Biomedical Engineering Reference
In-Depth Information
the consideration great similarities found between the chemical
composition and crystal structures of OCP and HA [148], one might
assume that the numerical values of surface diffusion distance for
HA should be close to the values for OCP.
Ionic detachment of some calcium results in removing of a local
positive charge from apatite, which is immediately compensated by
adsorption of other protons from the acidic solution. This conclusion
is based on the surface charge measurements: no information on
influence of dissolution time to values of the surface charge on
apatite has been found in literature [65-67, 73, 81-83, 137-141].
So, one may assume, that if a chemical composition of the solution is
kept constant the surface charge on apatite will also remain constant
during dissolution. Replacement of calcium by protons results in
formation of mono-atomic layer acidic calcium orthophosphates on
the crystal surface. Chemical phenomena of these transformations
are described by the chemical model [91-93] and the kinetics (it
strongly depends on the experimental conditions chosen)—by the
surface controlled model.
After
being
desorbed
from
the
surface,
calcium
cations
n−
) diffuse through the Nernst
layer into the bulk solution (the diffusion controlled model) [126,
140]. However, according to the calcium-rich layer formation model,
a part of the already dissolved calcium ions might be adsorbed back
onto apatite. Adsorption of calcium was suggested to happen at the
very beginning of dissolution only and resulted in formation of a
perm-selective layer of calcium cations with or without counter ions
(A
(presumably, coupled with anions A
n−
and/or acidic orthophosphate) [19, 44-48, 63]. In this case,
there should be a competition between protons and calcium cations
for the exposed ions of oxygen on the surface to be adsorbed onto.
Experimental data on a shift of the point of surface zero charge into
more basic solution pH in presence of calcium in a solution [141] are
in favor of such competition.
An idea on formation of some layer on the surface of apatite was
raised in several dissolution models. For example, there is either a
supposition (the polynuclear, diffusion, and kinetically controlled
models) or a description (the ion exchange, hydrogen catalytic
and chemical models) of a very thin virtual layer of acidic calcium
orthophosphates (more exactly presence of HPO
ions) on the
surface of apatite. Besides, according to the calcium-rich layer
formation model, dissolution of apatite happens by ionic diffusion
4
2−
Search WWH ::
Custom Search