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

Abstract

Molecular dynamics simulations and dielectric relaxation (DR) measurements in the frequency window, 0.2 ≤ ν/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τ1 ∼ 500 ps, τ2 ∼ 100 ps, τ3 ∼ 30 ps, and τ4 ∼ 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, ⟨τDR⟩, exhibit a pronounced fractional viscosity dependence, ⟨τDR⟩ ∝ (η/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 (⟨τ l ⟩) 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.

Computer Simulations of Deep Eutectic Solvents: Challenges, Solutions, and Perspectives

“…MD simulations were extensively used by Biswas’ group to prove the existence of dynamic heterogeneity in DESs [ 251 , 252 , 253 , 254 , 255 , 256 ]. Das et al [ 251 ] applied MD simulations to provide support for the fractional viscosity dependence of rotation rates of fluorescence probes observed in experiments for DESs based on acetamide (C 2 H 5 ON) and lithium bromide (LiBr) with an acetamide mole fraction of 0.78.…”

“…Reorientation dynamics was also studied by Rajbangshi et al [ 256 ] using MD in ChCl + urea DES. They simulated systems at 0.33 mol fraction of urea at six different temperatures between 293 and 333 K. Their results suggested evidence for strong temporal heterogeneity in DESs based on, e.g., displacement distributions and dynamic susceptibilities.…”

“…The above works use a single FF in MD simulations, the most popular ones being OPLS-DES [ 116 ] (in Ref. [ 256 ]), CHARMM [ 259 , 260 , 261 ] (in Refs. [ 248 , 249 , 250 , 251 ]) and its modified versions [ 261 , 262 , 263 ] (in Refs.…”

“…[ 254 , 255 ]). The atomistic models in these works were mostly validated by measuring density [ 252 , 253 , 254 , 255 , 256 ] and good agreement was found. The situation is different for dynamic properties.…”

“…Despite partial agreement with experiments regarding viscosity [ 249 , 255 ] and ionic conductivity [ 249 ] at a single state point, the temperature transferability of dynamical properties suffers from a lack of quantitative agreement. For example, Rajbangshi et al [ 256 ] have shown that the ratio of diffusion coefficients measured with pulsed-field gradient nuclear magnetic resonance to those estimated in MD simulations lay in the range of 0.58 to 4.82 for choline and 0.63 to 4.76 for urea in the temperature range 293 to 333 K, despite using a specific OPLS-DES FF [ 116 ]. This result emphasizes the importance of FF development and inclusion of polarizability.…”