Biomedical Engineering Reference
In-Depth Information
resistance and a diffusion jump) remain applicable for any case of
apatite dissolution. A similar approach is correct for the surface
phenomena: when dissolution occurs, adsorption and desorption
of ions, as well as chemical transformations (7.1)-(7.4) always take
place on the surface regardless the experimental conditions chosen.
After being delivered by diffusion to the solid/liquid interface,
ions of H
+
n−
are adsorbed onto the surface of apatite. A great
number of various surface complexes might be formed as a result [81-
84, 137-139]. According to the diffusion-controlled model, there is
an adsorption resistance for ions to be adsorbed onto the surface and
in order to overcome the resistance the ions make a diffusive jump
toward the surface to a distance corresponding to their size [126,
140]. Being charged positively, protons are adsorbed onto oxygen ions
of orthophosphate groups [53] as well as onto ions of fluoride (in the
case of FA) and hydroxide (in the case of HA). After the model by Wu
et al., the surface protonation of apatites proceeds via formation of �
POH surface groups (“�” stands for the surface) at 5 < pH < 7, while
apatite surfaces become fully protonated at pH < 5 [137]. One may
expect to find some differences in the adsorption kinetics of protons
onto fluoride, hydroxide and orthophosphate ions. Namely, due to
a higher electro negativity of fluoride (when compared to oxygen
ions of orthophosphate groups) and a higher basicity of hydroxide
(when compared to orthophosphate), adsorption of protons might
happen faster (or previously) onto these ions when compared with
orthophosphate. However, recent results of computer simulations
indicated a possibility of OH
and A
−
4
2−
protonation by the nearest HPO
ion [122], indicating to the fact, that orthophosphate ions might be
protonated faster (or previously) if compared to that for hydroxide
ions. Obviously, this topic needs to be clarified in future.
n−
may be adsorbed onto
calcium cations only. According to the ion exchange model, the
exchange process shows an adsorption of about one anion per unit-
cell of apatite [114-116]. On the other hand, the surface of apatites
is charged positively in aqueous acidic media and negatively in
basic solutions (the point of zero charge is at solution pH within
6.8-8.5) with an electric double layer formation at the solid/liquid
interface [65-67, 73, 81-83, 137-139, 141]. The latter points out
to a non-equivalent ionic adsorption of H
Anyway, negatively charged anions A
onto apatite: in
acidic solutions, adsorption of protons always exceeds that of A
+
and A
n−
n−
,
while in basic solutions the situation is opposite. Therefore, in acidic
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