Biomedical Engineering Reference
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crystallization of hydroxyl-rich areas of the amorphous phase to HA
[174]. Therefore, hydroxylated ACP regions were found to crystallize
more readily compared to the dehydroxylated ones. A heat of re-
crystallization of ACP to HA was calculated to be ~43 kJ/mol in still
another study [172]. Presumably, such variations are due to the
differences in the chemical composition of various ACP samples.
To conclude the thermal part, the results mentioned in this
section appear to be additional indirect confirmations of the fact
that ACP is not a single chemical compound but represents a special
class of calcium orthophosphate salts. Furthermore, as the first
approximation and for want of anything better, the first crystalline
product formed at heating of a specific ACP sample might become
an indicator of its type. Namely, if the first crystalline product was
β-TCP [197], the authors of that study dealt with an “amorphous
β-TCP.” Similarly, if the first crystalline product was α-TCP [147, 175,
176, 189], the authors of those studies dealt with an “amorphous
α-TCP,” etc. Certainly, this is a hypothesis only, which needs to be
investigated and/or verified both theoretically and experimentally
in future.
2.3.7
Amorphous-to-Crystalline Transformations in
Aqueous Solutions
As previously stated, in the vast majority of cases, ACPs are the first
solid phases to appear upon mixing of calcium- and orthophosphate-
containing aqueous solutions at pH > 7 and concentrations
sufficiently high to produce an immediate precipitation. The
spontaneous formation of ACPs is a kinetically driven process. A
rapid mixing of highly concentrated solutions creates sufficiently
strong stochastic interactions among the ions and they quickly
coalesce into irregularly coordinated highly hydrated clusters large
enough to separate from solution in a gel-like state before they had
a chance to be rearranged into orderly nuclei capable of growth
as crystals. This structural arrangement, however, is inherently
unstable. In addition to desolvating, ACPs kept in solution eventually
disappear, being supplanted by more stable crystalline phases such
as OCP or CDHA [20].
Watson and Robinson in their pioneering study on ACP [98] were
the first, who observed a transient nature of ACP when kept in contact
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