Adsorption And Dynamics Of Water And Ions On Carbon-based Surfaces With Aromatic Rings: A Theoretical Study

Systems composed of water, ions, and carbon-based surfaces are ubiquitous in nature. Understanding and controlling the behaviors of water and ions on the arbon-based surfaces are of great importance and general interest in water desalination and purification, nanofluidic manipulation, the design of biomimetic pores, and understanding biological activities in cellular transport phenomena, etc. The carbon-based surfaces strongly interact with cations, referred to as cation–π interactions. However, unfortunately, those interactions have not been well considered, i.e., the non-covalent interactions between hydrated ions and aromatic rings in carbon-based surfaces have not been included in previous theoretical analyses and simulations. It is clear that this will result in misleading theoretical predictions and misinterpretation of experimental results. Physically, correct description of the interactions between hydrated ions and carbon-based surfaces is urgently required to clarify the misunderstanding and give a correct explanation of the experimental observations. In this paper, we present an approach in this regard using the two notable systems of water desalination and human-made ion channel design based on carbon nanotubes（CNTs） that uses an improved description of the cation-π interactions based on combining classical simulations with density functional theory calculations.First, it has long been expected that narrow CNT could be used as an excellent seawater desalination membrane because of their experimentally confirmed ultrafast pure water flow and theoretically predicted ion rejection. However, to date, there is insufficient experimental evidence of adequate salt rejection for desalination, even though fabrication of CNT membranes has greatly improved. By properly considering the interactions between hydrated ions and carbon-based surfaces, we show that cations are easily adsorbed at the entrance of（6,6）-type CNTs because of these interactions, resulting in blocking of water flow through the nanotube. The key to this behavior is the strong non-covalent interactions between cations in solution and aromatic rings in CNT. Then we propose new methods to facilitate the water and ions transport across the CNT: functionalizing the CNT entrances with saturated groups（-CH₂CH₂-） or applying an electric field.Second, in recent experiments for the design of human-made biological ion channels, unexpected open/closed state switching of ion transfer behavior in wide CNT has been observed experimentally. Unfortunately, this open/closed state switching of ion transfer cannot be explained using classical molecular dynamics（MD） simulations. By explicitly considering the interactions between hydrated ions and carbon-based surfaces, our calculations show that Na+ ions are easily trapped in the interior of the（8,8）-type CNT, and an electric field above a threshold value will allow the cations to escape from the trapped state and move through the channel, inducing open/closed state switching of ion transfer. Moreover, simulations with K⁺, N（CH₃）₄⁺, and Cl^- are also consistent with experimental results..Our findings highlight the crucial role of cation–π interactions in the behavior of water and hydrated ions on aromatic-rich carbon-based materials, and we propose an improved description of the cation-π interaction. Based on this interaction, we clarify the main difficulties in water desalination and human-made ion channel design based on CNTs. These findings provide new insight for the understanding and design of desalination membranes, new types of nanofluidic channels, nanosensors, and nanoreactors based on a CNT platform.This study also reports a fine linear relationship between the microscopic substrate-water interaction potential and the macroscopic surface energy on SHSs. Such linear relationship is still suitable for the hydrophobic and hydrophilic surfaces, even for super-hydrophobic surface. This linear relationship exists on various types of substrates with different lattice lengths, and the slopes of the linear relationship for different types of substrates are dependent on the density of surface atoms. Those findings enrich our understanding of water behavior on solid surfaces, especially the water wetting behaviors on uncharged super-hydrophilic metal surfaces, provide new insight into the understanding and control of surface wettability, especially for designing new force fields for use when preparing materials with various or designated wettability without chemical detail in theoretical research.