Investigation On The Interaction Between Ionic Liquids [bmim][BF₄] And Water

Ionic liquids (ILs) have attracted increasing interest in many fields of chemistry because they are environmental friendly and designable. However, it is impossible to synthesis the huge amount of ILs and study their properties by experiments. Therefore, we developed a screening method to design ILs suitable to a specific task, in which the key is to establish the relationship between properties and molecular structure. However, the results by current method are not accurate enough compared with experiments. In this work, we investigated the mixtures of [bmim][BF4] and water by simulations as a case study. By using the force fields we recently developed, the excess volumes and self-diffusion coefficients agreed with the experimental values very well. The results can be further improved by introducing non-LB combining rules.First, we studied the interaction between the cation and anion of [bmim][BF4] by quantum mechanics approach. Two kinds of stable ion pairs were obtained by the optimization starting from various initial structures, which indicating the strong interactions between the anions and the hydrogens on imidazolium ring. We further investigated the interactions of anion-water, cation-water and IL-water. The results showed that the anion, cation and the ion pairs are able to form stronger hydrogen interaction with water.The macroscopic properties, such as thermodynamics properties, diffusion coefficient, viscosity, conductivity, are crucial in chemical engineering processes. However, the quantum mechanics calculations are heavily limited by computational power and only properties of small cluster of molecules can be obtained. Therefore, molecular dynamics simulations of [bmim][BF4] and water were performed to study the interplay of anions, cations and waters.Our recently developed force field (OPTUA), in which the polarizability was considered implicitly, was used in this work and compared with the former UA and AA force fields. We found that the OPTUA force field was improved evidently in prediction of the excess molar volume property. The results of UA force field were negative. In contrast, the positive values were reported by various experiments, which were well reproduced by OPTUA. In addition, non-LB combining rules were introduced in simulations and we found the accordance with experiments was much better. The calculated excess molar enthalpies agreed with the experiment well. To get some detailed microstructure information, we analyzed the RDF (radial distribution function) and SDF (space distribution function) of the mixtures of [bmim][BF4] and water. The diffusion coefficient we calculated on the pure ionic liquid [bmim][BF4] is lower by one order of magnitude. However, compared with the simulated results from other research group, the depiction of diffusion property is improved with the OPTUA force field.In summary, the quantum mechanics calculation, molecular dynamic simulation and experimental results are combined to investigate the interactions in pure ionic liquid and the mixtures of [bmim][BF4]/water. The results in this work, reflecting the property-structure relationship in the systems, are helpful in understanding the nature of ILs and the process designs in this field.