Biomedical Engineering Reference
In-Depth Information
Nevertheless, in slightly alkaline aqueous solutions, ACP might
have a well-defined chemical composition. For example, there is a
finding that ACP slurries over the pH range 7.4-9.25 appeared to
have a nearly constant solution ion activity product of ~1.6 × 10
−25
when the solid-phase composition is postulated to be Ca
(PO
)
3
4
, i.e. when the local chemical unit in ACP is postulated
to have a Ca/P molar ratio of 1.45, with ~10% HPO
(HPO
)
l.87
4
0.2
4
2−
, but without
OH
ions [207]. Thus, in spite of the absence of a LRO, this relative
constancy in the composition over such a wide pH range indirectly
suggests that ACP should have some well-defined local chemical
units. However, at solution pH exceeding 9.25, ACP does not
appear to have a nearly constant solution ion activity product. This
breakdown in the solution constancy suggests that the solubility-
controlling ions of ACP are subtly dependent compositionally on the
preparation conditions. Perhaps, at solution pH > 9.25 the content
of HPO
−
4
2−
ions no more remains constant and gradually decreases
with pH increasing [128]. Furthermore, at more acidic pH = 6.9,
ACP precipitates with Ca/P molar ratios as low as 1.15 have been
reported [211]. These latter precipitates are extremely unstable and
rapidly change over into crystalline DCPD. Again, a term “amorphous
DCPD” has not been introduced in that study.
Even after a lyophilization, solution-matured, spheroidal ACP
solids still retain ~15% water by weight [197, 212]. A temperature
programmed description analysis by Sedlak and Beebe indicated that
the most part (~75%) of this retained water was tightly bound inside
the solid, while the rest was a more loosely held surface water with
different activation energies of 20.0 and 10.5 kcal/mole, respectively
[213]. These results suggest that ACPs do not completely desolvate
in solutions but remain partially hydrated with about 3 water
molecules per formula unit. Other researchers found that water
occurred in regions those were only loosely associated with calcium
cations in ACP [5]. Furthermore, when prepared from carbonate-
containing solutions, ACPs can readily incorporate carbonate
anions [35-40, 214, 215]. The amount of carbonate incorporated
at any given pH increases with solution carbonate concentration.
At a given concentration, carbonate uptake also increases with pH.
Incorporating carbonate into ACP does not affect the HPO
4
2−
content
but raises the Ca/P molar ratio. At physiological pH, the carbonate
content of ACP precipitated from solutions containing 30 mmol/l
carbonates is ~3% by weight [182]. These data suggest that ACP, if
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