Biomedical Engineering Reference
In-Depth Information
motif not found or even permissible in any of the crystalline calcium
orthophosphates [20].
More to the point, mechanisms of calcium and orthophosphate
ion association in aqueous solutions were elucidated by means of
quantum and classical molecular mechanics simulations [235]. A
special focus was dedicated to the role of the protonation state of
orthophosphate ions and depronation of the hydrogenorthophosphate
ions appeared to be necessary during crystal growth. According to
the simulation results, a triple ion [Ca
2+
2−
2+
2+
could
form in aqueous solutions and subsequently yield another triple ion
[Ca
··(HPO
)
··Ca
]
4
2+
3−
2+
+
by releasing a proton. The latter ion suggested
to be the smallest stable aggregate, which contains an entirely
deprotonated orthophosphate ion [235]. Some indirect experimental
evidences in support of this hypothesis have been obtained
recently [236]. Nevertheless, a verification of the aforementioned
structural models of ACPs will require details of the chemistry
and the processing conditions together with viewing techniques
at the atomic scale. Unfortunately, no additional information on
the structure of high-temperature ACP phases has been found in
the references. Thus, for want of anything better, one is forced to
assume, that except of water molecules and possible presence of
HPO
··(PO
)
··Ca
]
4
4
2−
ions, all the aforementioned is valid for ACPs presented in
plasma-sprayed coatings. Undoubtedly, this assumption should be
verified and corrected in future.
To conclude the structural part, determination of the specific
surface area of ACPs generally leads to surprisingly low numbers.
This might be related to the larger spherical associations of Posner's
clusters and the apparent hindrance to nitrogen adsorption of these
inner surfaces [189, 212].
2.3.6
Thermal Properties
All types of ACP (presumably, except of ones presented in plasma-
sprayed coatings) are thermally unstable and sustain neither
calcining nor sintering. For example, if precipitated ACPs are
heated, first of all, they lose water. Two types of water loss occur,
corresponding to loosely bound water molecules adsorbed on the
surface of ACP agglomerates and more strongly bound internal
water molecules, respectively. The first loss is essentially reversible,
whereas the second is mostly irreversible [213, 237]. Furthermore, in
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