Biomedical Engineering Reference
In-Depth Information
findings cannot be underestimated because they have already made
a significant impact on understanding of the medical aspects of
both dental caries and osteoporosis. Nevertheless, there are still a
lot of elements missing in the overall explanation for the processes
involved. To clarify possible directions of further investigations on
apatite dissolution, a critical analysis of currently available models
has been made in this chapter followed by an attempt to combine the
data available into a general dissolution mechanism.
7.2
Critical Analysis of the Dissolution Models
of Calcium Apatites
7.2.1 Diffusion (or Transport) and Kinetically (or
Surface) Controlled Models
A diffusion (or transport) and a kinetically (or surface) controlled
[9-26] dissolution models, as well as the two-site model [15, 27-29],
are devoted to comparison of transport rates of chemical reagents
(H
+
and anions of acid) from bulk solution to the surface of apatite
crystals and those of products (Ca
2+
−
and orthophosphate anions)
obtained backwards with the kinetics of chemical interactions
on the crystal surface. According to the diffusion-controlled and
two-site models, a transport of either the chemical reagents and/
or the products of chemical reactions mainly determine the
overall dissolution rate, whereas, the kinetically controlled model
emphasizes chemical transformations on the surface as the limiting
factor (Fig. 7.1) [30]. An important consequence of the two types
of rate-controlling mechanism is that the solution immediately next
to the crystal surface is undersaturated with respect to apatite in
the surface controlled process and this solution is saturated in the
diffusion-controlled process [31]. Both models usually operate with
a so-called driving force which means either a concentration gradient
within the Nernst diffusion layer (the diffusion controlled model) or
a gradient of ionic chemical potentials between the apatite crystal
surface and bulk solution (the kinetically controlled model).
To investigate whether a dissolution process follows either of
these models, one should study the influence of hydrodynamics,
concentration of reagents, ionic strength, solution undersaturation,
, F
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