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.