Insight into the properties and structures of vapor-liquid interface for imidazolium-based ionic liquids by molecular dynamics simulations

The ionic liquids (ILs) have been widely used in many areas. However, because of its hydrophilicity, it is very difficult to be purified and recycled, which is one of the great puzzles to the scientific community. In order to study water absorption and desorption of ILs, the free energy of a water molecule crossing IL plate was calculated in this work. As the vapor-liquid interface is necessary for the hydrophilicity of ILs, the vapor-liquid interface for 1-butyl-3-methylimidazolium tetrafluoroborates([Bmim][BF4])-water system was also studied by molecular dynamics simulations. It is very interesting to find that there is a good linear relationship between surface tension and water content in the interface layer, although it was reported that there is a nonlinear relationship between the surface tension and the water content of ILs aqueous solution. Our calculation is based on the composition at the interface, while the latter is based on the total water content. It was worth mentioning that water tended to be distributed in the inner layer of the interface, while IL was distributed in the outer layer of the interface, and the alkyl chain pointed to the vapor phase. The above findings could explain the nonlinear change of surface tension with total water content in the literature. The change rule of interface structure with water content was investigated by means of number density distribution and angle distribution. Besides, the excess chemical potential of the vapor-liquid phase was calculated, and the free energy of one water molecule crossing the IL slab has also been calculated.