Molecular Dynamics Simulation Of The Relationship Between Hydrogen Bond And Viscosity For Two Types Of Ionic Liquids

The development of industry has greatly promoted the prosperity of human society.At the same time,it has also caused serious environmental pollution.Organic solvent is one of the most important components in industrial pollutants.With the concept of"green chemistry"proposed,searching for low-pollution and high-efficiency solvents has become one of the hot research topics.Ionic liquids?ILs?are regarded as possible replacement for organic solvents in various applications.However,the viscosity of many ionic liquids is higher than the common organic solvents by 2-3 orders,which greatly hinders their many practical applications including mass and heat transfer process and/or make it difficult for fluids pumping and stirring.By suitable combination of different ions,any ionic liquids with desired properties would be designed.However,it is impossible to measure everyone since the number of synthesized ILs is very large.If the relationship between structure and the viscosity of ionic liquid can be uncovered,then the relative order of viscosities can be judged qualitatively,and it could provide valuable clues for the screening and synthesis of the ionic liquid with desired properties.In this thesis,we devoted to studying the relationship between hydrogen bond and viscosity of pyridinium and quaternary ammonium ILs by combination of Molecular Dynamics?MD?and quantum chemistry.The order of viscosity for a serious of ionic liquids are evaluated.We hope that it would provide some valuable advice for the rational design of ILs with proposed viscosity.The following three parts are included in the thesis:1.The relationship between hydrogen bond and viscosity of four pyridinium ILs,1-butyl-2-methylpyridiniumtetrafluoroborate?[b2mpy][BF₄]?,1-butyl-3-methylpyridinium tetrafluoroborate?[b3mpy][BF₄]?,1-butyl-4-methylpyridinium tetrafluoroborate?[b4mpy][BF₄]?,N-butylpyridinium tetrafluoroborate?[bpy][BF₄]?,is studied to explore the effect of substituted group and position on the viscosity.First,the 512 pairs of ionic liquid were simulated by molecular dynamics?MD?30 ns.Radial distribution functions?RDFs?and spatial distribution functions?SDFs?were obtained.The position and relative strength of hydrogen bonds in ionic liquid were determined by the RDFs,in which the hydrogen bonds would be mostly like to be formed between F atom in[BF₄]^-anion and H atom near to the N atom in pyridine ring.According to the results of molecular dynamics simulations,the lifetime was calculated.The decreased order was[b2mpy][BF₄]>[b4mpy][BF₄]>[b3mpy][BF₄]>[bpy][BF₄].This sequence is exactly the same with the sequence of experimental viscosity.To further explore the effect of hydrogen bond,both single ion pair and two ion clusters were optimized by Becke's three parameter exact exchange-functional combined with Perdew and Wang?B3PW91?method with 6-31G?d,p?basis set.According to the results of AIM,the[b2mpy][BF₄]has not only much number of hydrogen bonds but also the stronger strength.As a result,the viscosity of[b2mpy][BF₄]is higher than other three pyridinium ILs.While[b3mpy][BF₄]and[b4mpy][BF₄]have the similar features for hydrogen bonds,which is consistent with their closer viscosity.Finally,the relationship between hydrogen bond and viscosity is built to predict the viscosity directly.The methyl group substituted in ortho position would greatly increase the viscosity since more hydrogen bonds would be formed with stronger strength.While the pyridinium ionic liquids with the substitution in meta and para position has the similar viscosity,which is consistent with the similar features of hydrogen bonds in them.2.The influence of hydrogen bonds on the viscosity for five pyridinium ILs,[EPy][OTf],[EPy][NTf₂],[BPy][OTf],[BPy][NTf₂],and[BPy][BF₄],with different alkyl chain lengths,cations,and anions are theoretically studied by combination of molecular dynamics and quantum chemistry.Under the NPT ensemble,they were simulated for 30 ns.To keep the ion mobility,an annealing method was used in the simulation.According to the results of the simulation,we can obtain radial distribution functions.The possibility to form hydrogen bond between the N,O and F atoms in the anion and the H atoms in the cation was analyzed by the radial distribution functions.The results show that their common feature is that the anion hardly forms hydrogen bonds with the H atoms at the end of the alkyl chain.Both single ion pair and two ion clusters were optimized by the B3PW91 method with 6-31G?d,p?basis set.The hydrogen bonds in the system were analyzed by the AIM and NCI methods.Both the lifetime of ion pair and the strength of hydrogen bond present that the viscosity of IL with[BF₄]^-anion is larger than that with[OTf]^-anion,which is further larger than that with[NTf₂]^-anion,which is attributed that the F atom in[BF₄]^-anion can form strong hydrogen bonds with the H atom in the cation.3.Based on the above studies,the relationship between the hydrogen bond and viscosity for three quaternary ammonium ionic liquids,TBA pyruvate,TBA L-lactate,and TBA L-malate was studied by the same method.Their cations are all quaternary ammonium cations,the anions are different but all contain oxygen atoms.By the RDFs,the primary information for their hydrogen bonds are obtained.The structures of single ion pair and two ion clusters were optimized at the B3PW91/6-31G?d,p?level.Using NCI and AIM methods,their intramolecular and intermolecular hydrogen bond strengths were analyzed.It turned out that the viscosity of the ionic liquid is determined by the stronger hydrogen bonds in the molecular,and is not directly related to the number of hydrogen bonds.In addition,their ion lifetimes and self-diffusion coefficients qualitatively determined their viscosity order,which is in agreement with experimental values.In general,there is a positive relationship between the viscosity and hydrogen bond strength.Therefore,it is possible to determine the order of viscosity for ionic liquids with similar structures by the strength of hydrogen bonds.Although this method can not obtain the accurate viscosity of ionic liquid,It is possible to adjust the viscosity of ionic liquid by variation of the hydrogen bond strength.