Intercalation of metal–aluminum layered double hydroxides with anionic surfactants: Experimental and density functional theory studies

Abstract

Metal–aluminum layered double hydroxides (LDHs) intercalated with bis(2-ethylhexyl)sulfosuccinate (EHS) and dodecyl sulfate (SDS) anions, formulated as EHS-MgAl-LDH 1, SDS-MgAl-LDH 2, EHS-CaAl-LDH 3, and SDS-CaAl-LDH 4, were synthesized using the co-precipitation method. The compounds were characterized using Fourier transform infrared spectroscopy, x-ray diffraction, transmission electron microscopy, scanning electron microscopy, and thermogravimetric analysis. The textural properties were studied using the Brunauer–Emmett–Teller method. The density functional theory method was used to perform computational calculations of the surfactants (EHS and SDS) and the Pb(II)-surfactant bonds (EHS-Pb and SDS-Pb) in the layered double hydroxides. Natural bond orbital (NBO) calculation for the investigation of the stabilization energy and charge transfer performed on the compounds showed significant donor–acceptor NBO interactions between π^*C7–O8 → σ^*C7–O8, LP(3)O13 → π^*C12–O14, LP(3)O3 → σ^*S1–O5, and π^*S1–O3→ σ^*S1–O2 natural bond orbitals, having second-order stabilization energies of 285.84, 92.84, 30.78, and 447.27 kcal/mol, respectively. The observed highest and least perturbation energies within the compounds are observed to occur between LP(3)O13 → π^*C12–O14 and π^*S1–O3 → σ^*S1–O2 interacting orbitals with stabilization energies of 92.84 and 447.27 kcal/mol, respectively. The HOMO–LUMO energy gap results showed an increase in the energy values as the surfactant interacts with the Pb(II) ions. This increase in the energy gap is responsible for the stability of the surfactant–metal complexes and demonstrates the potential of the surfactant for the removal of the heavy metal ions in solution. The compounds were used as adsorbents for the removal of lead(II) ions from wastewater.