Biomedical Engineering Reference
In-Depth Information
−
−
(for FA) located
on the surface are protonated as well and, of them, fluoride ions
exhibited much lower proton affinities (pK
simulations revealed that both OH
(for HA) and F
= 10.8, a weak base)
B
= 0, a strong base) [122]. Furthermore,
the surface protonation of apatites is intimately related to hydrogen
bonding. The details on the latter process are available in literature
[123].
compared to hydroxide (pK
B
Figure 7.4
A schematic representation of surface protonation of apatites
at different solution pH. Reprinted from Ref. [110] with
permission.
One can notice, that the authors of different dissolution models
(the diffusion and kinetically controlled [9, 10, 15, 18, 20] and
polynuclear [51-56] models) have suggested a similar possibility
of a catalytic effect of the adsorbed protons. At present, indirect
data only are available to support this hypothesis. Namely, at
neutral pH, dissociation of ions is penalized by more than 150 kJ/
mol giving rise to very stable apatite-water interfaces [122]. This
picture changes drastically with decreasing pH, as the protonation of
orthophosphate and hydroxide ions lowers the free energy of calcium
ions dissociation. An obvious trend in apatite dissolution is that the
dissolution rate is increased as the solution pH decreases, which is a
general phenomenon observed in dissolution of other minerals. This
indicates that dissolution proceeds by surface protonation, which
promotes breaking of Ca-O bonds.
In the course of proton uptake, the charge of the surface anions is
reduced and, the Coulomb attraction to the crystal surface is lowered
considerably, which might be considered as “catalysis”. Currently, no
precise information about a catalytic activity of adsorbed protons
on the dissolution of apatites is available. However, it does not
mean that this effect does not exist. Up to now, nobody has ever
succeeded to reveal a catalytic effect unambiguously due to a lack
of an experimental technique able to provide the experimental data
necessary. Obviously, the general idea on a catalytic activity of the
Search WWH ::
Custom Search