Comprehensive DFT and NBO study of hydralazine adsorption on pristine and COOH-functionalized SWCNTs

• DFT analysis revealed that functionalized SWCNTs demonstrate a more negative adsorption energy than pristine SWCNTs, indicating stronger and more stable binding. • Natural Bond Orbital (NBO) analysis revealed that functionalization promotes stronger donor-acceptor interactions between Hydralazine and the carbon nanotube surface. • Atoms in Molecules (AIM) calculations indicated that partial covalent interactions dominate in the adsorption complexes, with functionalized systems showing enhanced electron density at bond critical points. • The functionalization of carbon nanotubes with carboxyl groups reduces the energy gap and global hardness of the complex, leading to increased electronic reactivity and interaction propensity. • Thermodynamic analysis demonstrated that both pristine and functionalized SWCNT adsorption processes are spontaneous and exothermic, with the functionalized system exhibiting a more favorable Gibbs free energy change. The adsorption behavior of the vasodilator drug Hydralazine on pristine and COOH-functionalized single-walled carbon nanotubes (SWCNTs) has been investigated in this study using density functional theory (DFT) at the M06/6-31G* level. The electronic and thermodynamic properties were calculated based on optimized structures using Gaussian 09 software. The bonding analysis revealed that hydrogen bonding between Hydralazine and functionalized SWCNT (f-SWCNT), modified by the addition of a carboxyl group at one end, exhibits stronger adsorption of Hydralazine (-18.167 kcal/mol) compared to pristine SWCNT (-7.228 kcal/mol). The HOMO-LUMO gap decreased from 0.516 eV (pristine) to 0.433 eV (functionalized), indicating enhanced electronic interactions. Natural Bond Orbital (NBO) analysis confirmed stronger donor–acceptor charge transfer for f-SWCNT, while Atoms in Molecules (AIM) and NCI-RDG analysis indicated cooperative hydrogen-bonding and weak covalent contributions to the adsorption mechanism. Thermodynamic results (ΔH < 0, ΔG < 0) showed that the adsorption processes are spontaneous and exothermic, and the formed complexes are stable at standard conditions. These findings suggest that while both pristine and functionalized SWCNTs can act as effective adsorbents for Hydralazine, COOH-functionalized SWCNTs demonstrate superior adsorption capacity and potential as promising nanocarriers for drug delivery applications.