On The Interaction Mechanism Between Functionalized Carbon Nanotubes And Volatile Organic Compounds

Volatile organic compounds(VOCs)are a big family of pervasive carbonbased chemical pollutants.They are often discharged into ground-water system with the petrochemical and chemical waste.Due to their highly volatile nature,they can easily emit as gases into the air from aquatic systems.Certain VOCs play roles in the stratospheric photochemical reaction and ozone depletion,exacerbating the greenhouse effect.Besides,aromatic compounds have been confirmed to be highly toxic,teratogenic and carcinogenic,which can cause permanent diseases.Because of these negative effects,developing efficient detection and treatment techniques of detrimental VOCs has received extensive attentions all over the world.Although a variety of control technologies have been proposed and practiced,they focused on the adsorption of VOC molecules in gaseous status,which failed to eliminate the damage of VOC molecules radically.As an alternative,the early detection and removal of VOC molecules prior to their emission from aqueous solution may be more practical and reasonable.Motivated by this issue,carbon nanotubes(CNTs)were selected as the adsorbents of detrimental VOCs,benefitting from their high specific area.In order to explore the adsorption mechanism between VOC molecules and CNTs,molecular dynamics simulations were performed to systematically investigate the interaction behavior of five representative VOC species(methanol,acetone,ether,toluene and undecane)on CNTs.The results provide a deep understanding on the effects of chirality indices,diameter,hydroxylation and noncovalent functionalization on the recognition and adsorption.On the basis of extensive parametric studies and configuration analysis,a few important conclusions can be drawn as follows.(1)VOC molecules were adsorbed on pristine CNTs spontaneously.Different VOC species exhibit significantly different adsorption preferences on CNTs,which is dependent on the chirality indices and diameter of CNTs.The van der Waals interaction dominates the adsorption behavior of VOC molecules on both pristine and functionalized CNTs.Besides,the binding free energy calculations indicate that the adsorption affinity is positively correlated with the hydrophobicity of VOC molecules.For the same diameters,zigzag CNTs possess a higher adsorption affinity than the armchair ones.(2)Surface hydroxylation are significant for both adsorption morphology and adsorption affin-ity.The transition of the adsorption pattern of VOC molecules due to the introduction of hydroxyl groups was captured.Especially,the monolayer adsorption configuration of toluene and undecane molecules changes to a multilayered structure.The hydrogen-bond network near the adsorption region is also broken and then rebuilt for stabilizing the adsorption kinetics.The adsorption affinity of hydroxylated CNTs is inversely proportional to the concentration of hydroxyl groups.For the same diameters,armchair CNTs possess a higher adsorption affinity than the zigzag ones when hydroxyl-functionalized,which is opposite to the pristine case.(3)Four ss DNA oligonucleotides are induced to undergo a spontaneous conformational change and their sugar-phosphate backbones tend to wrap about CNTs into compact left-handed helices driven by the van der Waals interaction between ss DNA and CNTs.The left-handed helices are stabilized by the hydrogen-bond network within the sugar-phosphate backbone of ss DNA.In the DNA-CNT hybrids,the bases are compactly attached on CNTs sidewall via the π-π stacking interaction.Besides,the self-assembly behavior is also dependent on the chirality of CNTs.(4)Based on the DNA-CNT hybrids above,the adsorption of VOC molecules on DNA-CNT hybrids was systematically investigated.The electrostatic interaction between VOC molecules and ss DNA oligonucleotides provide additional driving force for the adsorption of VOC molecules except for the van der Waals interaction between VOC molecules and CNTs which is still predominant.The ss DNA oligonucleotides initially helically wrapped around the CNTs are forced to undergo obvious conformational changes,i.e.,decrease in pitch of ss DNA or elongation along the principal axis of CNTs.Besides,the binding free energy calculations indicate that the adsorption affinity of CNTs is weakened due to the presence of ss DNA,which is related to the bases of ss DNA.Aiming at the early detection and removal of VOC molecules prior to their emission from aqueous solution,the adsorption of five representative VOC species on pristine,hydroxylated and ss DNA-wrapped CNTs were systematically investigated by the virtue of molecular dynamics simulations.The effects of structure parameters of CNTs and functionalization methods on the adsorption of VOC molecules are emphatically probed.These results not only provide a detailed and direct insight into the current adsorption experiment of VOC molecules but also shed some lights theoretically on the design of functional adsorbent for the early removal of VOC molecules in aqueous solution.