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of acidic interaction with HA was found to involve two phases. In the
first phase, an acid bonds to surface calcium of HA with formation
of calcium-acid complexes. Depending on both a solubility and a
diffusion rate of the calcium-acid complexes from the HA surface to
a solution, the acid will in the second phase either remain attached
to the HA surface with only limited decalcification involved or
the calcium-acid complexes will detach, resulting in a substantial
decalcification effect. Rather similar data, in which carboxylic
groups of polyalkenoic acid were adsorbed on the surface of an HA
substrate, replaced orthophosphate ions and made ionic bonds with
calcium ions of HA, were obtained in another study [119].
total energy methods were used to study the
atomic structure and surface chemistry of HA [120]. All surfaces of
HA were found to react strongly with water and a loss of Ca from the
surface in exchange for two H appeared to be very favorable. Since
these calculations were performed for the case of water adsorption
on HA, it remained unclear whether they can be applied to the
acidic solutions. Nevertheless, other researchers found this effect
experimentally [81, 83]. Furthermore, another type of the exchange
was proposed for HA, in which one calcium and one hydroxyl were
replaced by one proton [82].
To conclude this model, the time independent ionic exchange of
orthophosphate by citrate was detected in dilute solutions only. In
more concentrated solutions it transferred into chemical reaction
(7.5) followed by time dependent precipitation of calcium citrate
when its solubility was exceeded [114, 116]. Application of this model
to HA dissolution in other acids revealed its dependency on both the
solubility and the diffusion rate of the calcium-acid complexes [117,
118]. Thus, new experimental data are necessary, to elaborate the
ion exchange model further.
Recently,
ab initio
7.2.8 Hydrogen Catalytic Model
A hydrogen catalytic model is based on a reasonable suggestion
about adsorption of protons onto negatively charged oxygen ions
of the surface orthophosphate groups of apatites [121]. Sorption
of protons results in partial transformation of surface PO
4
3−
groups
4
2−
into HPO
and catalyses the dissolution process [9, 10, 15, 18, 20,
48-56, 84]. A schematic representation of this process at different
solution pH is shown in Fig. 7.4 [110]. Besides, recent computer
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