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Molecular simulation methods were applied to study the effect of hydrophilicity on CO2/CH4separation using carbon nanotube (CNT) membranes. CNTs with a diameter of ~1 nm were functionalised by varying amounts of carbonyl groups, in order to achieve various hydrophilicity. The presence of –CO groups inside the CNT allow a significant gain in the diffusion selectivity of CO2, while in contrast the adsorption selectivity is hardly changed. The corresponding permeation selectivity increases as the hydrophilicity of the CNT-based membrane increases. However, the permeability of CO2decreases due to a combination of the intermolecular interactions between the gas and functional groups and the steric effects of the added functional groups. Considering both the permeation selectivity and permeability, it was found that the maximum separation performance is achieved in a certain hydrophilic CNT membrane. Moreover, the separation performance of hydrophilic CNTs for CO2/CH4mixtures breaks the Robeson upper bound. [ABSTRACT FROM AUTHOR]

The diffusion of a CO2/CH4mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO2and CH4are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO2and CH4molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO2in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO2and CH4in the pores. For CO2, the interaction energies (CO2–CO2and CO2–CNT) are larger than that of CH4in all cases. But only a very small difference in the diffusion coefficient was observed between CO2and CH4. Temperature has a negligible effect on the difference between diffusion coefficients of CO2and CH4in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO2/CH4in CNT bundle membranes. [ABSTRACT FROM AUTHOR]

Abstract: Molecular dynamics simulations were performed to study the hydration of Na+ inside carbon nanotubes (CNTs) under external electric fields from 0 to 0.1V/nm. The simulation results show that the hydration of Na+ varies with the electric field intensity (E). The coordination number in the first hydration shell of Na+ increases with the increase of E. The analysis of orientation distributions for water dipole shows the Na+ hydration is strengthened when the direction of external field is aligned with the electric field generated by Na+, and is weakened when these are opposite. When E is aligned with the electric field of Na+, the strength of hydration is non-monotonic as E and reaches its maximum when E is equal to 0.01V/nm. This finding provides some guidance for the application of electric field in ion-transporting channels. [Copyright &y& Elsevier]