Biomedical Engineering Reference
In-Depth Information
Figure 2.4
Ca/P molar ratios of washed (dashed line) and unwashed
(solid line) ACP precipitates as a function of their formation
pH. Reprinted from Ref. [137] with permission.
2.3.2.2 Non-aqueous solutions and solvents (sol-gel)
Besides, ACPs might be easily prepared in either non-aqueous or
solvent + water media [72, 131, 138-152]. The presence of organic
compounds and/or solvents results in decreasing of a dielectric
constant. Therefore, all ions in solutions appear to be less solvated
than those in water. The consequence of this is a strong decrease of
solubility and an increase in precipitation kinetics, which simplifies
amorphization [153]. Furthermore, in such systems, complexes
of calcium with organic agents can be formed. This favors ACP
formation, which is attributed to the coordinated complexing
agents remaining in the structure of ACP [144]. The influence
of the presence of organic solvents to the amorphization degree
of precipitated calcium orthophosphates is well illustrated in
Fig. 2.5a. In some cases, incorporation of organic compounds into
ACPs has been detected [131, 150]. Interestingly, but a replacement
of a freeze-drying stage of a wet ACP precipitate by an oven drying at
80°C resulted in its transformation to CDHA [139]. This process was
ascribed to an internal hydrolysis of a part of orthophosphate ions of
ACP to those of CDHA according to the schemes [139, 154]:
4 3−
Æ
4 2−
PO
+ H
O
HPO
+ OH
(2.1)
2
Æ
Ca
(PO
)
+ H
O
Ca
(HPO
)(PO
)
OH
(2.2)
9
4
6
2
9
4
4
5
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