Adsorption mechanisms investigation of methylene blue on the (001) zeolite 4A surface in aqueous medium by computational approach and molecular dynamics

The cationic methylene blue (CMB) and its pmtonated forms have been studied by computational tools to predict their adsorption behavior on zeolite 4A. The interaction nature has been exhaustively studied in terms of energy gap (Delta E-g), global reactivity descriptors, Fukui functions, adsorption energy, density of states and natural bond orbital analysis. It was found that the protonated CMB (N36N35) molecule is less stable, more electmphilic and very reactive. The highest contribution to the lowering of the stabilization energy (E-2) for all systems was mainly due to the intramolecular charge transfer from the lone pair of the nitrogen atoms as a donor orbital to pi* as an acceptor orbital. DFT calculations revealed that N36 and N35 site of CMB was faced with a lower energy gap (Delta E-g) and chemical hardness (eta) as well as a higher electron accepting power compared to other sites. Monte Carlo molecular modeling simulation (MCMM) confirmed the predominant of electrostatic interactions and p-p electron coupling. The studied compounds prefer to have configurations that facilitate p-p interaction, which is evident from the parallel orientation of CMB with zeolite surface. The dynamic parameters show that the adsorption of methylene blue is exothermic and spontaneous chemisorption.