Molecular Dynamics Simulations Study On Behavior Of Nanobubbles In Ionic Liquids

With the development of industry,the development and consumption of resources by human beings are also increasing,leading to the emergence of many environmental problems.More and more researchers have put forward their desire for green chemistry.Due to the serious pollution of traditional organic solvents in industry,the development of green solvents is also imminent.As an excellent new green solvent in the current era,ionic liquids have been paid more and more attention by researchers in the past decade.Nowadays,ionic liquids have been extended to many fields,especially in the fields of gas absorption and electrochemistry.In these fields,ionic liquid systems are always accompanied by gas.When the gas concentration in the solution reaches a certain value,bubbles will be generated.Bubbles grow from scratch,and nanobubbles are unavoidable.The nature of nanobubbles has broken the conventional understanding of bubbles.The study of nanobubbles in ionic liquids has become a key issue in this field.In this paper,the behavior of nanobubbles in ionic liquids is studied by molecular dynamics simulation.The main contents include the following parts.Firstly,aiming at the problem of low computational efficiency of all-atom model ionic liquids,a new coarse-grained force field is constructed.Based on the coarse-grained model of Martini force field,the bond length,bond angle and dihedral angle of the coarse-grained model are calculated based on the all-atom simulation data.Gaussian equation fitting is used to obtain more accurate parameters,and a variety of new imidazolium ionic liquid coarse-grained force fields such as C₂mim BF₄,C₄mim BF₄,C₆mim BF₄,C₈mim BF₄ and C₁₂mim BF₄ are established.The density,radial distribution function and mean square displacement of the coarse-grained model are compared with the all-atom model,the coarse-grained model in the literature or the experimental values to verify the accuracy of the new coarse-grained model.Subsequently,the structural characteristics of single nanobubbles in ionic liquids were studied.The single-phase nanobubbles in ionic liquids were simulated.LJ particles were used instead of gas molecules for simulation,and the parameter range of LJ particles forming bubbles in different ionic liquids was determined.The internal pressure of nanobubbles was obtained by the pressure change of the system.The pressure change was in line with the expectation of the Young-Laplace equation.The number density distribution was measured with the bubble center of mass as the center.It was found that the vertical arrangement of ionic liquid cations at the interface.At the interface,the cationic carbon chain points to the inside of the bubble,and the cation orientation angle was further measured.It was found that the overall trend was consistent but the orientation angle was relatively loose,not strictly arranged,with a certain degree of freedom.The structural characteristics of nanobubbles in ionic liquids were revealed by multi-angle studies.Finally,the simulation of the double bubble system was carried out.It was found that the double bubbles remained relatively stable in the long carbon chain imidazolium ionic liquid,while the double bubbles in the short carbon chain imidazolium ionic liquid quickly merged.In response to this phenomenon,by measuring the surface tension of the ionic liquid system,it is found that the change trend is different.When the thickness of the ionic liquid decreases,the surface tension will first decrease and then increase.Through the analysis of the number density distribution and the structure orientation distribution,it was found that the ionic liquid system had a layered phenomenon,and a multi-layer structure appeared in the long-chain ionic liquid.Through the analysis of the order degree of the system,it was found that the surface tension peak and valley were in a highly ordered state,which was a spontaneous process of the system.By analyzing the charge distribution of the system,it is found that there is an obvious dipole effect at the interface.With the decrease of the thickness of the system,the repulsion effect is enhanced,and various effects work together to cause a unique change trend of surface tension.