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

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Deep eutectic solvents (DES) often display heterogeneous relaxation dynamics.
• Previous measurements suggest fractional viscosity dependence is linked to DES dynamics.
• Understanding anion and temperature effects on DES dynamics is crucial.

Purpose of the Study:

• To characterize heterogeneous relaxation dynamics in acetamide-based ionic DES.
• To investigate anion and temperature dependencies of dynamics using molecular dynamics.
• To elucidate the microscopic origins of solution inhomogeneity in DES.

Main Methods:

• Extensive molecular dynamics simulations were performed for acetamide + LiX (X = Br-, NO3-, ClO4-) DES.
• Simulations covered a temperature range of 303 K to 370 K.
• Analysis included radial distribution functions, mean squared displacements, diffusion coefficients, and correlation functions.

Main Results:

• Simulations revealed micro-heterogeneous structures dependent on anion identity and temperature.
• Pronounced cage-rattling and dynamic heterogeneity were observed, persisting above the glass transition temperature (Tg).
• The bromide system exhibited the highest degree of dynamic heterogeneity, consistent with experimental findings.

Conclusions:

• Ionic acetamide DES exhibit significant dynamic heterogeneity and micro-heterogeneity.
• Local clustering, with correlated length scales of approximately 1 nm, likely causes the observed inhomogeneity.
• Simulation results provide microscopic insights supporting experimental observations on DES dynamics.