Molecular Dynamics Simulation Of Activities And Osmotic Coefficients For LiCl–NaCl–KCl–H₂O Solution Systems

As a typical‘liquid mineral resource’,the salt lake brine contains a large amount of salt-forming elements,which is the important source of multiple inorganic chemical reagents,providing key upstream products for the chemical industry,agriculture,light industry,metallurgy,construction,medicine,and other industries.Li,Na,K,andCl are important components of salt lake resources,among these resources,NaCl is the most basic industrial raw material and daily necessity;KCl is the most important form of potash fertilizer produced in our country,and is widely utilized in the food industry and pharmaceutical industry;Li is an inorganic element with high strategic value,which is used in the fields of energy,communications,functional materials,and nuclear science.While extracting and processing the related resources of salt lakes,various processes including evaporation,crystallization,floatation,reverse-floatation,membrane separation,and electrolysis are employed.When designing these processes and implementing the production,detailed research of the thermodynamic properties of solutions is required to increase the production efficiency and decrease resource waste and environmental pollution.Activity and osmotic coefficient are two of these basic and significant thermodynamic properties widely used in production and research.Determining activity and osmotic pressure in the laboratory cost a long period and acquire a strict limit of laboratory environment and equipment,in real practices,the idea of the theoretical calculation is often carried out to calculate and predict the activity and osmotic coefficient.In this paper,considering the basic composition characteristics of salt lake brine,the quaternary solution system LiCl–NaCl–KCl–H₂O and its three ternary subsystems and three binary subsystems are selected as the study object.Three different molecular dynamics simulation techniques and methods are employed to study the thermodynamic related properties and to calculate and predict the osmotic coefficients and activities,the results are compared with experimental data and thermodynamic model calculated data.The details are as follows:（1）The OPAS method was improved,the osmotic equilibrium simulation scheme of electrolyte solutions was established,and the design and important parameters of the scheme were optimized.The osmotic pressures,osmotic coefficients,and water activities of various compositions of each system at 298.15 K were calculated,and the calculated results were compared with the experimental values and the calculated values of the Pitzer model and the errors were analyzed.The results show that the simulated and calculated values of osmotic pressure and water activity are consistent with the experimental values and the calculated values of the Pitzer model,including the property relationship between different concentrations of the same solution and different solutions of the same concentration.The mean relative error of osmotic pressure calculation and water activity calculation is 10%and 0.5%.The method can be applied to the study of permeation properties and water activity of multi-electrolyte solutions,with good generality.（2）A cellulose triacetate membrane-solution simulation system was established to study the structure and diffusion properties of the solution in the membrane.According to the correlation between the diffusion coefficient and water activity,the water activities of each system under various compositions at 298.15 K were calculated.The results were compared with the experimental reference value and the error was analyzed.The mean relative error of the water activity calculation was 3%,which can reflect the trend of the water activity of the solution changing with the concentration,and the method has good generality.The structures of membranes and solutions are analyzed and discussed,the results show that the membrane-polyelectrolyte solution system has different structural features from dry membranes,pure water,and pure solutions.（3）Based on the Kirkwood–Buff statistical theory,according to the structure of the bulk solution,the water activities of the solutions of various compositions of each system at 298.15 K were calculated.The results were compared with the experimental reference values,and the errors were analyzed.The mean relative error of the water activity calculation was 0.6%.The method has good generality and scalability.A detailed discussion of structural and thermodynamic quantities shows that ion–water and ion–ion interactions in the 2–6（?）range are key factors affecting water activity.and in complex electrolyte solution systems,there might be a continuous correlation between the radial distribution and water activity.