Biomedical Engineering Reference
In-Depth Information
dissolution process might be composed of two stages: the first one
was found to be both surface and diffusion controlled, while the
second one was an exactly diffusion-controlled dissolution [24].
Therefore, the results obtained for these models are valid within
the experimental conditions studied only; no extrapolation can be
made outside the ranges studied. For example, after, say, an agitation
decreasing or temperature increasing, an initially kinetically
controlled dissolution might be controlled by diffusion [26].
Furthermore, in some cases, dissolution curves (a titrant volume
against time) reach plateaus prior to complete dissolution of HA, thus
indicating a creation of metastable states in which the reaction is
effectively terminated even though apatite crystals remain in contact
with the undersaturated solutions [37-48]. There is an opinion [49]
that this effect resulted from a false solubility product value for HA.
Thus, a high sensitivity to the experimental conditions used appears
to be the main drawback of these models.
7.2.2
Mono- and Polynuclear Models
The surface dissolution may be described by three different models
shown schematically in Fig. 7.2. As may be seen in Fig. 7.2A it is
possible to start dissolution from one center from which originates
the units detachment leading to step disintegration. This is a
mononuclear dissolution. It is also possible that detachment takes
place simultaneously from multiple centers (Figs. 7.2B and 7.2C)
which may be developed in one or more steps on the crystal surface.
This is a polynuclear dissolution. Alternatively, it has been suggested
that the crystal units' detachment takes place along spirals originating
from an active site on the crystal surface following an Archimedean
spiral (Fig. 7.2D). The unit detachment along the spirals takes place
at constant velocity [30].
Figure 7.2
(A) Mononuclear model. (B) Polynuclear model in one crystal
step. (C) Polynuclear model in multiple steps: birth and spread.
(D) Spiral model. Reprinted from Ref. [30] with permission.
Search WWH ::
Custom Search