On the adsorption of volatile organic compounds on hydroxyl-functionalized carbon nanotubes in aqueous solution

Volatile organic compounds (VOCs) are among the primary concerns due to their practical threatening on living environment and human health. This work aims at examining the physical adsorption behavior of four representative VOC species on hydroxyl-functionalized CNTs in terms of molecular dynamics simulations. Simulation results indicate that the adsorption affinities of these four VOC species follow the order of toluene > ether > acetone > methanol for both the unfunctionalized and hydroxyl-functionalized CNTs. This adsorption preference is endorsed by the binding energy calculations based on the umbrella sampling algorithm. The surface modifications due to the introduction of hydroxyl groups affect both the adsorption mechanism and equilibrium configuration. The hydrogen-bond network near the adsorption region is rebuilt for stabilizing the adsorption kinetics. Compared to the dominant van der Waals interaction, the electrostatic interaction between the charged hydroxyl groups and the VOC molecules are found to be much less significant. With the increase of hydroxyl concentration, the adsorption affinity decays for all four VOC species. This conclusion is confirmed by both the binding free energies and the number of successfully adsorbed VOC molecules. The fundamental mechanism is due to the reduction of the originally hydrophobic area of CNTs surface and the additional bonding competition from water molecules. This work presents a molecular insight on the interaction between VOC molecules and hydroxyl-functionalized CNTs in aqueous solution. It can also benefit the development and design of smart filtration devices of VOC molecules for their early removal in aqueous solutions.