Micro Mechanical Mechanism Of Ions Sieving And Gas Separation Through Low-dimensional Carbon Nanochannels

The low-dimensional carbon nanomaterials are widely adopted in the fields of nanodevice fabrication,nanopower harvest,water desalination and gas separation based on their excellent physical,mechanical and electrical stability properties.Buckyball,carbon nanotube（CNT）and graphene are the most representatives of zero-dimensional,one-dimensional and two-dimensional materials,respectively.And these materials display extraordinary mass transport properties.Investigating the process of ion sieving and gas separation is of great significance.On the one hand,it can deepen the understanding of special mass transfer phenomena in low-dimensional carbon nanochannels and provide theoretical guidance for the design of membranes made by low-dimensional carbon nanochannels.On the other hand,it promotes the upgrading and innovation of membrane materials,which plays an important role in seawater desalination,gas separation and other fields.This thesis focuses on the micro mechanical mechanism of ion sieving and gas separation through low-dimensional carbon nanochannels.Two key topics will be investigated with the combination of molecular dynamics（MD）simulations and theoretical analysis,i.e.,the competition mechanism between layered water structure and ion’s hydration structure,and the mechanism of gas transport.The major issues discussed are:（1）The diameter-dependent and thickness-dependent desalination performance within CNT nanochannels were explored to break the permeability-selectivity trade-off rule.Using MD simulations,we found that with the increase of CNT thickness,the water flux rate is compromised while the ion rejection is improved,such transitions lead to optimal desalination performance around the cross-over size.Further molecular analysis reveals that the effect of the thickness variable on desalination performance（i.e.,thickness effect）originates from the formation of two hydration shells and their competition with the ordered water chain structure.This section aims to explore the competition between the hydration shell and ordered water chain which induces thickness-dependent desalination performance in CNT,and interprets the mechanism of ion sieving from the molecular scale.Thus the confinement of the diameter effect and the thickness effect show a synergistic effect on the ion sieving,and it can be utilized to weaken the permeability-selective trade-off.It can also provide useful guidelines for establishing efficient ion sieving criteria for low-dimensional carbon nanochannels.（2）Using MD simulations to predict the desalination performance of monolayer fullerene,we find that monolayer fullerene enables 100%salt rejection and 1-2 orders of magnitude higher water permeance than that of commercial reverse osmosis（RO）membranes.The energy barrier difference between salt ions and water molecules is explored based on the inherently uniform and ordered nanoporous of monolayer fullerene,and the MD simulation exhibits that salt ions could not spontaneously enter the narrow path.Further molecular analysis shows that the energy barrier is mainly derived from ionic dehydration.Thus,it reveals the ion sieving mechanism based the on size exclusion effect.The transport properties of monolayer fullerene are compared with other nanochannels,and it clearly indicates the potential application of the new low-dimensional carbon nanochannel in seawater desalination,and provides a theoretical reference for practical desalination applications.（3）The gas separation performances of monolayer fullerene were also investigated by MD simulations.The simulation shows the monolayer fullerene has excellent selectivity for CO₂/N₂separation,a unique feature of monolayer fullerene in the separation of CO₂.It reveals the separation mechanism of CO₂/N₂.At the same time,the ability of gas molecules adsorption is reduced with the increase in temperature.In addition,we have also investigated gas separation between different gases,which shows that the kinetic diameter of gas molecules and the porous structure of the membrane directly affect the energy barrier of gas.At the same porous size of the membrane,the larger the kinetic diameter of gas molecules,the gas molecules need to overcome the higher energy barrier across the membrane.There is competitive adsorption during the diffusion of mixed gases.The gas which has the stronger adsorption capacity is easier to occlude the channel,thereby significantly affecting the permeation of other gas components.It demonstrates the excellent prospects of monolayer fullerene in gas separation for the first time.