Insight into the Inflection Point of Surface Tension in Aqueous Solution of Imidazolium-Based Ionic Liquids

An in-depth understanding of the interfacial structure is important to broadening the range of applications for ionic liquids (ILs). Interfacial properties of ILs depend on their intrinsic microstructure and interactions. The detailed structures at the interface can be obtained by molecular dynamics simulations at atomic levels. However, there are no clear conclusions about the liquid film thickness selection when modeling the liquid film systems. In this work, three aqueous solution systems with different liquid film thicknesses were investigated, and the relationship between the surface tension and water content was calculated and analyzed. The results indicate that a thicker liquid film leads to better agreement between the two relationships and the experimental data. Then, the thickest liquid film system was selected from the three different liquid film thicknesses. The bulk phase and interface of the IL aqueous system were treated separately. It is found that the IL content of the bulk phase was correlated with the surface tension. The results were more compatible with the experimental values. In addition, the work analyzed the atomic number density distribution and the angular distribution of alkyl chains in eight systems with varying concentrations. The concept of surface coverage of aqueous ILs was proposed and calculated with different concentrations. This helped to explain the reason for the inflection point in surface tension at low concentrations.