Physical insights into the molecular mechanisms governing structural and dynamic properties of choline chloride-based deep eutectic solvents

The present work studies the influence of the hydrogen bond donor (HBD) rigidity on the dynamic properties of Deep Eutectic Solvents (DES). We performed a series of all-atom molecular dynamics (MD) simulations of mixtures formed by choline chloride (ChlCl) with ethylene-glycol (ETG) or urea (UR) as HBD. The main aim is seeking for physical insights into the main molecular mechanisms that affect the transport properties of DES. We find a slower diffusion of cations occur when UR is the HBD, due to its high rigidity difficulties the overall DES molecular mobility. In other words, a rigid HBD decrease the local mobility. Hence, a flexible HBD enhances the molecular diffusion, promoting the stability of the liquid phase. Besides, we find a partial polarisation of the corresponding HBD due to the electrostatic interaction with the ions. Such behaviour is strongly reminiscent of the charge-screening phenomenon observed in classic electrolytes and could be relevant to the intrinsic stability of DES in the liquid phase at room temperature.