Biomedical Engineering Reference
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[97-106]. A theoretical basis for the pit formation and growth at
the surface outlets of dislocations includes detachment of single
ions from the kink sites. The bulk rate associated with dissolution
stepwaves arises quite naturally from the equations describing the
spreading of step trains from structure defects and densities [39].
However, until recently, the experimental investigations on pits
formation and growth on apatites were made when their dimensions
became at least 0.3-0.5 µm and even larger. Smaller dimensions of
pits were out of the range of old microscopes. That is why, the initial
stages of etch pits formation are not quite clear yet.
The investigations on apatite dissolution using atomic force
microscopy provided new data on this point [37-43, 61, 62, 109-
112]. For example, the growth process of a single pit on apatite was
followed since its dimensions were 50 nm [61, 62]. Therefore, one
may expect new results on even smaller dimensions of pits to appear
soon.
7.2.7 Ion Exchange Model
An ion exchange dissolution model is based on a supposition about
adsorption of protons [36] and anions of acid (e.g., citrate anions
[114-116]) from a solution onto the surface of apatite and removing
ions of calcium and orthophosphate into the solution instead.
An uptake of citric acid [114] and citrate ions [115] from dilute
(4-12.5 mM for citric acid [114] and 0.05-1 mM for sodium citrate
[115]) aqueous solutions by a synthetic HA was found to be time
independent and follow Langmuir isotherms rather good. However,
it was not the case when solutions of higher concentrations were
used. In more concentrated solutions (13-100 mM of citrate), this
interaction was “reactive” and not “adsorptive” [114, p. 1421].
Adsorption was found to take place by ionic exchange of
orthophosphate by citrate ions at the solid-solution interface,
caused by a higher affinity of citrate than orthophosphate species
for the Ca-sites on the surface. Citrate ions were proposed to interact
in two ways: C
7
3−
interacted weakly in a bi-dentate manner (1
citrate per 2 calcium sites), whereas C
H
O
6
8
interacted strongly in
a mono-dentate manner (1 citrate per 1 calcium site) [115].
Later this model was updated by investigation of apatite
dissolution in other acids and an adhesion-decalcification concept
was introduced [117, 118] According to this concept, the mechanism
H
O
7
2−
6
9
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