Carbon nanotubes (CNTs) mimicking the structure of aquaporins support fast water transport, making them strong candidates for building next-generation high-performance membranes for water treatment. The diffusion and transport behavior of water through CNTs or nanoporous graphene can be fundamentally different from those of bulk water through a macroscopic tube. To date, the nanotube-length–dependent physical transport behavior of water is still largely unexplored. Herein, on the basis of molecular dynamics simulations, we show that the flow rate of water through 0.83-nm-diameter (6,6) and 0.96-nm-diameter (7,7) CNTs exhibits anomalous transport behavior, whereby the flow rate increases markedly first and then either slowly decreases or changes slightly as the CNT length l increases. The critical range of l for the flow-rate transition is 0.37 to 0.5 nm. This anomalous water transport behavior is attributed to the l -dependent mechanical stability of the transient hydrogen-bonding chain that connects water molecules inside and outside the CNTs and bypasses the CNT orifice. The results unveil a microscopic mechanism governing water transport through subnanometer tubes, which has important implications for nanofluidic manipulation.

It is well-known that the aqueous-phase processing of chlorine nitrate (ClONO 2 ) plays a crucial role in ozone depletion. However, many of the physical and chemical properties of ClONO 2 at the air−water interface or in bulk water are unknown or not understood on a microscopic scale. Here, the solvation and hydrolysis of ClONO 2 at the air−water interface and in bulk water at 300 K were investigated by classical and ab initio molecular dynamics (AIMD) simulations combined with free energy methods. Our results revealed that ClONO 2 prefers to accumulate at the air−water interface rather than in the bulk phase. Specifically, halogen bonding interactions (ClONO 2 )Cl•••O(H 2 O) were found to be the predominant interactions between ClONO 2 and H 2 O. Moreover, metadynamics-biased AIMD simulations revealed that ClONO 2 hydrolysis is catalyzed at the air−water interface with an activation barrier of only ∼0.2 kcal/mol; additionally, the difference in free energy between the product and reactant is only ∼0.1 kcal/mol. Surprisingly, the near-barrierless reaction and the comparable free energies of the reactant and product suggested that the ClONO 2 hydrolysis at the air−water interface is reversible. When the temperature is lowered from 300 to 200 K, the activation barrier for the ClONO 2 hydrolysis at the air−water interface is increased to ∼5.4 kcal/mol. These findings have important implications for the interpretation of experiments.

Nanoporous graphene membranes with controllable pore size and chemical functionality may be one of the most desirable materials for water desalination. Herein, we investigate desalination performance of hydrogen-functionalized nanoporous graphene membranes. The charge values on hydrogen atoms (q H ) and carbon atoms at the pore rim are systematically adjusted. For q H > 0, the flow rate decreases as q H increases, whereas for q H < 0, the flow rate tends to increase first and then decrease with increasing q H , yielding a peak at ∼ −0.2 e. Moreover, nanopores with large dipole moments at the rim have little effect on the salt rejection. The calculated oxygen and hydrogen density maps, the potential of mean force for water molecule and salt ion passage through the nanopores, and the coordination number unveil the mechanisms underlying water desalination in nanoporous graphene. This work may inspire the design and improvement of two-dimensional membranes for water desalination.

The effects of water, formic acid, and nitric acid on N2 and N2O generation from NH2NO and NH2NO2 are studied using high-level quantum-chemical calculations. It is shown that the reaction barriers from isolated NH2NO and NH2NO2 are 35.5 and 38.1 kcal/mol, respectively. When the NH2NO and NH2NO2 reactions are examined in the presence of water, formic acid, or nitric acid, the energy barriers are different. The mechanisms of these reactions are revealed through reaction pathway calculations. The isomerization of intermediate HNNOH and HNNOOH molecules, which follows a group rotation mechanism, plays a key role in reactions of isolated NH2NO and NH2NO2. However, the presence of water, formic acid, or nitric acid changes the isomerization mechanism substantially. HNNOH or HNNOOH and the catalyzing molecule form a doubly hydrogen-bonded prereactive complex, which, in turn, facilities hydrogen atom migration (this is denoted as the hydrogen atom migration mechanism). This study demonstrates the feasibility of N2 or N2O generation from NH2NO and NH2NO2 in the presence of water, formic acid, or nitric acid.

Chemical processes involving chlorine nitrate (ClONO2) at the surface of stratospheric aerosols are crucial to ozone depletion. Herein, we show a reaction route for the formation of Cl2O, which is a source of stratospheric chlorine, in the ClONO2 + HOCl reaction at the air–water interface. Our ab initio molecular dynamics (AIMD) simulations show that the (ClONO2)­Cl···O­(HOCl) halogen bond plays a key role in the reaction and is the main interaction between ClONO2 and HOCl both at the air–water interface and in the bulk liquid water. Furthermore, metadynamics-based AIMD simulations reveal two pathways: (i) The OCl fragment of HOCl binds to the Cl atom in ClONO2, resulting in the formation of Cl2O and NO3 –. Simultaneously, the remaining hydrogen atom is transferred to a water molecule to form H3O+. (ii) HOCl acts as a bridge for Cl atom transfer from ClONO2 to the O atom of a water molecule, and this water molecule transfers one of its H atoms to another water molecule, forming two HOCl molecules, NO3 –, and H3O+. Free-energy calculations show that the former is the energetically more favorable process. More importantly, the free-energy barrier for Cl2O formation at the air–water interface is only ∼0.8 kcal/mol, and the reaction is exothermic. These findings provide insights into the importance of fundamental chlorine chemistry and the broader implications of the aerosol air–water interface for atmospheric chemistry.