Metastable equilibrium solubility of carbonated apatites in the presence of solution strontium and fluoride

The understanding of the physicochemical properties of biominerals is prerequisite to the understanding and treatment of certain pathological conditions (i.e., dental caries and osteoporosis). The present work uses synthetic carbonated apatites (CAPS) as model compounds for biominerals and investigates the anomalous solubility behavior of these compounds (termed metastable equilibrium solubility (MES)) in the presence of solution strontium and the combination of solution strontium and fluoride.; First, the MES behavior of a moderately crystalline CAP was examined in the presence of solution strontium. Important findings: The solubility behavior of the CAP was best described by a surface complex with a stoichiometry of Ca6Sr4(PO4)6(OH)2 when 60% or more of the solution calcium present in dissolution media was replaced with an equal molar concentration of strontium and by a stoichiometry of hydroxyapatite (HAP: Ca10(PO4)6(OH) 2) when 40% or less of the solution calcium was replaced by an equal molar concentration of strontium.; Next, the MES behavior of a moderately crystalline CAP was examined in the presence of solution strontium and fluoride. Important findings: The solubility behavior of the CAP was best described by a surface complex with a stoichiometry of Ca6Sr4(PO4)6F2 when 60% or more of the solution calcium present in dissolution media was replaced with an equal molar concentration of strontium and by a stoichiometry of fluorapatite (FAP: Ca10(PO4)6F2) when 40% or less of the solution calcium was replaced by an equal molar concentration of strontium.; Finally, solution strontium was used to probe the MES behavior of a high-crystallinity CAP (with properties similar to dental enamel) and a low-crystallinity (with properties similar to bone mineral). Important findings: (a) When 60% or more of the solution calcium present in dissolution media was replaced by solution strontium, the solubility behavior of the of the CAPS were best described by surface complexes with stoichiometries of Ca6Sr 4(PO4)6(OH)2 for the high-crystallinity CAP and Ca7Sr2(HPO4)(PO4) 5OH for the low-crystallinity CAP; (b) when 40% or less of the solution calcium was replaced by solution strontium, the stoichiometries best describing the MES behavior were HAP for the high-crystallinity CAP and calcium/hydroxide deficient (defective) hydroxyapatite (DHAP: Ca9(HPO4)(PO 4)5OH) for low-crystallinity CAP.