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Surface properties of various powdered hydroxyapatites.
L García Rodenas1, J M Palacios, M C Apella
1Unidad de Actividad Química, Centro Atómico Constituyentes, Comisión Nacional de Energía Atómica, Avenida General Paz 1499, 1650 San Martín, Provincia de Buenos Aires, Argentina.
Journal of Colloid and Interface Science
|June 21, 2005
Summary
The study measured electrophoretic mobilities of hydroxyapatite (HAP) nanoparticles, revealing surface complexation models accurately predict their behavior in solutions with varying pH and calcium concentrations. This offers insights into calcium phosphate interactions.
Area of Science:
- Materials Science
- Surface Chemistry
- Colloid Science
Background:
- Hydroxyapatite (Ca10(PO4)6(OH)2, HAP) is a crucial biomaterial with surface properties influencing its applications.
- Understanding the surface charge and ion interactions of HAP is vital for controlling its behavior in aqueous environments.
Purpose of the Study:
- To investigate the electrophoretic mobilities of synthetic and semisynthetic HAP powders with varying characteristics.
- To evaluate the suitability of Nernstian and surface complexation models in explaining HAP's surface behavior.
- To elucidate the role of pH, calcium concentration, and particle size on HAP surface charge.
Main Methods:
- Measurement of electrophoretic mobilities of HAP suspensions across a range of pH and calcium concentrations.
- Comparison of experimental data with predictions from Nernstian and surface complexation models.
- Utilizing Fourier-transform infrared spectroscopy (FTIR) to analyze surface phosphate interactions.
Main Results:
- A broad plateau of negative mobility was observed for HAP between pH 5-8, shifting to positive values with added calcium.
- A simple surface complexation model, considering Ca2+, PO43-, and OH- interactions, successfully reproduced the observed bell-shaped mobility profile.
- Nanoparticulate HAP exhibited faster ion exchange, suggesting it may not reach solubility equilibrium within experimental timescales.
Conclusions:
- Surface complexation models provide a superior framework for understanding HAP's surface charge and electrophoretic mobility compared to Nernstian models.
- The model successfully explains HAP's behavior and is applicable to other calcium phosphates like tricalcium diphosphate.
- Surface phosphate protonation is a key mechanism governing HAP's surface charge, as confirmed by FTIR analysis.