Quantum chemical calculations of the energy of adsorption of metal cations of second and third groups onto model ampholytic ion exchanger

Abstract

Ion exchangers of ANKB-50 type, in which iminodiacetic acid as the adsorbent fragments used are
promising materials for ion exchange chromatography. The object of the study are adducts of iminodiacetic
acid molecule and ions of metals of the second and third group of periodic table.
The energy adsorption of metal ions of the second and third groups of the periodic system. Influence
of water molecules on the energy of adsorption. The dependence of the adsorption energy on the charge of
the acid molecule. Changing the structure of the molecule during adsorption process. The establishment of
the quantum mechanism of adsorption and comparative analysis of the effects of adsorption of ions of
different metals.
Computer calculations of the molecular structure carried out for N-methyl-iminodiacetic acid
fragment simulating ion exchanger ANKB-50 by B3LYP method, based on density functional theory in the
basis 6-31G++(d, p)
It is found that all the above metal ions have positive bonding energy of the neutral molecule acid
and its anions. Adsorption of metal ions on the cation acids is really absend. The increase of negative charge
leads to an increase in bonding energy for all metal ions. The single-particle wave functions of adduct of acid and metal ion that are belonged to the second group show that between metal ions and acid occurs
predominantly ionic bond. Joining of the metal ion of the second group rotates carboxylic groups so that all
four oxygen atoms are close to the metal ion. For metals that are belonged to the third group bond is
characterized by a large contribution of covalent component. As a result, the metal ion contacts with three
atoms of oxygen. The presence of water molecules near adduct leads to increase energy of adsorption.
Calculations of binding energies of the ions of the second and third groups have shown that ion
exchangers based on iminodiacetic acid may be used for adsorption of these components from aqueous
solutions. The efficacy of these ion exchangers should increase with increasing pH of solution. The presence
of water as the solvent facilitates the process of adsorption because of the fact that it is an electron donor
molecule acid and increases the bond energy of metal ions with an acid. The chemical bond between the ions
of calcium (magnesium) and the acid has a pronounced ionic character, and for the ions of aluminum
(gallium) is characterized by the presence of a covalent component.