Atomistic insights on the adsorption of long-chain undecane molecules on carbon nanotubes: Roles of chirality and surface hydroxylation

The treatment of oily wastewater has been recognized as a global concern for decades, due to the severe envi-ronmental and ecological damage. Benefitting from their high specific area and strong hydrophobicity, carbon nanotubes (CNTs) have been employed as an effective adsorption and separation agent of the organic pollutant molecules. However, the underlying interaction mechanism and kinetics remain to be clarified. This work aims at investigating the adsorption behavior of the long-chain undecane molecules on both pristine and hydroxylated CNTs in terms of molecular dynamics simulations. Simulation results reveal that the equilibrium adsorption morphology is determined by the coupling competitions among undecane-CNT interaction, deformability of the long-chain undecane molecules, intermolecular interactions of undecane molecules and the local structural adaptation on the curved CNT surface. For both pristine armchair and zigzag CNTs, the adsorption affinity is found to be proportional to the CNT diameter. Moreover, zigzag CNTs show a stronger adsorption affinity than armchair ones. For hydroxylated CNTs, however, armchair CNTs are found to possess a higher adsorption af-finity. In addition, the originally monolayer adsorption configuration changes to a multilayered structure. Only some of the undecane molecules directly wrap around those unfunctionalized portion of the CNT surface. They serve as a substrate for the adsorption of the other undecane molecules. As a result, surface hydroxylation leads to the reduction of adsorption affinity. Simulation results and conclusions obtained in this work are able to offer molecular insights on the better filtration of long-chain molecules in oily wastewater.