Molecular Dynamics Study of the Effect of Water on Hydrophilic and Hydrophobic Ionic Liquids

We performed molecular dynamics (MD) simulations of ionic liquid (IL)-water mixtures to investigate the effects of water addition. The IL cation 1-butyl-3-methylimidazolium ([C₄mim]) and the four anions, nitrate (NO₃), tetrafluoroborate (BF₄), hexafluorophosphate (PF₆), and bis(trifluoromethanesulfonyl)imide (NTf₂), were used to examine the effects of differences in hydrophobicity and anion size. The radial distribution functions of water-water have two different water content dependences. In NO₃ and BF₄ systems, the effect of water-anion-water structures connected by hydrogen bonds due to the strong interaction of anion-water is large. The growth of water clusters changes the peak heights of the radial distribution functions in PF₆ and NTf₂ systems. The increase in the diffusion constant is small in the NO₃ system but is not small in the other systems. The relaxation time of the anion-water hydrogen bonds in the NO₃ system is much longer than those in the other systems. It is the reason for the low water content dependency of the diffusion constants of the NO₃ system. The water constant dependences of structures in NO₃ and BF₄ systems are similar, but that in the diffusion constant is not. The BF₄ system shows hydrophilic features in the structural change and hydrophobic features in the water content dependences of the diffusion constant. The addition of water molecules provides various hydrophobicity/hydrophilicity of anions.