Physicochemical characterization of variously packed porous plugs of hydroxyapatite: streaming potential coupled with conductivity measurements.
A homemade instrument was used for the measurement of the streaming potential, conductivity, and permeability of plugs packed in different densities with hydroxyapatite (HAP) particles at 25 degrees C and pH = 7.0 +/- 0.2. KCl solutions with ionic strength values in the range of 0.3-300 mM, equilibrated with HAP for 3 days, were forced to flow through the plugs. It was found that the particle volume fraction of the plug obtained from conductivity measurements was slightly higher than that obtained by weighing the solid. This suggested that, in addition to the volume of the solid itself, the volume of liquid trapped in the cavities of the particles does not contribute to the conductivity of the plug. The pH change recorded in the solution passed through the plug was attributed to the protonation/deprotonation of the HAP surface groups. Denser packing of the HAP crystallites resulted to higher surface conductivities. It was suggested that this trend was due to the easier interparticle ion transport in close-packed plugs. Considering zeta-potential, the values computed by neglecting surface conductivity were significantly underestimated, especially at low ionic strength values and at dense packing. More realistic values for the HAP zeta-potential were obtained taking into account the surface conductivity. These values were practically independent of the material packing during the plug preparation. Finally, the total surface conductivity was found to be limited behind the slipping plane of the electric double layer developed at the interface of HAP in contact with electrolyte solution.
Skartsila
Kyriaki
K
School of Science and Technology, Hellenic Open University, 262 23 Patras, Greece.
Spanos
Nikos
N
D017886:Durapatite / Q000737:chemistry*; D004553:Electric Conductivity; D004563:Electrochemistry; D008956:Models, Chemical; D016062:Porosity
D017886:Durapatite