Molecular Simulation Of The Influence Of Electric Field And Doping On Kaolinite Surface Hydration Under Temperature And Pressure

In recent years,the scope and depth of coal mining in my country has been continuously increasing,and it has gradually developed in the direction of deepening and large-scale development,and the engineering rock mechanics environment has become more and more complicated.Mudstone is one of the most widely distributed rock types in coal-measure strata.Affected by its large amount of clay minerals such as kaolinite,montmorillonite and illite,argillaceous rock is extremely sensitive to moisture.After the action of water,phenomena such as volume expansion,swelling expansion and strength reduction are likely to occur,which in turn induces engineering stability control problems.The problem of soft rock engineering has always been one of the severe challenges facing the safe and efficient production and construction of coal mines.The current research on soft rock engineering issues mainly focuses on the causes and characteristics of large deformation,engineering mechanics characteristics and support methods,etc.It is difficult to obtain the evolution law of water in nanopores through experiments.However,molecular simulation has become a computational simulation method that strongly explains the microscopic mechanism.Therefore,molecular simulation research on the microscopic mechanism of clay mineral hydration is very important.The main research object of this paper is kaolinite in soft rock clay minerals.The study focuses on the adsorption and diffusion process of clay mineral hydration at the molecular level,and reveals the microscopic mechanism of the effects of temperature,pressure,electric field and doping on the hydration properties of kaolinite.This paper clarifies the law of adsorption and diffusion of water on the surface of kaolinite,and provides a microscopic theoretical basis for the solution of soft rock hydration engineering problems.In order to elucidate the microscopic mechanism of the influence of water on the physical properties of argillaceous rocks,the macrocanonical ensemble Monte Carlo（GCMC）method was used to obtain the adsorption configurations of kaolinite adsorbed water molecules under different temperature,pressure,electric field and doping.Using molecular dynamics（MD）simulation method and programming Perl program to carry out kinetic calculation and analysis of adsorption configuration,the effects of temperature,pressure,electric field and doping on adsorption,diffusion and physical and mechanical properties of kaolinite surface hydration were obtained.The corresponding dynamic time-history analysis of molecular dynamics process was carried out.The main work and conclusions of the thesis are as follows:（1）Each adsorption configuration reached a dynamic equilibrium during the kinetic simulation process,and water molecules are more likely to be adsorbed on the（001）surface of kaolinite,and it is easy to form hydrogen bonds with surface hydroxyl groups.As the pressure increases,the system density and the longitudinal distribution density of water molecules increase,and the interaction energy increases in a logarithmic relationship,which is conducive to the generation of hydrogen bonds and the adsorption of water molecules,while the diffusion of water molecules becomes difficult.An increase in temperature is just the opposite.The interaction energy between water molecules and the surface of kaolinite is dominated by hydrogen bonding energy;the increase in temperature increases the displacement and movement range of water molecules,and the movement of water molecules appears as a jump between adjacent ring holes.（2）Electric field polarization will cause the adsorption of water molecules on the surface of the kaolinite to be in an orderly upside-down"V"shape,away from the hydroxyl groups on the surface of the kaolinite,and diffuse into the vacuum layer.When the electric field strength is 10¹⁰ V/m,the included angle of the H-O-H bond of water molecules decreases by 3°under the influence of Coulomb force.The diffusion of water molecules on the surface of kaolinite is anisotropic.The diffusion coefficients in the X and Y directions are similar and much larger than those in the Z direction.Water molecules are more likely to diffuse in the XY plane.The diffusion coefficient of water molecules increases with the increase of the electric field strength,and the positive and negative directions of the electric field have little effect on the diffusion coefficient of water molecules;the interaction energy between water molecules and kaolinite is mainly electrostatic interaction energy and hydrogen bond interaction energy.After the electric field is applied,the electrostatic interaction energy and the hydrogen bond interaction energy are greatly reduced,and the interaction energy is negatively correlated with the electric field strength.The electric field weakens the binding effect of water molecules on the surface of kaolinite,which is beneficial to the diffusion of water molecules on the surface of kaolinite.With the increase of the electric field intensity,the hydrogen bond between water molecules and the surface of kaolinite weakens,the number of hydrogen bonds decreases,and the hydrophilicity of the kaolinite crystal surface decreases.（3）At a temperature of 25°C and a pressure of 10 Mpa,the adsorption conformation of water molecules on the surface of kaolinite is distributed in two layers.The heat of adsorption is between 1.899 k J/mol～2.128 k J/mol,which is far below the critical value of 42 k J/mol for chemical adsorption.The interaction between kaolinite and water molecules is typical physical adsorption.The adsorption energy is a negative value.After kaolinite absorbs water,the system is thermodynamically stable.The adsorption energy value and the adsorption amount change law similarly,and both increase in a logarithmic relationship with the increase of pressure,while the doping effect makes them both decrease.The adsorption site is the frequency of water molecules appearing at a certain position on the surface of the kaolinite particles during the adsorption process;the doping effect enhances the interaction between the water molecules of the kaolinite,which is conducive to the adsorption of water molecules.