Entrance resistance of water transport into carbon nanotubes: Insights from molecular dynamics simulations

Ion Coupling, Bonding, and Transfer in Narrow Carbon Nanotubes

Narrow carbon nanotubes (nCNT) are unique mimics of biological channels with water-ion selectivity attractive for applications such as water purification and osmotic energy harvesting, yet their understanding is still incomplete. Here, an ab initio computation is employed to develop the full picture of ion transfer in nCNT including specificity and coupling between ions. The thermodynamic costs of ion transfer are computed for single ions and ion pairs and used to evaluate different local coupling scenarios including strong (pairing) and weak (free-ion) coupling as well as "electroneutrality breakdown" (EB), possible for cations only due to their chemisorption-like interaction with nCNT. The results also indicate that nCNT behaves as a highly polarizable metal-like shell, which eliminates the dielectric energy when CNT accommodates coupled cation and anion. This allows facile computation and comparison of the full transfer costs, including translation entropy, for different ions in different coupling modes to identify the dominant regime. EB transfer appears most favorable for K+, while anions strongly favor transfer as pairs, except for chloride which favors weak coupling and, at neutral pH, transfers as a trace ion coupled to both cation and OH-. The results demonstrate that, in general, observed ion permeation and conduction in nCNT, especially for anions, reflect a complex ion-specific and composition-dependent interplay between different ions.

Impact of Impure Gas on CO2 Capture from Flue Gas Using Carbon Nanotubes: A Molecular Simulation Study

We used a grand canonical Monte Carlo simulation to study the influence of impurities including water vapor, SO₂, and O₂ in the flue gas on the adsorption of CO₂/N₂ mixture in carbon nanotubes (CNTs) and carboxyl doped CNT arrays. In the presence of single impure gas, SO₂ yielded the most inhibitions on CO₂ adsorption, while the influence of water only occurred at low pressure limit (0.1 bar), where a one-dimensional chain of hydrogen-bonded molecules was formed. Further, O₂ was found to hardly affect the adsorption and separation of CO₂. With three impurities in flue gas, SO₂ still played a major role to suppress the adsorption of CO₂ by reducing the adsorption amount significantly. This was mainly because SO₂ had a stronger interaction with carbon walls in comparison with CO₂. The presence of three impurities in flue gas enhanced the adsorption complexity due to the interactions between different species. Modified by hydrophilic carboxyl groups, a large amount of H₂O occupied the adsorption space outside the tube in the carbon nanotube arrays, and SO₂ produced competitive adsorption for CO₂ in the tube. Both of the two effects inhibited the adsorption of CO₂, but improved the selectivity of CO₂/N₂, and the competition between the two determined the adsorption distribution of CO₂ inside and outside the tube. In addition, it was found that (7, 7) CNT always maintained the best CO₂/N₂ adsorption and separation performance in the presence of impurity gas, for both the cases of single CNT and CNT array.

Pressure-driven water flow through a carbon nanotube controlled by a lateral electric field

Tuning the water flow through nanochannels provides a key to many physicochemical phenomena, such as energy harvesting, desalination, biosensors and so on.