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.