Comparative theoretical analysis on the adsorption of cationic and anionic dyes on metal iodides in water

This study focused on the comparative analysis of the adsorption of cationic safranine O (SF⁺) and anionic acid blue 25 (AB⁻) on (1 1 0) surface of magnesium, manganese, zinc, and nickel metal iodides using DFT and molecular dynamics (MD) simulation. The nature of the interactions has been thoroughly investigated by the HOMO/LUMO energy gap, global reactivity descriptors, Mulliken charge distribution, molecular electrostatic potential (MEP) map, adsorption energy, and natural bond orbital (NBO) analysis. The reactivity of the two dyes was compared based on the LUMO and HOMO energy levels. It was found that SF⁺ with a LUMO value of −0.991 eV and lower energy gap of 1.184 eV exhibits an electrophilic characteristic and high ability to be strongly adsorbed on the MI₂. However, AB⁻ exhibits a higher energy gap of 5.854 eV, indicating its lower reactivity compared to SF⁺. Mulliken charge distribution of the dyes and their MEP map also showed strongly negative and strongly positive sites. Subsequently, the stabilizing interactions of hyper-conjugation and charge delocalization have been evaluated. In addition, the MD simulation was employed to elucidate the mechanism of the dye’s adsorption on the adsorbent surfaces. The results suggest that the dyes are adsorbed on the four metal iodides in a close parallel position with less adsorption energy for SF⁺ compared to AB⁻. Finally, it was found that the Van der Waals forces are predominant in the adsorption process suggesting a physisorption mechanism in accordance with RDF analysis.