Heterogeneous dynamics, correlated time and length scales in ionic deep eutectics: Anion and temperature dependence

Abstract

Heterogeneous relaxation dynamics often characterizes deep eutectic solvents. Extensive and molecular dynamics simulations have been carried out in the temperature range, 303 ≤ T/K ≤ 370, for studying the anion and temperature dependencies of heterogeneous dynamics of three different ionic acetamide deep eutectics: acetamide + LiX, X being bromide (Br−), nitrate (NO3−), and perchlorate (ClO4−). These systems are chosen because the fractional viscosity dependence of average relaxation rates reported by various measurements has been attributed to the heterogeneous dynamics of these systems. Simulations performed here attempt to characterize the heterogeneous relaxation dynamics in terms of correlated time and length scales and understand the solution inhomogeneity in microscopic terms. Additionally, simulation studies for pure molten acetamide have been performed to understand the impact of ions on motional features of acetamide in these ionic deep eutectic systems. The computed radial distribution functions suggest microheterogeneous solution structure and dependence upon anion identity and temperature. A significant plateau in the simulated time dependent mean squared displacements indicates pronounced cage-rattling and inhomogeneity in relaxation dynamics. Simulated diffusion coefficients for acetamide and ions show decoupling from the simulated viscosities of these deep eutectics. Calculated two- and four-point correlation functions reveal the presence of dynamic heterogeneity even at ∼180 K above the measured thermodynamic glass transition temperature (Tg). Further analyses reveal the existence of multiple timescales that respond strongly to the rise in solution temperature. The simulated dynamic structure factor and overlap function relaxations show strong stretched exponential relaxations. The simulation results support the experimental observation that the bromide system is the most dynamically heterogeneous among these three systems. Correlated length scales show much weaker anion and temperature dependencies with an estimated length of ∼1 nm, suggesting formation of clusters at the local level as the origin for the micro-heterogeneous nature of these ionic deep eutectics.

Temperature-Dependent Dielectric Relaxation in Ionic Acetamide Deep Eutectics: Partial Viscosity Decoupling and Explanations from the Simulated Single-Particle Reorientation Dynamics and Hydrogen-Bond Fluctuations

“…Our analyses of RDFs have also depicted that the anions can interact simultaneously with acetamide and Li + leading to the complexities in these mixtures. 95,97 The monodentate anion Br − interacts in a completely different way than the tridentate planar NO 3 − . ClO 4 − being tetrahedral has one more interacting site, but its bigger size introduces some excluded-volume effect.…”

“…Also, note that the structural heterogeneity of these mixtures has already been examined previously. 95,97 We investigated the structural heterogeneity part by a simulation of radial distribution functions and cluster size distributions among various interspecies. Although we did not calculate any order parameter 98 to quantify the structural heterogeneity, our analyses provided a molecular-level detailed description of the structural heterogeneity aspect.…”

“…This is understandable because the present measurements have employed a frequency window (0.2 ≤ ν (GHz) ≤ 50) that does not allow a detection of dynamics possessing sub-picosecond and multi-nanosecond relaxation time scales. It should also be recognized that the slow multi-nanosecond time scales in decays could arise from the use of the optimized potentials for liquid simulations–united atom (OPLS-UA) model potential itself because the translational diffusion coefficient predicted for neat molten acetamide by using the OPLS-UA force-field parameters has been found to be ∼5 times smaller than that predicted by the CHARMM force-field parameters. − In addition, temperature-dependent translational diffusion coefficients of acetamide in perchlorate and nitrate DESs reported by quasi-elastic neutron-scattering measurements are ∼4–30 times greater than those predicted for acetamide in these ionic DESs by the OPLS-UA potential parameters , (see Table S6, Supporting Information). This comparison indicates that the multi-nanosecond component predicted for decays by the OPLS-UA potential parameters is probably slower than what it should have been (particularly at lower temperatures), and one should exercise sufficient care in considering them for interpreting the experimental relaxation dynamics.…”

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

“…Biswas et al [ 243 , 244 ] performed MD microstructure simulations for a number of Li salts (Li + , Br − , NO 3 − , ClO 4 − ) and acetamide to identify the solution-phase microstructures in these media, and investigated the anion and temperature dependence of these microstructures. The authors showed that the presence of heterogeneity arises from the balance of the interactions between the various species.…”

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

“…Biswas et al [224,225] performed MD microstructure simulations for a number of Li salts (Li + , Br − , NO −3 , ClO −4 ) and acetamide to identify the solution-phase microstructures in these media, and investigated the anion and temperature dependence of these microstructures. The authors showed that the presence of heterogeneity arises from the balance of the interactions between the various species.…”