Biomedical Engineering Reference
In-Depth Information
3. chemical transformations on the surface;
4. desorption of products (ions of fluoride, calcium, and
orthophosphate) from the crystal surface;
5. their diffusion into the bulk solution.
All the steps mentioned above are likely to be much more
complicated. For example, processes 1 and 5 include chemical
transformations happen with the ions during diffusion, because
solution pH is known to depend on the distance from the solid/liquid
interface (in acidic solutions it is higher near the surface of apatite and
decreases when the distance increases) [127, 128]. Processes 2 and
4 include ionic diffusion along the surface both to (step 2a) and away
from (step 4a) the crystal steps, respectively, as well as a diffusive
jump (steps 2b and 4b) [126]. Finally, process 3 consists of several
successive chemical transformations (step 3a — chemical equations
(7.1)-(7.4)) [91-93] and includes ionic detachment from the kink
sites (step 3b) [126]. Moreover, for adsorption (step 2) to take place,
the following intermediate steps are necessary: (i) dehydration of a
surface site, (ii) partial dehydration of ions and (iii) a diffusive jump
toward the surface [126, p. 1692]. One can also expect three similar
intermediate steps (diffusion jump from the surface, hydration both
the ions and the surface site) for desorption (step 4).
As shown above, different models describe just diverse aspects
of the same phenomenon of apatite dissolution. Commonly, the
main distinction is made between the transport control and surface-
reaction control. For example, in the diffusion controlled, two-site
and polynuclear models steps 1 and 5 limit a reaction rate. On
the contrary, in the kinetically controlled, chemical and hydrogen
catalytic models a reaction rate is limited by step 3. In the cases of the
ion exchange and self-inhibition (or calcium-rich layer formation)
models, a reaction rate is likely to be controlled by both ionic
diffusion (steps 1 and 5) and kinetics of the adsorption/desorption
phenomena (steps 2 and 4). Processes of etch pit formation and
dissolution stoichiometry—are the experimental facts, not models.
Thus, they may be controlled by factors pertaining to any step from
1-5 and even all of them.
7.5
Brief Information on Apatite Structure
The crystallographic data on HA and FA are mentioned in Table 1.3,
while the crystal structure of HA is shown in Fig. 2.11. Briefly, crystals
Search WWH ::
Custom Search