Effects Of The Adsorption Of Ba2+ Ions On The Electrical Double Layer On The 4th Group Metal Oxide/electrolyte Interface
EFFECTS of the ADSORPTION of Ba2+ IONS on the ELECTRICAL DOUBLE LAYER on THE 4th GROUP METAL OXIDE/ELECTROLYTE INTERFACE
W. Janusz, A. Gałgan, E. Skwarek
Department Radiochemistry and Colloid Chemistry Maria Curie Sklodowska University Pl. M.C. Sklodowskiej 3; 20-031 Lublin, Poland
Fig. 1. Adsorption density of Ba2+ ions from solution of initial concentration 1*10-5 mole/dm3 at the metal dioxide/0.01 NaCl interface as a function pH.
Specific adsorption of Ba2+ ions at the titanium dioxide (rutile and anatase structure), zirconium and hafnium dioxide was investigated in by means of radioisotopic method as a function of Ba2+ and NaCl ions concentration and pH. The initial concentration of Ba2+ ions was ranged from 5*10–6 to 0.1 mole/dm3, pH was ranged from 3-10. As a background electrolyte NaCl solution was used at concentration 0.1, 0.01 and 0.001mole/dm3. Beside the adsorption measurements the potentiometric
titration of suspensions (to determine of surface charge) and electrophoretic measurements were carried out. The adsorption of Ba2+ as a function of pH has typical for cations "adsorption edge"[1,2]. It is characterized by pH at 50% ions adsorbs - pH50% and range pH where adsorption changed from 10 to 90% - pH10-90%. Adsorption density of Ba2+ions at the interface of 4th groups of metal dioxide/electrolyte interface is depicted on Fig1.
Adsorption edge of Ba2+ on the pH scale has the following sequence TiO2(anatase) @ TiO2(rutile)
A comparison of the data depicted on Figs 1 and 2 leads to the conclusion that specific adsorption of Ba2+ causes the increase the density of negatively charged groups. The decrease of surface charge at the zirconium dioxide/electrolyte interface begins at higher pH due to the appropriate position of adsorption edge at pH scale (Fig.1).
Fig. 2. Surface charge density at the TiO2(Rutile)/aqueous solution of Ba2+ - 0.001 mole/dm3 NaCl.
The electrophoretic measurement show that Ba2+ ions at low initial concentration cause the decrease of z potential, but with increase of concentration the CR2(CR-charge reversal) is observed at the z potential-pH dependence. This point was observed for the systems where hydrozable ions were present[1]. Presence of CR points in the system with nonhydrozable ions supports Schindler opinion that the CR may result of the overcharging of compact part of the electrical double layer[1].
References
1. Robertson A.P., Leckie J.O., Cation binding predictions of surface complexation models: Effects of pH, ionic strength, cation loading, surface complex and model fit. J Colloid Interface Sci., 188, 444 (1997).
2. Brown G.E., Parks G.A., and O’Day P.A., in Mineral Surfaces, Vaughan D.J., and Pattrick R.A.D. Eds, Chapman & Hall, London 1995, p.129.
3. James R. O. and Healy T. W., J. Colloid Interface Sci., 40, 42, (1971).
4. Schindler P.W., In: Adsorption of Inorganic at Solid/Liquid Interfaces. Ann Arbor, MI: Ann Arbor Sci., 1981, pp 1-49, 1981.
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