The Influence of Carbonate Ions on the Structure of the Electrical Double Layer at the Interface of Hydroxyapatite/Electrolyte Solution

The aim of the work was to investigate the changes in the double electrical layer at the hydroxyapatite containing different amount of carbonate ions/electrolyte solution. Besides, the main properties of the edl (electrical double layer), i.e. surface charge density and zeta potential were determined by electrophoresis measurements and potentiometer titration, respectively. The synthesized adsorbents were characterized by the following methods: X-ray diffraction, FTIR (Fourier Transform Infrared Spectroscopy), adsorption and desorption of nitrogen and scanning electron microscopy. The analyzed samples had different structures and particle sizes. It was proved that increase in the carbonate groups content is connected with the decrease of apatite grain sizes and crystallinity reduction. The characteristic parameters of the electric double layer were also different: pHpzc (point zero of charge) of hydroxyapatite was 6.5 whereas for carbonate apatite was higher – pHpzc = 8. In both cases determination of precise pHIEP (isoelectric point) was not possible but it is known that its value is lower than 4.