Molecular Features of Reline and Homologous Deep Eutectic Solvents Contributing to Nonideal Mixing Behavior

Deep eutectic solvents based on choline chloride and a series of urea derivatives are studied by molecular dynamics simulations with the aim to identify molecular features contributing to nonideal mixing behavior of these compounds. In case of reline, a mixture of choline chloride and urea in 1:2 ratio, urea molecules provide sufficient hydrogen bond donor sites to take up the chloride anions into their polar network. Replacing any of the hydrogen atoms of urea by a methyl group strongly pushes the anion to interact with these alkyl chains, resulting in a positive deviation of the activity coefficients of choline chloride compared to reline. Furthermore, the oxygen atom of urea can interact with the nitrogen atom of the cation. This enables the chloride anion to move off-center of the cation toward the hydrogen atom of its hydroxyl group, possessing stronger directional Coulomb interactions than the nitrogen atom of the cation. The substitution of urea's hydrogen atoms in cis position to the carbonyl group as in 1,3-dimethylurea, pushes the newly introduced nonpolar alkyl chains toward the nitrogen atom of the cation. This effect can be responsible for the experimentally observed increase of the activity coefficient of the urea derivative compared to urea. Additionally, indications for formation of nonpolar domains within the liquid and, thus, nanoscale segregation is visible as soon as one hydrogen atom of urea is replaced by an alkyl group.