Inhibitory mechanism of mimosa tannin using molecular modeling and substitutional adsorption isotherms

A semiempirical approach involving quantum chemical calculations was used to rationalize the mimosa tannin inhibitory action. The molecular spatial structure, atomic charges, dipole moment, HOMO-LUMO energy gap and HOMO density of four types of flavanoid units that constitute mimosa tannin, have been theoretically investigated using HyperChem 5.11 computer package. Optimization of the molecular geometry yielded a non-planar structure with the electron charge density distributed non-uniformly. The HOMO-LUMO energy gap was used to determine the electron donating ability of the molecule. The highest values of the HOMO density was found in the vicinity of the functional groups indicating them as most probable adsorption centers. A schematic representation of the different modes of adsorption of mimosa tannin is presented. The projected molecular area for a particular molecular orientation was compared to the area occupied by X water molecules displaced from the surface in the process of adsorption. X was determined from the Flory-Huggins, Dhar-Flory-Huggins and Bockris-Swinkels substitutional adsorption isotherms applied to the data obtained from the gravimetric experiments performed on low carbon steel specimen in 0.1 M H2SO4 solution at 30+/-1 degreesC. The value of X approximate to 4, and most probable vertical orientation tannin molecules in the adsorbed state were deduced from these considerations. (C) 2002 Elsevier Science B.V. All rights reserved.