Coupling Transport of Water and Ions through a Carbon Nanotube in a Pressure Difference: The Relation between Dynamics and Ion Structures

Understanding the coupling relation between water and ions in their transport through nanochannels is relevant to many physicochemical process and biological activities and should be the preliminary knowledge that nanofluidic devices depend on. This work is devoted to revealing the coupling transport relation between water and ions through a (10,10) carbon nanotube under the drive of pressure difference. By using extensive molecular dynamics simulations, we compare different salts from monovalent to trivalent cations as well as the monovalent salt concentration. An interesting phenomenon is that the distinct cation valence will result in different channel blockage that ultimately leads to the dynamics bifurcation of water and ions. The strong coupling relation between water and ions can be uncovered by their dynamical similarity, such as the linear flux and power law translocation time behaviors with the change of pressure difference. Furthermore, with the increase of salt concentration, the dynamics of water and ions exhibit unique behaviors, such as the monotonous decrease of water flux, the maximum behavior of ion flux, as well as the translocation time increase for both of them. We also find that the ions can travel through the channel in distinct structures that depend on the ionic conditions, such as single, couple, triple, quadruple, and even more complex clusters. These ion structures provide a key to decipher the dynamical behaviors of water and ions. Our results provide essential information on the coupling relation between water and ions, and the revealed connection between dynamics and ion structures is promising for future study.