Molecular perspective on charge-tunable adsorption of volatile organic compounds on carbon nanotubes.

Volatile organic compounds (VOCs) have been identified as highly toxic and carcinogenic pollutants threatening both human health and the living environment. Their recognition and removal are important issues. Carbon nanotubes (CNTs) have been proposed as a promising agent for the adsorption of detrimental VOC molecules. Due to the intrinsic nanoscale nature of such processes, details of molecular interactions and the adsorption mechanism remain to be clarified. This paper aims to provide a molecular perspective on the adsorption behavior of VOC molecules on both neutral and electrically charged CNTs by the means of molecular dynamics simulations. Simulation results indicate a strong correlation between the adsorption affinity and hydrophobicity of acetone, ether, methanol and toluene molecules. VOCs possessing a higher hydrophobicity demonstrate greater adsorption affinity. The adsorption of toluene and ether molecules is quite stable around the CNT surface. In contrast, hydrophilic molecules such as acetone and methanol can only be unstably adsorbed. For neutral CNTs, the van der Waals interaction is responsible for the adsorption affinity. For electrically charged CNTs, however, electrostatic attraction or repulsion with the charged groups in VOC molecules significantly affects the adsorption behavior. As a result, the introduction of charges on the CNT surface can help to optimize the adsorption process of VOC molecules. Calculations on the potentials of mean forces support the same reasonings. Simulation results about acetone, ether, methanol and toluene clearly indicate that customized strategies are needed for precisely controlling the adsorption of different VOC molecules on CNTs. The results reported in this work should be helpful for the better development of sensing and removal systems of detrimental VOC molecules.