Biomedical Engineering Reference
In-Depth Information
For acidic dissolution of apatites, net-reaction (7.5) represents the
direct chemical interaction between the surface of solid apatite and
an acidic solution. However, Pearce suggested that, in principle, the
dissolution process might also happen according to net-reaction
(7.6) followed by a chemical interaction in the solution among the
ions of apatite and acid near the crystal surface [95]:
5Ca
2+
4
3−
−
−
+
2+
4
−
O (7.7)
In spite of a lack of the experimental techniques, able to follow
the detachment of single ions, a reasonable ionic description of
apatite dissolution as a system of chemical equations (7.1)-(7.4)
was created from the indirect measurements [91-93]. According to
this model, chemical dissolution of apatite occurs step by step on the
surface via intermediate formation of a mono-atomic layer of some
virtual compounds, like Ca
+ 3PO
+ F
, OH
+ 7H
= 5Ca
+ 3H
PO
+ HF, H
2
2
[91-93]. Both latter
substances are known as precursors of apatite formation during
chemical crystallization [3, 96]. Moreover, based on the sequence of
ionic detachment (first fluorine for FA or hydroxide for HA, followed
by calcium and afterwards by orthophosphate) it is suggested that
dissolution of apatites is always incongruent (non-stoichiometric) at
the atomic (ionic) level. However, a lack of the precise experimental
verification transforms the chemical model into a reasonable
hypothesis only.
To conclude this model, one may assume that some new
relations between the kinetically controlled and chemical models
might be established in future. Before the chemical model was
created, chemistry of apatite dissolution had been always described
by equations (7.5) or (7.6) + (7.7). For this reason, the kinetically
controlled model was also adapted for these equations. For example,
the overall rate constants for net-reactions (7.5), (7.6), and (7.7) are
still used for calculations, whereas they are probably equal to either
the lowest of the rate constants of (7.1)-(7.4) (the rate-determining
step) or some combination.
(PO
)
and CaHPO
3
4
2
4
7.2.6 Etch Pit Formation
A process of etch pit formation describes acidic dissolution of
apatite crystals containing structural defects (dislocations and
inclusions) [37-43, 87, 97-106]. A radius of dislocations outlets
(hollow cores) on apatite was calculated to be within 8.3-20 Å for
screw dislocations and 18-43 Å for edge ones [100]. The presence
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