Modeling of the functionalization of single-wall carbon nanotubes towards its solubilization in an aqueous medium

A theoretical study of the functionalization of some single-walled carbon nanotubes (SWCNT) is presented using density functional theory. The pristine SWCNT consists of a finite, open (5, 5) nanotube with all the dangling bonds at the tips saturated with hydrogen. The structural and electronic properties of the pristine tube, with formula C80H20, are compared to those of a SWCNT with a vacancy defect at the sidewall, providing insight into the reactivity induced by the presence of those defects. The nanotubes were functionalized with some organic molecules: (a) formic acid, as a model carboxylic acid, (b) aminotriethylene glycol, as a model amide, and (c) ethylenglycol, as a model of the corresponding polymer. We study the effects of functionalization on both the pristine SWCNT and the SWCNT with a vacancy at the wall. Structures and electronic properties (dipole moments, ionization potentials, electron affinities, electronegativities, chemical hardnesses and HOMO-LUMO gaps) of both pristine and functionalized nanotubes are calculated, as well as the charge transfer and the binding energies of the organic radicals to the nanotubes. Binding to defects is thermodynamically favorable. The electrical dipole moments increase with the functionalization, and this enhances the solubility of the nanotubes in water, as shown by the favorable changes in the free energies of solvation. This should improve the biocompatibility of the nanotubes and lower their toxicity