Hydrogen Bond Behavior Of Ethylammonium Nitrate Ionic Liquid And Its Mixtures With Water Explored By Molecular Dynamics Simulations

Recently,ionic liquids(ILss)have attracted extensive attention due to their unique properties such as near-zero vapor pressure,high thermal conductivity and thermal stabILsity,nonflammab ILsity,high conductivity and so on.More importantly,the physical and chemical properties of ILs can be easily tailored by changing the combinations of cations and anions.In addition,their chemical and physical properties can be drastically adjusted by the addition of a small amount of water,which further expands their range of applications.Whether pure ILs or their mixtures with water,their unique physical and chemical properties essentially depends on the interactions among anions,cations,and water molecules,mainly including the electrostatic and van der Waals interactions,as well as hydrogen bonds(HBs).Compared to other two kinds of interactions,furthermore,experimental observations of the HBs often fraught with enormous difficulties owing to its localized and directional characters.As a powerful complementary analysis tool,molecular dynamics(MD)simulation can offer a direct and deep insight into structures,dynamics,and HBs of pure ILs and ILs/water mixtures at a molecular level,which is very helpful for scientists to deeply understand the relevant experimental results.In this paper,therefore,we mainly employs MD simulations to investigate structures,dynamics,and HBs of pure ethylammonium nitrate(EAN)protic ILs and its mixtures with water.First,a series of MD simulations have been performed to systematically investigate structures,dynamics,HBs of pure EAN protic ILs in the temperature range between 300 K and 400 K.Our simulation results show that the temperature has a little influence on the structures of EAN ILs in the range from 300 K to 400 K,suggesting the electrostatic and the van der Waals interactions between ions are almost independent of the temperature.Unlike the structures of EAN ILs,the temperature has a considerable influence on the relevant dynamics properties.Our simulation results shows that the strength of HBs between anions and cations decreases significantly as the temperature increases from 300 K to 400 K,so that the corresponding diffusion coefficient and rotational relaxation time of both anions and cations increase by more than one order of magnitude.Furthermore,an obvious sub-diffusive behavior is observed for both anions and cations since these ions in bulk EAN ILs are trapped by the cage formed by their neighbors with opposite charges.Additionally,the lifetime and the structural relaxation time for the ion-pair dissociation between cations and anions are found to decreases significantly by approximately one order of magnitude due to weakened HBs between anions and cations from 300 K to 400 K.Then,we extend our research system from pure EAN ILs to more complex EAN ILs/water mixtures.Sim ILsarly,a series of MD simulations have been carried out to investigate structures,dynamics,HBs of various EAN ILs/water mixtures with 19 different water concentrations(the water mass fraction from 2.04 wt% to 83.33 wt%).The simulation results demonstrate that 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.Meanwh ILse,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 concentrations(<10 wt%),wh ILse the corresponding order is cation-anion > cation-water > anion-water > water-water at high water concentrations(>10 wt%).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 mob ILsity and ionic concentration leads to that the ionic conductivity of EAN/water mixtures initially increases with the water mass fraction and follows a sharp decrease beyond 70 wt%.Therefore,our simulation results have provided a microscopic understanding of temperature-dependent structures and dynamics of pure EAN ILs,revealing that the HBs between anions and cations play an essential role in determining the dynamics properties of protic ILss compared to the electrostatic and van der Waals interactions.Furthermore,our simulation results have revealed the concentration-dependent HB behavior of protic ILs/water mixtures at a molecular-level,as well as their relationship with unique structures and dynamics.Our simulation results in this work can be of great theoretical benefit for the experimental scientists to study the unique physical and chemical properties of ILss and ILs/water mixtures.