MD Simulations On The Transport Behaviors Of Mixed Na~+ And Li~+ In A CPNT Under An Electric Field And Thermal Stabilities Of CPNTs

In this work,the transport behaviors of Na+ and Li+in the 8 ×(WL)4/POPE-CPNT from a NaCl/LiCl solution under different electric fields and thermal stabilities of two CPNTs with diverse hydrophilic/hydrophobic characteristics have been explored by molecular dynamics simulations.Li+faces a higher dehydration barrier than Na+ at the entrance of the 8 ×(WL)4/POPE-CPNT.It can enter the channel from a NaCl/LiCl solution only under an electric field?0.3 V nm-1.The number of Na+entering the channel increases with the augment of the electric field strength.In the space-strained channel,both Na+and Li+can form two solvation shells.The electrostatic interaction with water,and the orientations of the neighboring water molecules both refer to a more compact solvation structure and stronger hydration of Li+.Na+exhibits stronger hydration in an a-plane zone,while Li+does so in a midplane region.Na+ tends to stay in an a-plane zone and Li+in a midplane region,with the first solvation numbers of 5.1 and 4.1,respectively.Na+may coordinate with the carbonyl oxygen(Oc=o)atoms of the CPNT.The increase of the electric field strength significantly accelerates two cations' axial diffusions.The axial diffusion of Li+with a relatively small hydration structure is slightly higher than that of Na+,and the residence time is relatively shorter in the channel.The presences of ions and an external electric field hardly disturb the original water array-1-2-1-2 file,but may bring disorder somewhat.As the electric field strength increases,channel water molecules in the whole channel tend to take positive dipole states.MD simulations show that the increase of temperature decreases the stability of a CPNT,described as the increases of the RMSD of the backbone atoms'positions and the channel length,and the decrease of the H-bonds between two adjacent cyclic subunits.The hydrophilic QT-CPNT was speculated to be more stable than the hydrophobic WL-CPNT,due to the co-existence of the H-bonds between the backbones and between the side chains.