Evolution of microscopic heterogeneity and dynamics in choline chloride-based deep eutectic solvents

Stephanie Spittle ,Mark Dadmun ,Mark E Tuckerman ,Carla Cecilia Fraenza ,Burcu Gurkan ,Jing Peng ,Tyler Cosby ,Charles Kolodziej ,Edward J Maginn ,Ramez A Elgammal ,Madhusudan Tyagi ,Steve Greenbaum ,Luke Heroux ,Henry Squire ,Clemens Burda ,Brian Doherty ,Derrick Poe ,Thomas A Zawodzinski ,Sahana Bhattacharyya ,Md Ashraful Haque ,Yong Zhang ,Joshua Sangoro

doi:10.1038/s41467-021-27842-z

Abstract

Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to exploit the large design space and tunability of DESs for specific applications. Here, we employ a range of computational and experimental techniques that span length-scales from molecular to macroscopic and timescales from picoseconds to seconds to study the evolution of structure and dynamics in model DESs, namely Glyceline and Ethaline, starting from the parent compounds. We show that systematic addition of choline chloride leads to microscopic heterogeneities that alter the primary structural relaxation in glycerol and ethylene glycol and result in new dynamic modes that are strongly correlated to the macroscopic properties of the DES formed.

Highlights

Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies
(i) how do the local structures and microscopic dynamics evolve with a variation of the composition of the mixtures approaching the eutectic concentration, and (ii) how do spatial and temporal heterogeneities influence the macroscopic properties? To answer these questions, we start with canonical hydrogen bond donor (HBD), namely glycerol and ethylene glycol (EG), and track the evolution of structure and dynamics as the hydrogen bond acceptor (HBA), choline chloride (ChCl), is added using a combination of experimental and computational techniques spanning length-scales from molecular to macroscopic as well as broad ranges of time-scales from picoseconds to seconds
We start with well understood HBDs, glycerol, and EG, and track the evolution of structure and dynamics as the HBA, ChCl, is added up to the eutectic composition (33 mol% ChCl in glycerol and EG, namely Glyceline and Ethaline, respectively) using an array of complementary experimental and computational techniques capable of spanning lengthscales from molecular to macroscopic and timescales from sub-picoseconds to seconds and beyond

Summary

Introduction

Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. We start with canonical HBDs, namely glycerol and ethylene glycol (EG), and track the evolution of structure and dynamics as the HBA, choline chloride (ChCl), is added using a combination of experimental and computational techniques spanning length-scales from molecular to macroscopic as well as broad ranges of time-scales from picoseconds to seconds. These results together provide detailed insights into the observed increase in glass transition temperature depression, dc ionic conductivity, fluidity, and the mean rates of orientational dynamics as ChCl concentration is increased These fundamental insights form a scientific basis for the understanding of the dominant correlations in structure-property relationships of DESs needed for rational design and tunable applications

Methods

Results

Conclusion