Adsorption of methylene blue cationic dye onto brookite and rutile phases of titanium dioxide: Quantum chemical and molecular dynamic simulation studies

This study reports the statistical physics modeling of the methylene blue cationic dye (MBCD) adsorption in aqueous solution on TiO2 surface. In order to comprehend the adsorption performance, theoretical investigations were done to collect extensive information about the chemical and physical properties of the molecules using quantum chemical parameters (QCP). Also, molecular dynamic and quantum descriptors were applied to better understand the dye adsorption on different phases of TiO2 (i.e., bmokite and rutile). The molecular structures of MBCD acts as good electron donators to an empty d orbital of TiO2 due to the availability of electrons in both the lone pair on the N atoms and 't electrons of the aromatic rings. The reactivity of optimized molecular structures was confirmed by the quantum chemical descriptors. Modeling results showed that the adsorption was performed with a horizontal position of MBCD molecules on both surfaces. Overall, the TiO2 showed the highest adsorption energies for protonated MBCD (N36N35) (-886.533 kcal/mol (MBCD/rutile-TiO2)) > -626.428 kcal/mol (MBCD/brookite-TiO2)), which implies that these sites prompt the adsorption on both surfaces. The analysis of the adsorption energy values showed an exothermic chemisorption process, which could be governed by steric parameters according to the results obtained with the molecular dynamics modeling.