Biomedical Engineering Reference
In-Depth Information
The authors of calcium-rich layer formation model suggested
that the rate control of apatite dissolution was diffusion of calcium
and/or orthophosphate ions through an interfacial surface layer
formed during the initial portion of the dissolution process. This
interface would be composed of the Nernst layer that would be
adjacent to the calcium-rich layer. This last one would present low
permeability making the dissolution process slower [19, 44-48]. It
is important to notice that these conclusions were made from the
experiments on following calcium concentration and solution pH
only; no measurements of the surface composition of apatites were
performed.
As said by this model, ionic diffusion in solution results in a
situation when ionic concentration near the surface layer of apatite
is always higher than that in the bulk solution. Calcium concentration
in the solution is also inhomogeneous: it decreases when the
distance from the liquid/solid interfaces increases [19, 44-48].
Therefore, the calcium-rich layer appears to be undoubtedly rich in
calcium when compared with the bulk solution only. However, when
compared with the bulk of solid apatite (Ca/P = 1.67) this layer
appears to be calcium-rich (Ca/P > 1.67) when the counter anions
A
n−
n−
(it is assumed, that apatite is dissolved in acid H
A, where A
n
is an anion) adsorbed from the solution are not taken into account.
If the counter anions are calculated, a numeric value for Ca/(P+A)
molar ratio on the surface will be around 1.67 in the case of zero
charge on the surface.
On the other hand, ions of calcium might be adsorbed without the
counter anions. This results in an electrical double layer formation
on the solid/liquid interface with positive charge on apatite. Indeed,
apatite is charged positively in acidic solutions. This effect was
explained by adsorption of either protons only [65] or protons and
calcium cations from the solution [66, 67]. Certainly, the presence
of a positive charge on apatite surface is in favor for the calcium-
rich layer model but one should not forget that this charge might
be caused by adsorption of protons only [65]. The dissolution rate
reduction found by the authors and explained by the state of the
interface (adsorption of calcium [48]) might also be explained by
adsorption of some impurities those always present in the solution.
Other explanations are also available [37-43]. Thus, unless the direct
measurements of the calcium content on the surface are made, this
model cannot be considered as experimentally proven.
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