Biomedical Engineering Reference
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consist of ions of either calcium or orthophosphate. However, the
edges parallel to the
−
c
-axis, in principle, may consist of X-ions (F
,
OH
). The last case is the simplest one. According to the chemical
model [91-93], dissolution of apatite starts with protonation
and detachment of X-ions, followed by removing of calcium (one
should remind here that “walls” of the channels where X-ions are
located consist of Ca(2) ions [87]) and afterwards by protonation
and removing of orthophosphate. In the case, when the edges and
corners of apatite mainly consist of calcium, again, according to the
chemical model, detachment of weakly bounded calcium precedes
protonation and removing of orthophosphate. Finally, if the edges
and corners mainly consist of orthophosphate, the dissolution
will start by chemical interaction of protons with these weakly
bounded orthophosphate groups. The latter decreases electrostatic
attraction forces between orthophosphate anions and neighboring
calcium cations (attraction forces between Ca
−
, Cl
−
2+
4
3−
and PO
are
2+
evidently stronger when compared with those between Ca
and
). These forces become still weaker after adsorption of the
second proton onto the given orthophosphate anion, causing its
transformation to H
HPO
4
2−
4
−
PO
followed by detachment.
2
7.7.2 The Influence of Dislocations and Surface Defects
Before now, no data on crystal defects have been used. For this
reason, the above description is valid for dissolution of the perfect
single crystals without defects and with the molecularly smooth
surface. However, like other solids, crystals of apatite always contain
both surface irregularities (e.g., steps, missing ions, and dislocation
outlets) and structural defects inside the bulk of crystals (e.g.,
dislocations and inclusions). The schematic depiction of a crystal
surface is shown in Fig. 7.5. This model is known as the “Kossel
model” [151]. In principle, any surface irregularities may act as
dissolution nuclei because sometimes even one missing ion might
become the critical nuclei (the polynuclear model) [55, 56].
Each dislocation outlet on apatite was found to be a hollow core
with radius within 8.3-20 Å for the screw dislocations and even
18-43 Å for the edge ones [100]. Dimensions of the hexagonal unit-
cell of apatite are:
≈
≈
6.88 Å (Table 1.3). If the numeric
values for the dislocation outlets were calculated correctly, their
a
9.43 Å,
c
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