Comparative study of cationic Nile blue and anionic methyl orange dyes adsorption in water on the (110) surface of metal chlorides by DFT and MD approaches

Abstract

Computational studies using density functional theory (DFT) and molecular dynamics simulations (MD) provide a valuable way to evaluate the removal of dyes by different adsorbents. In this study, Nile blue (with a positive charge; NB+) and methyl orange (with a negative charge; MO−) were used to examine the adsorptive performance of metal chlorides MCl2 (where M = Mg, Mn, Zn or Ni) from water. The calculated gap energies of NB+ and MO− were 5.333 and 2.395 eV, respectively, suggesting that the NB+ molecule is more reactive, electrophilic and can adsorb strongly on the MCl2 surface compared to MO−. Frontier molecular orbitals, Mulliken charge and an electrostatic potential map were used to explain the redox reactive sites of the investigated molecules. In addition, possible donor–acceptor interactions of molecules were assessed by natural bond orbital analysis. The negative values of back-donation energy ΔEback-donation for both dyes reflected that back donation for both dyes was energetically favorable. Furthermore, the most stable energy configuration of NB+ and MO− adsorption on MCl2 was determined by MD simulations. These simulations showed that both dyes can be adsorbed onto the MCl2 (110) surface in a nearby horizontal position. The radial distribution function peak values for all systems occurred at distances greater than 3.5 Å, indicating that the dyes were physically adsorbed onto the MCl2 surfaces through van der Waals interactions. The negative values of adsorption energy for both studied dyes indicated spontaneity of the adsorption process. Furthermore, MD simulations revealed that the adsorption energies between NB+ and MCl2 were much higher than between MO− and MCl2.