaxation timescales and viscosity dependence. This study provides insights into the microscopic origins of the observed dynamics and the deviation from traditional laws.