Adsorption and structural properties of hydroxy- and new lacunar apatites

Design of novel phosphorus apatites, K2Pb6Zn2(PO4)6 and K2Pb4Ca4(PO4)6, zinc hydroxyapatite Ca8Zn2(PO4)6(OH)2, as well as hydroxyapatite Ca10(PO4)6(OH)2, and its corresponding calcined powder were conducted by means of two different synthesis protocols, i.e. solid state and wet chemical methods. The prepared materials adopt P63/m (No 176) as a space group. Structural refinements for K2Pb6Zn2(PO4)6 and K2Pb4Ca4(PO4)6 lacunar apatites, and that for the calcined hydroxyapatite, were performed using Rietveld method. For K2Pb6Zn2(PO4)6, the refinement results showed that the majority of Pb2+ cations occupy (6h) sites. This study examines the impact of the synthesis conditions on the physico-chemical properties and on the surface reactivity of the prepared materials. The analysis of the interactions of the samples with respect to a protein model (Bovin Serum Albumin), evaluated under the same experimental conditions, revealed a fast-kinetic process for the precipitated samples, compared to the crystals obtained by solid chemical reaction. Thus, the equilibrium conditions were reached in less than 20 min for the former while 6 h of contact were requested for the latter. However, all the obtained isotherms show a Langmuirian shape. The difference in the adsorption parameters of the materials, i.e. constant affinity and amount adsorbed at saturation, is discussed in terms of surface characteristics, chemical composition, crystals size and specific surface area. It is concluded that the interaction with the Bovin Serum Albumin (BSA) molecules is highly sensitive to the microstructure of the crystals.