Microheterogeneity of Calcium Chloride-Based Deep Eutectic Solvents

Metal salt-based deep eutectic solvents (DESs) are finding numerous applications in various technological processes, such as metal electrodeposition and nanoparticle synthesis, to name but a few. In particular, calcium chloride hexahydrate is used as a building block for inexpensive, biodegradable DESs containing a controlled amount of hydration water. Although such systems are being actively researched for their practical applicability and some of their bulk thermophysical properties have been measured, their microstructure and molecular-level dynamics have been poorly characterized. In this work, two DESs based on CaCl₂·6H₂O─a type II DES made with choline chloride (DES1) as a hydrogen bond donor and a type IV DES containing ethylene glycol (DES2)─are studied using molecular dynamics simulation with a polarizable force field. The ethylene glycol molecular model was developed for this study, and the entire force field was validated on test systems with well-known properties. The structures of both DESs are found to be strongly microheterogeneous with interweaving organic and aqueous phases interacting via calcium chloride domains acting as linkers. While DES2 contains continuous microphases of water and ethylene glycol, in DES1, choline forms distinct separable domains in a connected aqueous environment. The latter system also shows an increased tendency to form multicenter <i>x</i> Ca²⁺·<i>y</i> Cl^- complexes compared with DES2. Both systems are characterized by a rich and diverse hydrogen bond network involving all DES components. Ultimately, molecular dynamics simulations with explicitly polarizable models are shown to be a promising method to elucidate the microstructure of hydrated DESs based on metal salts.