3.2. Adsorption energies in the gas and solvent
One of the most significant parameters to determine the strengths of
interactions is adsorption energies (Ead). As previously
mentioned, the adsorption energies in both gas and solvent phase
obtained from the Gibbs free energies (ΔGad), and these
values (shown in Table 2) for all interactions of CO molecule with CNT
and DNTs in the gas phase were calculated by
6-31G* basis set for optimizations.
Moreover, regarding solvation effects, adsorption energies in three
solvents (water, acetone, and cyclohexane) were calculated by PCM using
6-31G* basis set, and the results are illustrated in Table 2. It should
be noted that, the negative values of ΔGad in both gas
and solvent phase demonstrate that the interaction of CO molecule with
simple CNT-p (between -23.23 and -23.13 kcal/mol) and NCNT-d (between
-8.34 and -8.03 kcal/mol) was energetically favorable and the adsorption
process was effective. In the gas phase, the ∆Gad values
are between -23.13 and 30.41 kcal/mol for CNTs and between -8.30 and
45.58 kcal/mol for DNTs. These values are in the range of 18.14 and
27.49 kcal/mol for SCNT complexes, -8.30 and 45.58 kcal/mol for NCNT
complexes and 8.44 and 8.45 kcal/mol for BCNT complexes. Based on the
results, CO molecule has a decent adsorption on BCNTs in comparison to
other DNTs. In the solvent phase, all adsorption energies are between
-23.22 and 30.15 kcal/mol for CNTs and between -8.03 and 46.23 kcal/mol
for DNTs. So, the values of ∆Gad in solvent phase showed
that solvent increased the ability of simple CNTs to adsorbed CO
molecule. Also, climbing the values of ∆Gad in solvent
phase for all DNTs indicate that DNTs have become solvated by the
solvent molecules, and this matter affected their ΔGadvalues. In addition, the adsorption of CO molecule on the surface of CNT
and DNTs in cyclohexane needed less energy compared to the other
solvents (water and acetone) since the molecules of cyclohexane is
nonpolar, and their solvation effects are low.