Molecular insight into flow resistance of choline chloride/urea confined in ionic model nanoslits

Choline chloride/urea (1:2) is the most widely used deep eutectic solvent, which has attracted much attention due to its excellent advantages of low cost, environment friendly and easy synthesis. In this work, nanofriction-based molecular dynamics simulations were performed to investigate the effect of interfacial hydrophilicity on the flow resistance of Choline chloride/urea (1:2) confined in ionic model nanoslits. Simulation results showed that the flow resistance of the choline chloride/urea system increases with the increasing interfacial hydrophilicity. Urea molecules form a preferential adsorption layer on the wall. As the interfacial hydrophilicity increases, the number of urea molecules in the interfacial adsorption layer increased, whereas the stability decreased. Unique confined spatial distributions of urea molecules greatly contribute to ionic association between choline cations and chloride anions. Furthermore, with the increase of interfacial hydrophilicity, orientation distributions of urea molecules in the adsorption layer are more orderly, then causing a decrease in the average hydrogen bond number (N_HB) of urea molecules. Moreover, the more the N_HB of urea molecules, the better is the stability in the interfacial adsorption layer, which in turn results in less flow resistance.