Room Temperature Ionic Liquids And Water Interaction Theory Research

Ionic liquids (ILs), which are liquid at ambient temperature, are completely composed of organic cations and inorganic anions. They have many attractive properties, such as low melting point, negligible vapor pressure, nonflammability, high thermal stability and wide electrochemical window. ILs as important green solvents and electrolytes have shown important application. Many ILs can easily soak up water. The presence of water could dramatically affect their physical and chemical properties, such as viscosity, density, polarity and conductivity. Therefore, it is of great significance to investigate the interaction between the water molecules and ILs.The interaction of F", Cl-, Br-, BF4-, PF6-,NO3-, CF3CO2-, CF3SO3-anions with water molecules was studied at B3LYP/6-31+G(d,p) level. It was found that the interaction between water molecules and F-is strongest, and the weakest interaction is PF6-, which illustrates that the anion of PF6-is hydrophobic. The interaction sequence between anions and water molecules is F-> CF3CO2-> Cl-> Br-> NO3-> CF3SO3-> BF4-> PF6-. Water molecules prefer to connect with oxygen of CF3CO2-, CF3SO3-anions and surround oxygen atom with increasing the number of water molecules. The results of these calculations are in good agreement with the experimental results.The geometries, infrared spectra and interaction energies of the seven different possible configurations of complexes formed with1-ethyl-3-methylimidazolium ([Emim]+) and H2O were studied at B3LYP/6-31+G(d,p) level. It was shown that water molecules mainly interact with H of imidazole ring and the strongest interaction is between water molecules and H of N=C(H)=N. The complexes formed by water molecules with1-alkyl-3-methylimidazolium [Cnmim]+(n=1-5) cations were also analyzed. It was found that the interaction between water molecules and [Cnmim]+cations is weakened with the longer alkyl group of cation.The B3LYP/6-31+G(d,p) method was used to study the geometries, interaction energies and infrared spectra of [Emim]Cl,[Cnmim]NO3(n=1-7),[Emim]BF4and [Emim]PF6ILs as well as their interacting with the different number of water molecules (n=1-5). It was found that the influence of water molecules on interaction between anion and cation for [Emim]Cl become larger with increasing the number of water molecules. It was shown that the water molecule interact with NO3-and the interaction is enhanced with the longer alkyl group of cation for [Cnmim]NO3-(H2O) complexes. The water molecules mainly interact with anion when the number of water molecules is less than2. The water molecules are observed to get together and begin to interact with cation for [Emim]BF4when the number of water molecules is more than3. The water molecule can interact with both anion and cation for [Emim]PF6, but the interaction is rather weak.The interaction of water molecules with BF4-, PF6-anions,[Emim]+cations and [Emim]BF4,[Emim]PF6ILs were studied at B3LYP/6-31+G(d,p) level. The interaction sequence among water molecules and BF4’,[Emim]+,[Emim]BF4is H2O-BF4-> H20-[Emim]+> H2O-[Emim]BF4. The interaction sequence of water molecules with PF6-,[Emim]+and [Emim]PF6is H2O-[Emim]+> H2O-[Emim]PF6> H2O-PF6-. The investigations not only reveal the inner relation for microstructure interaction between water molecules and ILs, but also help to understand the nature of ILs with other solvents.