Superficial Properties Of Calcite: Adsorption Model With The Orbital Control
Superficial properties of calcite: adsorption model with the orbital control
N. V. Nikolenko
Ukrainian State Chemical Technology University, Gagarin Av., 8, Dniepropetrovsk 49005,Ukraine
The forecasting of surface activity of substance is most perspective from comparison of adsorbate and adsorbent electronic structure. From this point of view chemical nature of substance can be considered as the minor factors. An orbitals symmetry, degree of orbital overlapping, MO energy and electronic density will be general parameters determining result of superficial interaction [1].
In the present work we compared adsorptive properties more 20 aromatic and heterocyclic nitrogen- and oxygen-containing hydrocarbons adsorbed in static conditions from water solutions on a surface of CaCO3. Utilized CaCO3 had crystal structure of calcite that was confirmed by radiographic researches.
It is established that indole, benzaldehyde, N, N-dimethylaniline, 3-methylaniline, 2-aminopyridine and methylnaphthalene are not adsorbed on calcite. The greatest meanings of adsorptive constants of the Langmure isotherm have a-naphthylamine (290 l/mol) and aniline (260 l/mol). Superficial activity of others investigated compounds can be arranged in a number: 1,2,3-azimidobenzene > acrylonitrile > 4-methylpiridine > quinoline > 1,3-dinitrobenzene, 2,4,6-trinitrophenol, benzamid, 2,6-dimethylpiridine, nitrobenzaldehide, 2,4-dinitrotoluene, a-naphthol, pyridine, cyanobenzene.
No correlations between adsorption and chemical composition, solubility or salvation properties of the investigated compounds were found. From the perturbation MO theory there is nothing surprising because the obtained data should be compared first of all with an energy and symmetry molecular orbitals of adsorbate and adsorbent [1].
As parameter of correlation between electronic and adsorptive properties of organic molecules it is offered to utilize the vertical ionization potentials and energies of vacant orbitals [2]. For approximation of vacant orbital energy we utilize the energy of electronic transferring HOMO ® LUMO.
It is found that for the majority of the investigated compounds (except for aniline, a-naphthylamine, 4-methylpiridine, 2,6-dimethylpiridine and quinoline) the correlation between LUMO energy and adsorption of their molecules is observed. It is offered the adsorption model including two types of interactions: 2-electron donor-acceptor interaction HOMO(adsorbent) ® LUMO(adsorbate) and 4-electron three-orbital interaction HOMO(adsorbent) + OMO(adsorbate) [3]. It is found that the molecules of all organic compounds adsorbed on CaCO3 and not submitting correlation of adsorptive constants with LUMO energies have MO with potentials of ionization 9.25±0.35 eV. The conclusion about strengthening of the chemical bond at concurrence of occupied MO energies of adsorbate and surface is made.
It was established that adsorption of organic amines is accompanied by blue shift of n®p* absorption bands. According to the numerous data, such shift testifies to participation of nitrogen in an intermolecular hydrogen bridge. However, if to compare shifts of bands for adsorbed amines on SiO2, Al2O3 and CaCO3, the displacement of n®p* bands of amines practically is not observed or does not exceed 100 cm-1. Apparently the atoms of nitrogen test approximately identical perturbation both on hydroxyl surfaces of oxides and for calcite. At the same time, for aniline, a-naphthylamine, 4-methylpiridine, 2,6-dimethylpiridine and quinoline the rather large change of energy p®p* benzene rings bands is established. It testifies to participation of their p-electrons in adsorptive interaction with calcite. This interaction is selective and defined by an individual electronic structure of molecules of adsorbates.
The obtained results allow assuming that the adsorption of polar organic compounds from water solutions on calcite is caused by weak chemical interaction. In formation of superficial bonds participate electrons both heteroatoms and aromatic rings of the molecules.
References
1. Hoffmann R. Solids and Surfaces: A Chemist’s View on Bonding in Extended Structures. – N.Y.: VCH Publishers.- 1988, 139 p.
2. Николенко Н.В., Верещак В.Г., Пархоменко Н.В.// Журн. физ. химии. 1994. Т.68. №2. С.301.
3. Nikolenko N.V., Smirnova H.S., Shmarova I.A.//Adsorp. Sci. Technol. 1999. V.17. N 10. P.827.
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