Abstract
This study was on the influence of the mineral phase crystallite microstrain (CM) on the nature of the surface complex (SC) governing the metastable equilibrium solubility (MES) behavior of carbonated apatites (CAPs) in aqueous acidic media (0.10 M acetate buffers, with and without fluoride, 0.50 M ionic strength maintained with NaCl). The MES behavior of a set of four CAPs (synthesized at 85 °C by a precipitation method) of increasing CM and therefore of increasing MES (CAP4 > CAP3 > CAP2 > CAP1) was quantified. The following were the findings. For CAP1 and CAP2, the SCs deduced were Ca
10(PO
4)
6(OH)
2 and Ca
10(PO
4)
6F
2 for the nonfluoride and the fluoride cases, respectively. For CAP3 and CAP4, the SCs deduced were Ca
9.5(PO
4)
6OH or Ca
9.5(HPO
4)(PO
4)
5(OH)
2 and NaCa
9.5(PO
4)
6F
2 for the nonfluoride and the fluoride cases, respectively. These results together with that from an earlier limited study show that the Ca/P ratio of the SC decreases from 1.67 to 1.58 to 1.50 with increasing CM of the CAPs; this relationship inversely correlates with the chemistry of maturation of aqueously precipitated defective apatites. Also the SCs do not appear to exist as a continuous series and only a few SCs may account for the MES behavior over a wide range of CAP preparations.
Surface complexes governing carbonated apatite solubility behavior: with increasing crystallite microstrain, solubility increases and the surface complex formula changes from that of stoichiometric hydroxyapatite to those with decreasing Ca/P ratios.
*For comparison purposes, the CAP solubilities are at 50% dissolved and pH 5.7, and are expressed by the ion activity
product
=
(
a
Ca
)
10
(
a
PO
4
)
6
(
a
OH
)
2
.
**This formula is indistinguishable from Ca
9.5(HPO
4)(PO
4)
5(OH)
2.