Coupling Transport of Water and Ions Through a Carbon Nanotube: The Role of Ionic Condition

Control of water and ion transport through nanochannels is of primary importance for the design of novel nanofluidic devices. In this work, we use molecular dynamics simulations to systematically analyze the coupling transport of water and ions through a carbon nanotube in electric fields. We focus on the role of ionic conditions, including the salt species and concentration, which can significantly regulate the ion and further the water transport. We find that the coupling of water–anions is stronger than water–cations, and thus anions play a dominant role in determining the water transport. Specifically, the water and ion flux both exhibit a linear increase with the field strength, in agreement with recent experimental observations; while the water and ion translocation time show a linear and power law decrease, respectively, yielding to the Langevin predictions. These results strongly depend on the salt species, demonstrated by the ion binding. More surprisingly, with the increase of salt concentration, the anion flux displays a maximum behavior, inducing a similar maximum for water flux; while the cation flux increases almost linearly. These unordinary ion flux behaviors should be due to the channel confinement, since a wider channel exhibits similar behaviors with the previous simulation and experimental work. Detailed discussion based on Poisson-Nernst-Plank equation are further presented for ion transport. Our results reveal deep insights into the coupling transport of water and ions, especially the important role of ionic condition, and are helpful for the design of desalination, ion separation and high flux nanofluidic devices