Theoretical Study On The Microstructure Of [C₂mim][Tf₂N]/Water Binary Mixtures And Their Application In Extracting Cd²⁺ And Capturing CO₂

Ionic Liquids（ILs）,as a new type of green materials,are widely used in the fields of organic synthesis,electrochemistry,and material preparation.It is found that even hydrophobic ILs inevitably absorb water molecules from the atmosphere.In the presence of water,physical and chemical properties of the ILs such as density,viscosity,and polarity will change significantly.It is important to study the interaction between ILs and water.In this paper,we discuss the microstructure of1-ethyl-3-methylimidazole bis（trifluoromethylsulfonyl）imide（[C₂mim][Tf₂N]）/water mixed system,and further extends to the application of the interaction between[C₂mim][Tf₂N]-H₂O-[Cd Cl₄]^2-and[C₂mim][Tf₂N]-H₂O-CO₂.Molecular dynamics（MD）simulation method is used to study the[C₂mim][Tf₂N]/water mixed system microstructure and the state of water molecules under different water mole fractions by analyzing the density,radial distribution functions（RDFs）,the average number of hydrogen bonds and the distribution of water clusters.As the water content increases,the density value of the mixed system gradually decreases and the average number of hydrogen bonds between water molecules gradually increases.RDFs show that at low water content,water molecules are mainly located in the[C₂mim][Tf₂N]interstice and interact with the oxygen atoms in the anions,which formed a homogeneous mixed system.Gradually adding water will slightly destroy the[C₂mim][Tf₂N]interstice.When the water mole fraction is0.25,the water molecules reach saturation in[C₂mim][Tf₂N].It can be clearly observed in the anion-cation RDFs that the water molecules slightly weaken the anion-cation interaction.As the water concentration increase,the interaction between water molecules is significantly enhanced,and there exist a liquid-liquid biphase system.Statistical analysis of the water clusters shows that water molecules gradually transition from small clusters to large clusters.The density functional theory is used to study the structure and interaction energy of[C₂mim][Tf₂N]/H₂O/[Cd Cl₄]^2-at the levels of B3LYP-D3/6-311++G（d,p）and B3LYP-D3/lanl2DZ.The results show that the interaction energy between[C₂mim][Tf₂N]-H₂O is greater than[Cd Cl₄]^2--H₂O,hence[C₂mim][Tf₂N]is easier to extract Cd²⁺from water.The MD simulation method is used to study the mechanism of[C₂mim][Tf₂N]extracting Cd²⁺in the[C₂mim][Tf₂N]/water liquid-liquid biphase system.The results show that when the water mole fraction x_w=0.92,[C₂mim]⁺and Cd²⁺exchange reaction occurs,[C₂mim]⁺enters the water area from the ILs phase,and Cd²⁺enters the ILs-H₂O interface from the water phase.Atmospheric greenhouse gases such as CO₂ are considered to be the main reason of global warming.ILs have become a research hot spot due to their high efficiency in capturing CO₂ and no pollution to the environment.The MD simulation method is used to calculate the RDFs,mean square displacement（MSD）and diffusion coefficient（D_i）of the[C₂mim][Tf₂N]/water/CO₂ system.The results show that CO₂mainly interacts with[C₂mim][Tf₂N]anions.The interaction between the system components is the strongest at x_w=0.1,and the diffusion coefficient of CO₂ is the smallest,indicating that at this water content is beneficial to the CO₂ adsorption by ILs.