Study On Mass Transfer Characteristics Of Osmosis And Thermo-osmosis Through Nanopores

The shortage of clean water supply is one of the global challenges.Among the approaches to obtain clean water,the membrane-based reverse osmosis(RO)technology is the most widely used.Membrane permeability and selectivity are important properties related to membrane performance and are dominated by the membrane pores.Molecular-level design of pores is an important approach to improve the membrane performance,which is a great challenge for the currently used polymer membrane.Nanopores,including graphene,carbon nanotube(CNT),are treated as potential membrane materials,with high sieving and small flow resistance.The structure of these nanopores is similar to several protein pores on biomembranes.For instance,aquaporins(AQPs)play an important role in maintaining the balance of water in cells.By adjusting the structure and functional groups inside the pore,aquaporins can exhibit highly screening effect of specific solutes.Osmosis is the main mass transfer process in these processes.Therefore,understanding the osmosis in these nanopores is helpful to explore their roles in desalination and physiological processes.Bionic design of membrane pores may also benefit from the study.In this thesis,we analyze the sieving effect of the pore entrance,the osmotic transport in single-file pores and the thermal effect in osmosis with theoretical and numerical analysis.The main contents of this thesis are as follows:1.The application of nanoscale slit pore in desalination.Circle pores is the simplest and widespread pore structure and have been extensively investigated by theoretical and simulation studies.However,another simple pore,slit pore,is always ignored.By analogy the solutes in dilute solution to ideal gases,the probabilities of solute being rejected by a thin slit and penetrating the pore are quantified.Then achieved osmotic pressure are obtained.The results are compared with the Kedem-Katchalsky theory based on nonequilibrium thermodynamics.The approach to analysis the entrance effect of thin pores stems from the analysis of circular pores,AQPs.The results show that the two reflection coefficients of a slit in Kedem-Katchalsky theory do not have equal values,which is similar to the properties of a circular pore.We verified the model through molecular dynamics simulations.Compared with circular pores,the reflection coefficients of slit pores are slightly smaller than those of circular pores,but the permeability of slit pores are much higher.The discovery will help to guide the structural design of nanopores.2.Modeling investigation of osmosis in single-file pores.The length of biological pores and artificial pores is usually not negligible and particles inside them may not able to penetrate each other,which refer great potential in engineering.This kind of pore is called the single-file pore.We propose a discrete model to discuss the osmosis through single-file pores.The model shows a good complement to the existing physical models.The key factors that influence the transport of solvent and solute are systematically investigated,including the solute-pore attraction and the translocation rates of solute inside the pore.It is found that the probability of solute being rejected by the pore entrance determines the osmotic pressure and the solute inside the pore becomes the flow resistance.At last,we discuss the osmotic effect of an unpenetrable non-uniform pore with its entrance accessible to solute,which is similar to many protein pores.The results can explain why the flux induced by solutes that cannot pass the membrane is less than the ideal value.Also,the “ideal” osmosis should be redefined: the solute should not enter the pore and the solute-pore attraction should be limited.3.Osmotic effect of multi-solute in single-file pores.Realistic osmotic systems usually consist of multiple solutes.It is straightforward to generalize the one-solute osmotic model to the situation with multiple solutes by linear superposition.Therefore,we discuss the single-file osmosis induced by two kinds of solute with the previous discrete model.The results show the effectiveness of linear superposition for solutes without strong attraction between them and the pore.The role of different solutes in osmosis in independent of each other.Otherwise,if the attraction between one solute and the pore is strong enough,its large entrance rate and small exit rate can turn the diffusion of this solute into a driving force.The flux of all species can vary nonlinearly as the concentration of the attractive solute increases.The pore can even be chocked in extreme situations.Meanwhile,the osmotic flow may drive the solute flow against its concentration gradient,which provides a method of uphill flow without external force or energy.The mechanism may have certain physiological significance,such as the recovery of urea in the kidney.4.The influence of temperature on osmosis and the applications of thermo-osmosis.Increasing the bulk temperature can help increase transmembrane flux and reduce energy consumption by reducing the viscosity and increasing the osmotic pressure.Meanwhile,the temperature gradient may also become a driving force,considering the molecular dynamics simulation results about desalination with a temperature gradient and carbon nanotubes.Based on the boundary layer theory and nonequilibrium thermodynamics theory,we discuss the thermal effect in an osmotic system consisting of a gap-filled vertically aligned CNT membrane.It is found that the thermal effect of ultrahigh flux can still be ignored in isothermal conditions.While in nonisothermal conditions,it is evident that increasing the temperature of two bulk solutions can both promote transmembrane flux.Heating the solution with high solute concentration may also drive the solvent flow against the osmotic pressure gradient.Many factors should be required to achieve this goal,including a large thermo-osmotic coefficient,a large equivalent thermal resistance of the membrane,a large temperature gradient and a small solute concentration gradient.The discussion will help to explore the role of temperature in osmosis through nanopores and provide an alternative method to promote the separation performance with low-grade waste heat.