Study Of Ion And Water Transport In Low-dimensional Carbon Nanochannels

With the development of micro and nano fabrication technology,it is now possible to fabricate individual artificial channels with nanometric and even subnanometric size with manifold geometries.Taking advantage of size confinement and diverse interactions between ion/molecules and nanochannels,numerous applications can be implemented with nanochannels such as separation,controllable gating or rectification,energy storage or conversion,molecule sensing,and so on.As water scarcity becomes more common,it has become greatly important to use nanochannels for the ions separation and seawater desalination.Among a variety of nanochannels,low–dimensional carbon nanochannels,including the two–dimensional graphene nanochannels and one–dimensional carbon nanotubes,are most promising for building separation membranes due to their super–high mechanical strength and low–frictional resistance as well as the outstanding properties in ion and water transport.For the requirements of practical application,with combining experimental methods and simulation technologies,we studied the ion and water transport in low–dimensional carbon nanochannels as well as the regulation mechanism,and we obtained the following main findings.1.The ion transport mechanism in graphene nanochannels was revealed.It was found that ion pairing and surface trapping played a critical role in reducing the ion mobility inside a nanochannel.The mobility for both cations and anions decreased with the reduced channel sizes because it was easier to form ion pairs in sub–5 nm diameter nanochannels with a smaller diameter.Inside the charged nanochannels,besides the ion pair formation,the surface charges reduced the counterion mobility through surface trapping.With both effects,it was uncovered that the mobility of sodium ions increased first and then decreased with the surface charge density,while the mobility of chloride ions had the opposite trend.2.The selective ion transport in graphene nanochannels was realized.The selectivity between cations and anions could be regulated by modifying the geometry and surface properties of graphene nanochannels.With the help of surface charge exclusion,graphene nanochannels with diameter as large as 3.5 nm still had a very high ion selectivity.Increasing surface charge density was beneficial to enhance the ion selectivity.However,there existed a critical value for the surface charge density.Once the surface charge density exceeded the critical value,charge inversion occurred inside the nanochannel,and further increasing the surface charge density would deteriorate the ion selectivity.Beside the surface charge density,the channel length also affected the ion selectivity.3.The selective anion transport in carbon nanotubes was explored.The intrinsic permeabilities of chloride,bromide,iodide and thiocyanate ions through 10 nm–length and 0.8 nm–diameter carbon nanotube porins(CNTPs)were determined.These measurements revealed unexpectedly strong differential ion selectivity with permeabilities of different ions varying by up to three orders of magnitude.Removal of the negative charge from the nanotube entrance increased the anion permeability by only a relatively small factor,indicating that electrostatic repulsion was not a major determinant of CNTP selectivity.It was found that the ion permeability had strong correlations with the ion hydration energies,which revealed that the origin of this strong differential ion selectivity was partial dehydration of anions upon entry into the narrow CNTP channels.4.The potassium ion transport mechanism in carbon nanotubes was explored.The potassium ion diffusion coefficient and mobility in CNTP channels were experimentally determined,which indicated that Nernst–Einstein relation breaks down by three orders of magnitude in these channels.Molecular dynamics simulations revealed that the breakdown of Nernst–Einstein relation in CNTPs was attributed to two distinct microscopic mechanisms of ion transport in the extreme confinement.The extreme confinement of ions in the single–file water wire filling the nanotube led to the expected slowdown of the ion diffusion.For the electrophoretic transport under the electric field,the water wire broke down and formed a distinct ion–water cluster that was able to traverse the nanotube pore at much higher speeds.5.The fast water transport mechanism and water/salt permselectivity in carbon nanotubes were explored.The water and ion permeability through CNTPs were measured under optimized experimental conditions.It was found that water molecules could transport very fast with a rate that was comparable to the value when transport in aquaporins.Measured activation energy of water transport through the CNTPs agreed with the energy barrier values typical for single–file water transport.Simulations also showed similar activation energy values and provided molecular–scale details of the mechanism for water entry into these channels.CNTP channels showed very high water/salt permselectivity values,indicating its potential application in fabricating desalination membranes.