Theoretical Study On Diffusion And Manipulation Of Water Molecules Between Carbon Nanotubes

Water is the source of life.Water resources,especially fresh water resources,play a decisive role in the survival and sustainable development of mankind.Although the earth is rich in water,only 2.53 percent of it is fresh water for human consumption,and most of it is in the form of snow and ice in the north and south poles,mountain glaciers,continental ice sheets and permafrost.Because of the uneven distribution of fresh water,human beings are always faced with the crisis of shortage of fresh water.And how to efficiently desalinate seawater is the key to solve the problem.At present,there are many ways to desalinate seawater,among which the most commonly used is the design of ultrafiltration device using water molecules transported in nanotubes.There are a variety of materials used in present studies.Carbon nanotubes,boron nitride,graphene,graphene oxide and black phosphorus are the main materials commonly used in theoretical simulations and experiments.Carbon nanotubes(CNTs)are often used in seawater desalination research due to their physical properties of ultra-fast transport of water molecules.Controlling the diffusion of water molecules in nanoscale channels and improving the diffusion rate of water molecules are important methods for efficient desalination.The most direct approach to manipulate the transport behavior of water molecules is to change the nanotube surface properties.For example,phonon induced oscillations can be used to reduce friction between water molecules and the wall of nanotubes to enhance the diffusion rate of water molecules in nanochannels or adjust the interaction between nanotubes and water molecules by modifying the charge of nanotubes to adjust the diffusion rate of water molecules in nanochannels.The latest research shows that by controlling two unconnected nanotubes,water molecules can form a water bridge between the gaps,and the diffusion speed of water molecules in the water bridge can also be increased.In this paper,we study the diffusion process of water molecules between two disjoint CNTs that are connected by a water bridge under external electric field by molecular dynamics simulations.The extended length of the water bridge under different electric filed strength could adjust the diffusion process of the water molecules crossing the two disjoint nanotubes and the diffusion coefficients could be remarkably enhanced up to 4 times larger than the value in the bulk water.By analyzing the structure of the water bridge,it is found that the diffusion enhancement originates from the strengthened interactions and the increase of hydrogen bonds between the water molecules due to the restrained reorientation from the external electric field.In addition to using water bridges to control the diffusion speed of water molecules,we try to adjust the concentric tube length difference(?)L of the double-walled nanotubes(DWNTs)to manipulate the capillary imbibition by molecular dynamics simulations.Three configurations are considered,i.e.,(?)L=0(the common DWNT structure and labeled as configuration?),(?)L<0(configuration?,the outer tube is longer than the inner tube),and(?)L>0(configuration?,the outer tube is shorter than inner the tube).By comparing with single-walled nanotube,it is found that the imbibition velocities of the water molecules in the DWNT are weakened 9.1%in configuration? and enhanced30%-46.5%in configuration? and 10%in configuration?.By analyzing the microscopic structure of the water molecules,it is found that the imbibition velocity adjustment originates from the potential energy difference of the water molecules in the water reservoir and at the entrance of the nanotube at the nanotube cylindrical pore.An exponential relation between the imbibition velocity v _L and the potential energy difference (?)E is obtained as v_L(t)?e~-((?)E/(k_B)T).Therefore,we can control the diffusion rate of water molecules by adjusting the length difference between the inner and outer tubes of the concentric double-walled nanotubes.Our research results can not only play an important role in seawater desalination,but also greatly help in hydrotherapy,nano-exchangers,engine catalysts,ultrafiltration,and transmission of osmotic energy.