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Research Of Molecular Simulation For Water Cluster

Posted on:2018-04-04Degree:MasterType:Thesis
Country:ChinaCandidate:Q W JiangFull Text:PDF
GTID:2381330596468613Subject:Chemical Engineering and Technology
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Nowdays,the problem of high injection pressure in low permeability reservoirs is prevailing.In order to reduce the injection pressure,removing the impurity in water and changing the surface wettability of rock have been widely adopted,but the existence of water cluster is also a critical inner factor resulting in high injection pressure,so the research of water clusters has important academic value and potential application prospects.As the liquidity of the water,the hydrogen bonds keep disappearing and regeneraton constantly,which results in the disappearing and regeneraton of water clusters,so the direct observation and separation of water clusters are impossible by experiment methods.The researchs of water cluster are mainly composed of theoretical simulation in this work.The work conducts research mainly from aspects of microstructure and macro properties of water clusters.DFT is used for the studies of microstructure of water clusters and GGA/PBE functional is used for specific calculations.Molecular dynamics simulation is used for the studies of macro properties of water.Traditional TIP4P used for water phase transition exists limitations as a forcefield to calculate hydrogen bond.So Dreiding forcefield combining customised charges is successfully used for the molecular dynamic simulation of water phase transition.The results shows that average hydrogen bond energy has a linear relation with the the mean length of hydrogen bonds in ice melting or supercritical phase change.It also shows that hydrogen bond energy has a linear relation with hydrogen bond numbers in water liquefaction.Based on the distribution of water clusters,the lowest energy structure of(H2O)n(n=2-50)were built.The results show that as the cluster size increasing,the cluster structures evolve from a mono-ring motif to bilayer stacking motifs and multilayed stacking motifs,and then to cage structures.Internal water molecules begins to appear in some large cage clusters(H2O)n(n>20).Average number of hydrogen bond,average length of hydrogen bond and average binding energy could reflect the changing trend of structures.The analyses of binding energy and second order difference energy have confirmed that(H2O)4,(H2O)8,(H2O)10,(H2O)15 and(H2O)21 are magic number clusters.In order to find effective methods to reduce the size of water clusters,the impact of temperature,external electric field,ions and the surface of rock on the water clusters have been studied.Among the factors above,the impact of ions on the water clusters was studied deeply.By analyzing the electric structure of ionic water clusters,the diffusion coefficient of water and the injection pressure in the micron grade core flowmeter,the results show that I-can significantly damage the association of hydrogen bonds in liquid water while Ca2+and SO42-can obviously promote the association of hydrogen bonds in liquid water.Furthermore,the analyses of hydrated ions show that ionic kosmotropes exhibit strong hydration effect but ionic chaotropes exhibit weak hydration effect.This article has also established rock-water-rock coarse graining channel models and the coarse graining force fields.The effect of channel scale on diffusion coefficient was investigated.The results show that the diffusion coefficient of water is obviously influenced by rock pore diameter range(2-5nm),but with the increasing of the rock pore diameter,the influence begins to decrease.
Keywords/Search Tags:Water Cluster, Hydrogen bond, Density functional method, Magic number, Diffusion coefficient
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