Biomedical Engineering Reference
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below. It is just assumed that dissolution of apatite occurs in
acid H
n−
is an anion.
5. Only soluble compounds are formed during the apatite
dissolution in acids. No precipitates are obtained.
6. Hydration effects for all the ions and molecules involved as
well as those for crystal surface of apatite [90, 131-134] are
omitted for simplicity.
7. All crystal faces of a solid apatite are equal and have a similar
ionic arrangement. Although, it was found that HA {001} facets
could be more resistant against acidic dissolution than {100}
ones [135], no specific influence of different crystal faces on
the dissolution process is considered.
8. Since the afore-described dissolution models have been
elaborated for micron-sized apatite crystals, while the
nanodimensional calcium orthophosphates possess the special
properties (chapter 2), to eliminate the size effects, dissolution
of large crystals is described. No specific influence of crystal
dimensions on the dissolution mechanism is considered.
Based on these assumptions and limitations, an attempt to create
the general description of the apatite dissolution mechanism at the
atomic (ionic) level is given below.
A, where A
n
7.7
Creation of the General Dissolution
Mechanism
7.7.1 Atomic (Ionic) Description for the Perfect Crystals
Immediately after being placed to acidic aqueous solutions, the
surface of apatite crystals becomes in contact with water molecules
and ions of the acid. Transport of the chemicals (H
+
n−
) to a
solid/liquid interface always happens by ionic diffusion through the
Nernst diffusion layer (Fig. 7.1). A thickness of this layer depends
on both solution hydrodynamics and dimensions of the crystals
dissolving: it decreases when agitation intensity increases and crystal
sizes decrease. However, in no cases the thickness becomes equal to
the zero [136]. Thus, a concentration gradient is always present near
the solid/liquid interface, the diffusion laws always remain valid,
and all findings of the diffusion-controlled model (e.g., an adsorption
and A
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