Biomedical Engineering Reference
In-Depth Information
structure. Replacing the OH
with F
is thought to release the strain so that the
subsequent stabilization of the crystal structure reduces the solubility of the solid
phase. Another likely cause of structural stabilization is that the replacement of
the polar group OH
by the nonpolar group F
affects the local charge balance.
The solubility of ACP is not shown in Fig.
3.1
a. However, as mentioned above, the
phase transformation from ACP to HAP has been observed. Therefore, at least near
a neutral pH, ACP has a higher solubility than HAP. Although the solubility of all
calcium phosphates rapidly increases with decreasing pH, the rate of increase differs
greatly depending on the type of salt. Compared to HAP, OCP, and TCP, the pH-
dependence of solubility for DCPA and DCPD is relatively small. For this reason,
the solubility of HAP exceeds that of DCPA at pH
4.5, and DCPA becomes the
most stable phase. The form of the solubility curves for each calcium phosphate in
Fig.
3.1
a, as well as the pH at which DCPA becomes the most stable phase, vary to
some extent when there are ionic species other than calcium and phosphoric acid in
the solution. However, it is thought that HAP is the most stable at a near-neutral pH
and that DCPA or DCPD is the most stable at a low pH. However, questions have
been raised that challenge this idea [
25
].
The conventional method for measuring solubility is to inject an excess amount
of the solid phase of the target calcium phosphate salt into solution, and after
sufficient stirring, measure the ionic concentrations and calculate the equilibrium
concentrations. Pan and Darvell pointed out a shortcoming in this method—
solid calcium phosphate in solution undergoes incongruent dissolution, which in
turn triggers a phase transformation that results in large errors in the calculated
equilibrium concentrations. They measured the solubility of HAP using a unique
solid-titration method and found that it was considerably lower than conventionally
reported values (Fig.
3.1
b). They also measured the solubility of other calcium
phosphate salts using the solid-titration method and compared the results. They
concluded that HAP represented the most stable phase for pH
4.2 and not DCPD,
as was generally believed (Fig.
3.1
c). In explaining why DCPD was believed to
be the most stable phase, they mentioned the stability of DCPD and the nucleation
frequency and concluded that DCPD is far more likely to nucleate than HAP at low
pH. In titration experiments using solid DCPD, the only phase observed at a pH of
3.60 or 4.50 was DCPD; however, when a minute amount of HAP was added to this
equilibrium state, the DCPD disappeared and highly crystalline HAP formed at a
pH of 3.30, 3.60, and 4.47.
3.3
Crystal Growth Mechanisms in an Aqueous Solution
System: Historical Review
Given that the growth of HAP occurs in an aqueous solution environment, the
history of research on the mechanisms of crystal growth in aqueous solutions will
be reviewed before the growth mechanisms of HAP are discussed.
