hydroxyapatite particles
It is shown in Section 3.8 that regular particles of hydroxyapatite could be produced
without the addition of fluoride. To test if the regular shape of these hydroxyapatite particles was as sensitive to the pH level in the gel as fluorapatite, gels set to di↵erent pH levels were tested.
Little change in the shape of the particles produced was seen at various gel pH
(Fig.3.9). This was unexpected and contrary to the results found when fluorapatite was
made. The apatite crystals had the same appearance when formed from solution gel set to various pH tested from 5.4 to 6.4. The rod-shaped particles made of hydroxyapatite appeared to be less solid than those made of fluorapatite reported in this work.
There was some di↵erences seen with changes to the pH. It was observed that at
a lower pH the particles were larger, as shown in Fig. 3.9(a). This was not reported
before. It is unknown why no change was seen at the various pH tested in the gel, but possibly a more comprehensive test across a larger range of pH might provide greater insight.
3.10
The control of fluorapatite particle morphol-
ogy by adjustment of the solution gel pH
Fluorapatite was characterized by XRD and Zeta and the results shown in Fig. 3.10 These results were consistent for all methods of apatite synthesis tested in this work if the solution gel was doped with fluorapatite. The two factors that appeared to control the morphology of fluorapatite particles were the pH and the temperature conditions in the gel [17]. These conditions
also appeared to be interrelated, each having an e↵ect on the other. Two experiments
to test this were run. In one test, the pH of the gel was set between 3.6 and 7.6. In the other test, particles produced from a gel set at both a low pH and incubation
52An investigation into the optimization of the hydrothermal method
(a) (b)
(c)
Figure 3.9: The pH of the solution gel has much less of an e↵ect on the shape of particles of hydroxyapatite produced than it does on particles of fluorapatite. At a lower pH, as seen in Fig.3.9(a), the particles were larger. At a high pH, as seen in Fig.3.9(b) and in Fig.3.9(c), they were smaller and more solid. By observing the SEM images, some of the particles appear to be less solid i.e. to have a porosity, an internal fugitive structure to them then other more solid less porous particles.
temperature was compared to one set at both a high pH and incubation temperature.
It was observed in Fig.3.11, that as the pH of the gel tested rose from low to high,
the shape of the fluorapatite particles also changed. It was also found that at a low or high pH, the synthesis of the particles was compromised and an amorphous precipitate was produced. At a mid-pH, particles were at their largest and, as the pH rose in the gel from low to high, the particles produced became less rod-like and more whisker-like [17].
The fluorapatite particles produced were irregular at pH 3.6 (Fig. 3.11(a)) and
3.10 The control of fluorapatite particle morphology by adjustment
of the solution gel pH 53
(a)
(b)
Figure 3.10: Fig.3.10(a)XDR analysis of precipitate indicated fluorapatite. Fig.3.10(b) The particle size distribution indicated that the particles in solution were about 1 micron in size.
54An investigation into the optimization of the hydrothermal method
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
(j) (k)
Figure 3.11: The pH of the solution gel is a major influence on the shape of fluorapatite
particles. Irregular particles were produced at a pH of 3.6 (Fig. 3.11(a)), 7.2 (Fig. 3.11(j))
and at 7.6 (Fig. 3.11(k)). They were regular between a pH of 4.0 (Fig. 3.11(b)) to 6.8
3.10 The control of fluorapatite particle morphology by adjustment
of the solution gel pH 55
to 6.8 (Fig. 3.11(i)). They had an almond shape at pH 3.6 (Fig. 3.11(a)), and 7.2
(Fig. 3.11(k)), and a rod shape at pH 4.0 (Fig. 3.11(g)) to pH 6.0 (Fig. 3.11(b)).
They had a whisker shape at pH 6.4 (Fig. 3.11(h)). They were largest at pH 4.8
(Fig. 3.11(d)) and 5.2 (Fig.3.11(e)). They were smallest at pH 3.6 (Fig. 3.11(a)), and
pH 6.8 (Fig.3.11(i)) to 7.6 (Fig. 3.11(k)).
In a second test, shown in Fig. 3.12, fluorapatite made at a low temperatures and
pH, were found to be similar in shape to particles made at a high pH and temperature. Specifically, particles made at 150 C and a pH of 5.2 matched those made at 180 C and a pH of 7.2. Similarly, particles made at 160 C and a pH of 6.0 matched those made at a 180 C and a pH of 7.7. The pH and temperature of the gel appear to be interrelated. It is commonly known that as the temperature of a gel rises, its measured pH drops.
For further information on the phenomena of the pH of a gel increasing as it is heated, the Author refers the reader to the following literature [31,33,34]. This general physical phenomena has also been reported in the literature specifically in relationship to the synthesis of calcium phosphates.
These results suggest that it was possible, when making fluorapatite, to control the size, shape and regularity of particles by changing the solution gel pH. Similar results were reported elsewhere [37,40], but this work is the first systematic test of the e↵ect of
di↵erent gel pH and incubation temperature on the shape of the crystals of fluorapatite
produced.
A change in the shape of the particle as the pH level was increased might be
explained, and is illustrated in Fig. 3.13, by an increase in the dipole moment that
surrounds each particles as it grows [30]. The presence of an electric dipole field created as the calcium cations coalesce with the phosphate anions might align the citric ions parallel to the c-axis of a forming fluorapatite crystals.
56An investigation into the optimization of the hydrothermal method
(a)
(b)
Figure 3.12: The pH and temperature of the solution gel were interrelated in their
determination of the shape of the particles produced. As shown in Fig. 3.12(a), whisker
shaped particles were produced at a pH of 5.2 and 50 C. As shown in Fig. 3.12(b), similar
shaped particles were produced at a higher pH of 7.2 and an incubation temperature of 180 C.