Effect Of Driving Force And Its Direction On The Transport Of Water And Ions:A Molecular Dynamics Simulation Study

Due to the rapid population growth and advancement of industrialization,the world is inevitably facing the problem of water shortage.It is urgently needed to develop desalination and sewage treatment technologies to deal with the crisis.The deionized ultrafiltration semipermeable membrane is the key and bottleneck.The polymer membrane currently used has some problems of high energy consumption,low efficiency and easy pollution due to its complicated internal structure and tortuous water channels.Nanochannels such as carbon nanotubes(CNTs)and graphene oxide sheets have ultrafast fluid transport properties because of their smooth interface,and exhibit excellent performance in ion filtration.Especially,carbon nanotubes with appropriate pore diameters can transport water molecules while rejecting large hydrated ions,achieving selective transport property similar to biological channel proteins.A large number of theoretical calculations and experiments have confirmed that the slip length between water and hydrophobic nanotube interface is the main reason for its ultra-fast transport,and the transport rate is several times higher than that predicted by the classic Hagen-Poiseuille equation.The research on the transportation of fluid in nanochannels mainly focuses on two aspects: the study of the dynamic properties of water and ions through the nanotube and the interpretation of the internal mechanism;the driving of directional flow of fluid along the nanotube and the design of nanometer water pump.It is the goal of researchers to fully understand the transport mechanism of water and ions in nanotubes at the nanoscale to promote their industrial applications.Through molecular dynamics(MD)simulations,this thesis focuses on the effects of different external driving forces and their directions on the transport properties of water and ions through carbon nanotubes`.The main contents are as follows:The first chapter introduces the water crisis and solutions,especially the membrane separation method.The key to membrane separation method is the selective semipermeable membranes and the corresponding technology of directional fluid transport.Next,it introduces the CNTs,that researchers have high expectations on.Also,it introduces the kinetic characteristics and transport properties of water and ions in the tubes,including the ultra-fast transport properties,unique structures,selectivity for ions,and directional transport for fluids.The second chapter is the method description,which briefly introduces the basic method of MD simulation that is used for the study.It includes the basic principles,molecular force fields,simulation process and common skills.In chapter three,a new nanofluidic device is introduced by using the graphene sheet laying on the membrane.Based on the competitive effect of water’s spontaneous infiltration of two sides of a CNT,the water pump is designed by making use of the natural permeability.According to MD simulations,continues net flux is observed.The motion mode of the sheet is the key for the performance.For the pure Brownian motion without any dynamical load,two water molecules per nanosecond flux is observed,while the flux induced by the unidirectional motion can be several times enhanced,depending on the external force.The Brownian motion is similar to the physical mechanism of osmotic pressure,and the unidirectional motion shows great performance that has huge applications for reverse osmosis.Our work creatively proposes a new strategy to pump water molecules crossing though a nanochannel,inspiring for nanofluidic device designers.In chapter four,the long-term osmotic process lasting more than one hundred nanoseconds is systematically studied.Osmosis are essential for not only the application of nanofluidic devices but also the understanding of working principles of biological transmembrane proteins.Despite considerable experimental interests,comprehensive simulation work is still lacking,possibly because of the periodic boundary condition that inevitably leads to the spontaneous exchange of two side reservoirs.To eliminate this disadvantage,herein we design a simple model system by introducing a dipalmitoyl-phosphatidylcholine bilayer into a common CNTbased setup,which allows long time osmotic simulations.Interestingly,the osmotic water flux exhibits an excellent linear relation with the concentration gradient and an Arrhenius relation with the temperature,which highly coincides with recent experimental observations.Furthermore,the osmotic flux can be quantitatively comparable to not only the experimental CNTs and protein channels but also the theoretical estimation from the Hagen-Poiseuille equation.The designed simulation model could open a new window for future studies on osmosis.In chapter five,the effect of additional lateral pressure on the transport of water and ions across the membrane in the common pressure-driven nanowater pump is studied.A surprising desalination phenomenon is found when tuning the pressure direction for the ionic transport through(10,10)CNT.A series of MD simulations manifest that under a given longitudinal pressure,the water flux exhibits interesting maximum behaviors with the increase of lateral pressure;while the ion flux shows an excellent linear reduction even to nearly zero.This indicates an optimized pressure choice for not only the permeability but also the desalination performance,which can be attributed to the competition between the enhanced water motion and reduced water occupancy inside the CNT.The ion translocation time has a linear decay that follows the water,and their values are quite close to each other,suggesting the strong coupling motion of them inside the CNT.Furthermore,with the increase of ionic concentration,the flux of water and ions exhibit an opposite change,indicating their transport competition.Although the lateral pressure promotes the motion of water and ions inside the CNT,the higher concentration can always slow down the motion,since the ion clusters can have significant blockage effect.Our results imply a new method for controlling the transport of water and ions,and should have great implication for the desalination technology.