Development Of A Universal Tool For Constructing Polyamide Reverse Osmosis Membranes And Related Molecular Dynamics Study

The use of reverse osmosis technology for seawater and brackish water desalination is very promising to alleviate and even solve the human crisis of freshwater resources.The most important part in reverse osmosis technology is the reverse osmosis membrane,which greatly affects the application range and operating cost of the whole reverse osmosis process.The aromatic polyamide thin film composite membrane is by far the most popular reverse osmosis membrane.The top-active polyamide layer predominantly determines the membrane's water flux and separation properties.Thus,the investigation of top-active polyamide layer will benefit for developing new high-performance reverse osmosis membrane.However,it is well-known that the performances of most polyamide membranes fall within a “trade-off” relationship between permeability and selectivity.To really solve the trade-off problem,the physical mechanism that control the transport of water and salt ions through reverse osmosis membranes should be fully understood at the atomistic level.Unfortunately,at this microscopic scale,the motion of the solutes in the polyamide layer occurs at high speed,and it is difficult to observe it in real time by current experimental methods.This has led to many analyses of the reverse osmosis process in the polyamide layer that rely primarily on conjecture and deduction,and lack of systematic cognition.Therefore,if we can solve the above problems through theoretical simulation,it will be very beneficial to deepen our understanding of some key properties and characteristics in the polyamide layer,and then help experimental scientists to design a better comprehensive reverse osmosis composite membrane.Nowadays,computer technology,including hardware and software,is at a stage of rapid development,and computer simulation is gradually playing a critical role in the scientific research.As a bridge between experiment and the theory,computer simulation can verify and even acquire some important theoretical knowledge by performing real systems.Up to now,the lab research on polyamide membrane based reverse osmosis process has made great progress,but due to the complexity of the polyamide layer structure,there has been a great challenge in its model construction,which makes related theoretical simulation study still in its infancy.Therefore,it is very important to develop an efficient and reliable modeling tool for the polyamide layer and to carry out corresponding theoretical simulations to reveal the motion law of the solutes in the polyamide layer during reverser osmosis process.In this study,the language we use to develop modeling tools is C++ and Python.With the help of this tool,we use molecular dynamics(MD)to study the physical and chemical mechanisms involved in the reverse osmosis process that occurred in the polyamide layer.The main research contents of this thesis include:1.Development of MembrFactory: a force field and composition double independent universal tool for constructing polyamide reverse osmosis membranesThe all-atom polyamide layer model is crucial for the application of MD simulation in the field of membrane based reverse osmosis.However,it is difficult to conveniently construct a reliable model due to the lack of a universal tool,which greatly limits the application prospects of MD technology in this field.In order to break through the bottleneck,we have developed MembrFactory,a tool for the construction of polyamide membrane models for MD simulation of the reverse osmosis process.Firstly,with the commercial FT-30 reverse osmosis membrane as the target,the reliability of the MembrFactory was confirmed by comparing the basic properties of the model constructed under different molecular force fields with experimental data.Then,under the AMBEAR force field,three new polyamide layer models were constructed with MembrFactory.The internal structural factors affecting the water flux of the polyamide membrane were revealed by analyzing the content of the connected percolated free volume in the trans-membrane direction.2.Equilibrium MD simulation of water molecules and salt ions in polyamide reverse osmosis membraneIt is well-known that the performance of most polyamide membranes falls within a“trade-off” relationship between permeability and selectivity.To address it,the physical mechanism that controls the transport of water and salt ions through reverse osmosis membranes should be fully understood at the atomistic level.With the help of equilibrium MD simulation,the structure and kinetic behavior of water molecules and salt ions in the polyamide layer were analyzed.Through the analysis of radial distribution function,it was found that in the polyamide layer,the oxygen atom and the hydrogen atom in the carboxylic acid group and the oxygen atom in the amide bond are the main interaction sites between the polyamide layer and the water molecules,and the structure of the salt ions' hydration shell is destroyed due to the coordination interaction with the functional groups of the polyamide chains.By analyzing the tetrahedral parameters and partial structure factors,it is revealed that the tetrahedral structure of water molecules in the membrane will be distorted,and there is a long-range force existed between water molecules.Through the study of the self-diffusion behavior of water molecules and salt ions,it was revealed that the huge difference between their mobility leads to the high salt rejecting properties of the polyamide layer.3.Non-equilibrium MD simulation of polyamide reverse osmosis membrane at different temperaturesOperating temperature has a great impact on the performance of polyamide reverse osmosis membranes during the actual production process.Although some reports demonstrated that the performance of polyamide RO membranes is very sensitive to the temperature of water inlet,the underlying mechanism behind it is still difficult to understand at an atomistic level.Therefor using the non-equilibrium MD simulation,the temperature effect on the reverse osmosis process was studied.It is revealed that in the presence or absence of salt ions,the concentration of the aggregated pores in the polyamide membrane exhibits a different trend with increasing temperature.In addition,by analyzing the largest water clusters in the membrane,we also found that the temperature affects the water flux of the polyamide layer from two different directions,the positive effect is by increasing the mobility of water molecules in the polyamide layer,while the negtiveeffect is carried out by destroying the water clusters in the polyamide layer.4.Theoretical construction of gradient crosslinked polyamide reverse osmosis membrane and simulation of its antifouling behaviorThe polyamide reverse osmosis membrane is essentially inhomogeneous,that is,its structural properties vary with its depth.However,although many models have been constructed to date,their structural properties lack gradient changes along the thickness direction,and can only be used to describe the structure of the inner dense region of the polyamide layer.In order to better reflect the structural characteristics of the polyamide layer and to more accurately describe the transport behavior of the solute in this layer,based on MembrFactory,we constructed a fully atomistic model of a polyamide layer with the crosslinking degree gradiently changed along the thickness direction.Using MD simulation,it was found that the pore size distribution increased with the cross-linking degree from the innermost region to the interfacial region of the polyamide layer model,and this is closely related to the self-diffusion capability of water molecules within it.After grafting polyethylene glycol chains onto the surface of the polyamide layer,we introduced two common organic pollutants into the system.It was found that the size and hydrophobicity of the contaminant molecule,and the coverage of the hydrophilic coatings affect the contamination behavior of the polyamide reverse osmosis membrane.