Carboxylated carbon nanotubes can serve as pathways for molecules in sandwich-like two-phase organic-water systems

Molecular dynamics simulations of pristine and carboxylated (6,6) carbon nanotubes (CNTs) immersed in water-chloroform, water-formaldehyde and water-nitromethane mixtures were performed at ambient conditions. The selected co-solvents differed in their degree of polarity (chloroform < formaldehyde < nitromethane) and proticity (formaldehyde is a polar aprotic solvent, while nitromethane is a polar protic solvent). The hydrophobic character of the CNTs was altered via functionalizing one of the nanotube endings with three -COO- groups, or by decorating both endings with three -COO- groups on each side. The immiscibility of water with chloroform and nitromethane leads to a sandwich-like liquid-liquid structure of the solvent mixture. In both cases the pristine CNT preferred to be fully burred inside the organic phase, while partial or complete carboxylation of the CNT openings oriented the nanotube in such a way as to form a channel-like pathway through the organic layer (i.e. -COO- groups touching aqueous phase and the hydrophobic part immersed in the organic phase). Since the nonpolar chloroform molecules were too large to enter the interior of the (6,6) CNT, the nanotube presented a pathway for water transport between the layers. In the case of water-formaldehyde mixture (no phase separation) the hydrophobic character of the CNT was the only decisive factor for the preferential solvation of the tube. Here, -COO- groups were predominantly hydrated, while formaldehyde molecules preferentially solvated the hydrophobic parts of the CNTs exterior and interior. (C) 2019 Elsevier B.V. All rights reserved.