微孔/表面及复杂流体的分子动力学模拟研究
摘要
近年来分子动力学模拟飞速发展,在化工、材料等相关工业上发挥着重要作用。它不仅可以模拟超临界、过冷等极端条件下流体的结构和动力学性质,也可以获得微孔或无定形材料表面受限流体的微观行为,这些数据用实验测量的方法难以直接得到。此外,分子动力学模拟也是研究水溶液氢键结构的有力工具。本文用分子动力学模拟方法就几个复杂体系进行研究。
1. 稠密二氧化碳在不同宽度的粘土狭峰孔中的结构性质和扩散行为的研究。稠密二氧化碳在多孔材料中的结构性质和动力学性质对材料合成和吸附分离起重要作用。我们的研究有助于理解粘土矿物中功能分子的引入和超临界二氧化碳去除粘土中杂质的过程机理。稠密二氧化碳在粘土固体表面形成明显的高密度层。受限二氧化碳的密度分布和结构随孔宽而变化,在宽孔中只有接触层位置的流体才表现出长程有序的结构。接近粘土表面的二氧化碳的氧原子的结构类似于固体。研究二氧化碳的扩散系数和取向相关时间,探讨二氧化碳分子在孔中的详细行为。
2. 稠密二氧化碳在无定形二氧化硅表面行为的分子动力学模拟。二氧化硅作为半导体材料在电子工业上得到广泛应用,稠密二氧化碳以其特殊的物理化学性质在微电子处理上有重要作用,因此二氧化碳在固体表面的结构和扩散性质的研究有重要意义。从模拟结果可以看出二氧化碳在二氧化硅表面形成一个高密度区域。二氧化碳的密度分布和自由能分布有一个明显的“镜面对称”的结构,高密度区域对应自由能的深阱。二氧化硅中的二氧化碳的自扩散行为是各向异性的。与宏观流体相比,二氧化硅表面二氧化碳的平动扩散受到阻碍,取向扩散则显著加强。在靠近二氧化硅表面位置处,二氧化碳分子的停留时间几乎为中间层的五倍。
3. 研究乙醇/水混合物的微观结构和扩散性质。醇水溶液的物理化学性质在传质的理论研究和工业应用方面有重要意义,从微观的角度来看醇水溶液的结构行为是阐述该混合物扩散现象的基础。用全浓度范围内的O-O 和O-H 径向分布函数来分析二元混合物的局部结构,发现混合物中乙醇-乙醇的氢键结构而随着水浓度的增加逐渐被打破。乙醇和水分子间的关联性较强,导致乙醇-水的结构
Molecular dynamics simulation plays an important role in chemical engineering and material science in recent years. It can simulate equilibrium structure and dynamics properties of fluids under extreme conditions such as supercritical fluids and supercooled fluids. Behaviors of confined fluids in micropores or on amorphous surfaces can also be obtained by molecular dynamics simulations, which are difficult by direct access to experimental measurement. Moreover, molecular dynamics simulation is a powerful tool in investigating the hydrogen bonding structure of aqueous solutions at a molecular level. In this thesis, several complicated systems are studied by molecular dynamics simulation.
1. Structure property and diffusion behaviors of dense carbon dioxide confined in a series of clay slit pores are studied. The structural and dynamics properties of the dense carbon dioxide are important in material synthesis and adsorptive separation. This study will assist in understanding the mechanism and process in the intercalation of functional molecules into clay mineral and the containment removal from clay sediment by using supercritical carbon dioxide. It is observed that the confined CO2 fluids form obvious layers near the clay surface. The density profile and structure of carbon dioxide vary with pore width and only the fluids in the contact layer show long-range order. Near the clay surface the oxygen atoms of carbon dioxide form a solid-like structure. The self-diffusion coefficient and the orientation correlation time are calculated to investigate the detail behaviors of confined carbon dioxide.
2. Molecular dynamics simulations are performed to study the behaviors of dense carbon dioxide on amorphous silica surface. Silica is used extensively as semiconductor material in electronic industry and the application of dense carbon dioxide on microelectronics processing are attractive for its special physical chemistry properties. Thus, the studies of structure and dynamics properties of dense carbon dioxide on silica surface are required in order to develop the process. From the simulation, the carbon dioxide molecules form a high density layer in the proximity of
the silica surfaces. The density profile and the free energy profile of carbon dioxide have a “mirror symmetry”, where the high density coincides with the free energy well. The diffusion coefficients of carbon dioxide near the silica surface are anisotropic. The translational diffusivities of the carbon dioxide at the silica surface are hindered and the rotational diffusivities are enhanced compared to bulk fluids. The residence time of carbon dioxide near the silica surface is almost five times the size of that in middle layer. 3. The structure and diffusion properties of ethanol/water mixtures are studied by molecular dynamics simulation. The physical chemistry properties of the alcohol solutions play important roles in theoretical study and technological application involving mass transfer, and the solution structure behaviors is very fundamental to understand the mixture diffusion phenomenon. Analyzing the local structure by the O-O and O-H radial distribution functions obtained from the simulation, the ethanol-ethanol hydrogen bonding structures are broken as the water concentration increases. The strong correlations between ethanol and water molecules lead to an enhancement in the ethanol-water structure with ethanol concentration increasing. The simulated self-diffusion coefficients of water and ethanol under different concentrations are larger than the experimental data. The self-diffusion coefficient of water is described by the combining contribution of the “bound”and “free”water molecules. The mutual diffusion coefficients obtained by the MD simulation are in fair agreement with the experimental data in the ethanol/water mixture. The effects of concentration and microstructure of mixtures on the diffusion coefficient are investigated by the simulation data.
1. 稠密二氧化碳在不同宽度的粘土狭峰孔中的结构性质和扩散行为的研究。稠密二氧化碳在多孔材料中的结构性质和动力学性质对材料合成和吸附分离起重要作用。我们的研究有助于理解粘土矿物中功能分子的引入和超临界二氧化碳去除粘土中杂质的过程机理。稠密二氧化碳在粘土固体表面形成明显的高密度层。受限二氧化碳的密度分布和结构随孔宽而变化,在宽孔中只有接触层位置的流体才表现出长程有序的结构。接近粘土表面的二氧化碳的氧原子的结构类似于固体。研究二氧化碳的扩散系数和取向相关时间,探讨二氧化碳分子在孔中的详细行为。
2. 稠密二氧化碳在无定形二氧化硅表面行为的分子动力学模拟。二氧化硅作为半导体材料在电子工业上得到广泛应用,稠密二氧化碳以其特殊的物理化学性质在微电子处理上有重要作用,因此二氧化碳在固体表面的结构和扩散性质的研究有重要意义。从模拟结果可以看出二氧化碳在二氧化硅表面形成一个高密度区域。二氧化碳的密度分布和自由能分布有一个明显的“镜面对称”的结构,高密度区域对应自由能的深阱。二氧化硅中的二氧化碳的自扩散行为是各向异性的。与宏观流体相比,二氧化硅表面二氧化碳的平动扩散受到阻碍,取向扩散则显著加强。在靠近二氧化硅表面位置处,二氧化碳分子的停留时间几乎为中间层的五倍。
3. 研究乙醇/水混合物的微观结构和扩散性质。醇水溶液的物理化学性质在传质的理论研究和工业应用方面有重要意义,从微观的角度来看醇水溶液的结构行为是阐述该混合物扩散现象的基础。用全浓度范围内的O-O 和O-H 径向分布函数来分析二元混合物的局部结构,发现混合物中乙醇-乙醇的氢键结构而随着水浓度的增加逐渐被打破。乙醇和水分子间的关联性较强,导致乙醇-水的结构
Molecular dynamics simulation plays an important role in chemical engineering and material science in recent years. It can simulate equilibrium structure and dynamics properties of fluids under extreme conditions such as supercritical fluids and supercooled fluids. Behaviors of confined fluids in micropores or on amorphous surfaces can also be obtained by molecular dynamics simulations, which are difficult by direct access to experimental measurement. Moreover, molecular dynamics simulation is a powerful tool in investigating the hydrogen bonding structure of aqueous solutions at a molecular level. In this thesis, several complicated systems are studied by molecular dynamics simulation.
1. Structure property and diffusion behaviors of dense carbon dioxide confined in a series of clay slit pores are studied. The structural and dynamics properties of the dense carbon dioxide are important in material synthesis and adsorptive separation. This study will assist in understanding the mechanism and process in the intercalation of functional molecules into clay mineral and the containment removal from clay sediment by using supercritical carbon dioxide. It is observed that the confined CO2 fluids form obvious layers near the clay surface. The density profile and structure of carbon dioxide vary with pore width and only the fluids in the contact layer show long-range order. Near the clay surface the oxygen atoms of carbon dioxide form a solid-like structure. The self-diffusion coefficient and the orientation correlation time are calculated to investigate the detail behaviors of confined carbon dioxide.
2. Molecular dynamics simulations are performed to study the behaviors of dense carbon dioxide on amorphous silica surface. Silica is used extensively as semiconductor material in electronic industry and the application of dense carbon dioxide on microelectronics processing are attractive for its special physical chemistry properties. Thus, the studies of structure and dynamics properties of dense carbon dioxide on silica surface are required in order to develop the process. From the simulation, the carbon dioxide molecules form a high density layer in the proximity of
the silica surfaces. The density profile and the free energy profile of carbon dioxide have a “mirror symmetry”, where the high density coincides with the free energy well. The diffusion coefficients of carbon dioxide near the silica surface are anisotropic. The translational diffusivities of the carbon dioxide at the silica surface are hindered and the rotational diffusivities are enhanced compared to bulk fluids. The residence time of carbon dioxide near the silica surface is almost five times the size of that in middle layer. 3. The structure and diffusion properties of ethanol/water mixtures are studied by molecular dynamics simulation. The physical chemistry properties of the alcohol solutions play important roles in theoretical study and technological application involving mass transfer, and the solution structure behaviors is very fundamental to understand the mixture diffusion phenomenon. Analyzing the local structure by the O-O and O-H radial distribution functions obtained from the simulation, the ethanol-ethanol hydrogen bonding structures are broken as the water concentration increases. The strong correlations between ethanol and water molecules lead to an enhancement in the ethanol-water structure with ethanol concentration increasing. The simulated self-diffusion coefficients of water and ethanol under different concentrations are larger than the experimental data. The self-diffusion coefficient of water is described by the combining contribution of the “bound”and “free”water molecules. The mutual diffusion coefficients obtained by the MD simulation are in fair agreement with the experimental data in the ethanol/water mixture. The effects of concentration and microstructure of mixtures on the diffusion coefficient are investigated by the simulation data.
引文
[1] 胡英, 刘洪来. 分子工程与化学工程.化学进展[J]. 1995, 7(3): 235～250.
[2] Allen M P, Tildesley D. J. Computer Simulation of Liquids[M]. Oxford: Clarendon Press, 1987.
[3] Frankel D, Smit B. Understanding Molecular Simulation. From Algorithms to Applications. San Diego: Acadamic Press, 1996
[4] 李以圭, 刘金晨. 分子模拟与化学工程. 现在化工[J]2001, 21(7): 10～13.
[5] McCabe C, Kalyuzhnyi Y V. Thermodynamic Properties of Freely-jointed Hard-sphere Multi-Yukawa Chain Fluids: Theroy and Simulation. Fluid Phase Equilibria[J]. 2002, 194: 185～196.
[6] Vasquez M, Nemethyt G, Scheraga H A. Conformational Energy Calculations on Polypeptides and Proteins Chem Rev[J]. 1994, 94: 2183～2239.
[7] Jiang J W, Liu H L, Hu Y. Polyelectrolyte Solutions with Stickiness Between Polyions and Counterions. J Chem Phys[J]. 1999, 110: 4952～4962.
[8] Xiao J, Lu J F, Chen J, et al. Molecualr Dynamics Simulation of Diffusion Coefficient of Oxygen, Nitrogen and Sodium Chloride in Supercritical Water. Chin Phys Lett[J]. 2001, 18(7): 847～849.
[9] SeséG, Palomar R. Molecular Dynamics Studies of Supercooled Ethanol. J Chem Phys[J]. 2001, 114: 9975～9981.
[10] Rocha S R P, Johnston K P. Molecular Structure of Water-Supercritical CO2 Interface. J Phys Chem B[J]. 2001, 105: 12092～12104.
[11] Schoen M, Diestler D J, Cushman J H. Fluids in Micropores. I. Structure of A Simple Classical Fluid in A Slit-Pore. J Chem Phys[J]. 1987, 87: 5464～5476.
[12] Abraham F F. The Interfacial Density Profile of A Lennard-Jones Fluid in Contact with A (100) Lennard-Jones Wall and its Relationship to Idealized Fluid/Wall Systems: A Monte Carlo Simulation. J Chem Phys[J]. 1978, 68: 3713～3716.
[13] Snook I K, Megen W. Solvation Forces in Simple Dense Fluids. I. J Chem Phys[J]. 1980, 72: 2907～2913.
[14] Magda J J, Tirrell M, Davis H T. Molecular Dynamics of Narrow, Liquid-Filled Pores. J Chem Phys[J]. 1985, 72: 1888～1901.
[15] Bitsanis I, Somers S A, Davis H Ted, et al. Microscopic Dynamics of Flow in Molecularly Narrow Pores. J Chem Phys[J]. 1990, 93: 3427～3431.
[16] Cao D P, Zhang X, Chen J, et al. Local Diffusion Coefficient of Supercritical Methane in Activated Carbon by Molecular Simulation. Carbon[J]. 2003, 41: 2686～2689
[17] Bhatia S K, Tran K, Nguyen T X, et al. High-Pressure Adsorption Capacity and Structure of CO_2 in Carbon Slit Pores: Theory and Simulation. Langmuir[J]. 2004, 20: 9612～9620.
[18] Samios S, Stubos A K, Kanellopoulos N K. Determination of Micropore Size Distribution fron Grand Canonical Monte Carlo Simulation and Experiment CO2 Isotherm Data. Langmuir[J]. 1997, 13: 2795～2802.
[19] Vishnyakov A, Ravikovitch P I, Neimark A V. Molecular Level Models for CO2 Sorption in Nanopores. Langmuir. 1999, 15: 8736～8742.
[20] Park S-H, Sposito G. Do Montmorillonite Surfaces Promote Methane Hydrate? Formation Monte Carlo and Molecualr Dynamics Simulations. J Phys Chem B[J]. 2003.107: 2281～2290
[21] Boek E S, Coveney P V, Skipper N T. Monte Carlo Molecular Modeling Studies of Hydrated Li-, Na-,and K-Smectites: Understanding the Role of Potassium as Clay Swelling Inhibitor. J Am Chem Soc. 1995, 117: 12608～12617.
[22] Skipper N T, Refson K, McConnell J D C. Computer Simulation of Interlayer Water in 2:1 Clays. J Chem Phys[J]. 1991,94: 7434～7445.
[23] Greathouse J A, Refson K, Sposito G. Molecular Dynamics Simulation of Water Mobility in Magnesium-Smectite Hydrates. J Am Chem Soc[J]. 2000 , 122: 11459～11464.
[24] Cavalho R F L, Skipper N T. Atomistic Simulation Clay–Fluid interface Colloidal Laponite. J Chem Phys[J]. 2001, 114: 3727～3733.
[25] Arab M, Bougeard D, Smirnov K S. Structure and Dynamics of the Interlayer Water in An Uncharged 2 :1 Clay. Phys Chem Chem Phys[J]. 2003, 5: 4699～4707.
[26] Brodka A, Zerda T W. Molecular Dynamics of SF6 in Porous Silica. J Chem Phys[J]. 1991, 95 : 3710～3718.
[27] Riccardo J L, Steele W A. Molecular Dynamics Study of Tracer Diffusion of Argon Adsorbed on Amorphous Surfaces. J Chem Phys[J]. 1996, 105 : 9674～9685.
[28] Puibasset J, Pellenq R J-M. A Comparison of Water Adsorption on Ordered and Disordered Silica Substrates. Phys Chem Chem Phys[J]. 2004, 6: 1933～1937.
[29] Marmier A, Hoang P N M, Girardet C, et al. Transfer of A Pollutant Molecule Through A Water Film on A Single Crystal Surface. J Chem Phys[J]. 1999, 111 : 4862～4864.
[30] Warne M R, Allan N L, Cosgrove T. Computer Simulation of Water Molecules at Kaolinite and Silica Surfaces. Phys Chem Chem Phys[J]. 2000, 2: 3663～3668.
[2] Allen M P, Tildesley D. J. Computer Simulation of Liquids[M]. Oxford: Clarendon Press, 1987.
[3] Frankel D, Smit B. Understanding Molecular Simulation. From Algorithms to Applications. San Diego: Acadamic Press, 1996
[4] 李以圭, 刘金晨. 分子模拟与化学工程. 现在化工[J]2001, 21(7): 10～13.
[5] McCabe C, Kalyuzhnyi Y V. Thermodynamic Properties of Freely-jointed Hard-sphere Multi-Yukawa Chain Fluids: Theroy and Simulation. Fluid Phase Equilibria[J]. 2002, 194: 185～196.
[6] Vasquez M, Nemethyt G, Scheraga H A. Conformational Energy Calculations on Polypeptides and Proteins Chem Rev[J]. 1994, 94: 2183～2239.
[7] Jiang J W, Liu H L, Hu Y. Polyelectrolyte Solutions with Stickiness Between Polyions and Counterions. J Chem Phys[J]. 1999, 110: 4952～4962.
[8] Xiao J, Lu J F, Chen J, et al. Molecualr Dynamics Simulation of Diffusion Coefficient of Oxygen, Nitrogen and Sodium Chloride in Supercritical Water. Chin Phys Lett[J]. 2001, 18(7): 847～849.
[9] SeséG, Palomar R. Molecular Dynamics Studies of Supercooled Ethanol. J Chem Phys[J]. 2001, 114: 9975～9981.
[10] Rocha S R P, Johnston K P. Molecular Structure of Water-Supercritical CO2 Interface. J Phys Chem B[J]. 2001, 105: 12092～12104.
[11] Schoen M, Diestler D J, Cushman J H. Fluids in Micropores. I. Structure of A Simple Classical Fluid in A Slit-Pore. J Chem Phys[J]. 1987, 87: 5464～5476.
[12] Abraham F F. The Interfacial Density Profile of A Lennard-Jones Fluid in Contact with A (100) Lennard-Jones Wall and its Relationship to Idealized Fluid/Wall Systems: A Monte Carlo Simulation. J Chem Phys[J]. 1978, 68: 3713～3716.
[13] Snook I K, Megen W. Solvation Forces in Simple Dense Fluids. I. J Chem Phys[J]. 1980, 72: 2907～2913.
[14] Magda J J, Tirrell M, Davis H T. Molecular Dynamics of Narrow, Liquid-Filled Pores. J Chem Phys[J]. 1985, 72: 1888～1901.
[15] Bitsanis I, Somers S A, Davis H Ted, et al. Microscopic Dynamics of Flow in Molecularly Narrow Pores. J Chem Phys[J]. 1990, 93: 3427～3431.
[16] Cao D P, Zhang X, Chen J, et al. Local Diffusion Coefficient of Supercritical Methane in Activated Carbon by Molecular Simulation. Carbon[J]. 2003, 41: 2686～2689
[17] Bhatia S K, Tran K, Nguyen T X, et al. High-Pressure Adsorption Capacity and Structure of CO_2 in Carbon Slit Pores: Theory and Simulation. Langmuir[J]. 2004, 20: 9612～9620.
[18] Samios S, Stubos A K, Kanellopoulos N K. Determination of Micropore Size Distribution fron Grand Canonical Monte Carlo Simulation and Experiment CO2 Isotherm Data. Langmuir[J]. 1997, 13: 2795～2802.
[19] Vishnyakov A, Ravikovitch P I, Neimark A V. Molecular Level Models for CO2 Sorption in Nanopores. Langmuir. 1999, 15: 8736～8742.
[20] Park S-H, Sposito G. Do Montmorillonite Surfaces Promote Methane Hydrate? Formation Monte Carlo and Molecualr Dynamics Simulations. J Phys Chem B[J]. 2003.107: 2281～2290
[21] Boek E S, Coveney P V, Skipper N T. Monte Carlo Molecular Modeling Studies of Hydrated Li-, Na-,and K-Smectites: Understanding the Role of Potassium as Clay Swelling Inhibitor. J Am Chem Soc. 1995, 117: 12608～12617.
[22] Skipper N T, Refson K, McConnell J D C. Computer Simulation of Interlayer Water in 2:1 Clays. J Chem Phys[J]. 1991,94: 7434～7445.
[23] Greathouse J A, Refson K, Sposito G. Molecular Dynamics Simulation of Water Mobility in Magnesium-Smectite Hydrates. J Am Chem Soc[J]. 2000 , 122: 11459～11464.
[24] Cavalho R F L, Skipper N T. Atomistic Simulation Clay–Fluid interface Colloidal Laponite. J Chem Phys[J]. 2001, 114: 3727～3733.
[25] Arab M, Bougeard D, Smirnov K S. Structure and Dynamics of the Interlayer Water in An Uncharged 2 :1 Clay. Phys Chem Chem Phys[J]. 2003, 5: 4699～4707.
[26] Brodka A, Zerda T W. Molecular Dynamics of SF6 in Porous Silica. J Chem Phys[J]. 1991, 95 : 3710～3718.
[27] Riccardo J L, Steele W A. Molecular Dynamics Study of Tracer Diffusion of Argon Adsorbed on Amorphous Surfaces. J Chem Phys[J]. 1996, 105 : 9674～9685.
[28] Puibasset J, Pellenq R J-M. A Comparison of Water Adsorption on Ordered and Disordered Silica Substrates. Phys Chem Chem Phys[J]. 2004, 6: 1933～1937.
[29] Marmier A, Hoang P N M, Girardet C, et al. Transfer of A Pollutant Molecule Through A Water Film on A Single Crystal Surface. J Chem Phys[J]. 1999, 111 : 4862～4864.
[30] Warne M R, Allan N L, Cosgrove T. Computer Simulation of Water Molecules at Kaolinite and Silica Surfaces. Phys Chem Chem Phys[J]. 2000, 2: 3663～3668.