Concentration-Dependent Hydrogen Bond Behavior of Ethylammonium Nitrate Protic Ionic Liquid-Water Mixtures Explored by Molecular Dynamics Simulations

The detailed hydrogen bond (HB) behavior of ethylammonium nitrate (EAN) ionic liquid (IL)-water mixtures with different water concentrations has been investigated at a molecular level by using classical molecular dynamics simulations. The simulation results demonstrate that the increasing water concentration can weaken considerably all cation-anion, cation-water, anion-water, and water-water HBs in EAN-water mixtures, and the corresponding HB networks around cations, anions, and water molecules also change significantly with the addition of water. Meanwhile, both the translational and the rotational motions of anions, cations, and water molecules are found to be much faster as the water concentration increases. On the other hand, the order of their HB strength is found to be cation-anion > anion-water > cation-water > water-water at low water mole fractions (<38%), while the corresponding order is cation-anion > cation-water > anion-water > water-water at high water mole fractions (>38%). The opposite orders of anion-water and cation-water HBs at low and high water concentrations, as well as the different changes of HB networks around cations and anions, should be responsible for the increasing deviation in diffusion coefficient between cations and anions with the water concentration, which is favorable to the cation-anion dissociation. In addition, the competing effect between ionic mobility and ionic concentration leads to that the ionic conductivity of EAN-water mixtures initially increases with the water mole fraction and follows a sharp decrease beyond 90%. Our simulation results provide a molecular-level concentration dependent HB networks and dynamics, as well as their relationship with unique structures and dynamics in protic IL-water mixtures.