Biomedical Engineering Reference
In-Depth Information
TABLE 3.2
Selected pH Values
of Isoelectric Points for Oxide Powders
Oxide
pH
iep
Reference
MgO
12.4
34
La
2
O
3
10.4
34
CuO
9.5
34
Al
2
O
3
9.4
25
ZnO
8.8
25
Cr
2
O
3
7.0
34
CeO
2
6.7
34
Fe
2
O
3
6.7
25
Fe
3
O
4
6.5
25
TiO
2
6.0
25
SnO
2
5.4
25
Amorphous SiO
2
2.5
25
Source:
Usui, S.,
Powder Technology Handbook
, Marcel Dekker, New York,
1991; Lewis, J.A.,
J. Am. Ceram. Soc.
, 83, 10, 2341-59, 2000, With
permission.
These concepts are illustrated schematically in Figures 3.5 and 3.6. In effect, this model
attempts to provide a phenomenological interpretation of the effect of pH and charge
anisotropy on the zeta potential. However, Figure 3.6 is an idealized curve that is appli-
cable to any suspended particle, regardless of whether the surface charge is isotropic
or anisotropic. The model merely clarifies the change in behavior in terms of physical
parameters.
The nominated values of zeta potential (±30 and ±60 mV) are based on well-known
observed thresholds for suspension stability. The terms
overly stable
and
appropriately stable
(right side of Figure 3.6) and
E
particle repulsion
and
E
external field
(left-hand side of Figure 3.6) are
relevant to electrophoresis, where it is possible for the suspension to be too stable because
pH range
Zeta potential range
B
C
A
-60 to -30 mV and
+30 to +60 mV
> +60 and
< -60
-30 to +30 mV
Attraction
Strong repulsion
Weak repulsion
+
+
-
+
+
-
-
-
-
+
-
+
-
+
-
+
-
-
-
-
-
+
-
+
-
-
+
+
+
+
-
-
-
-
-
-
-
-
-
+
-
+
-
+
+
-
+
+
+
+
Particle surface charge
schematic
-
Low pH
-
-
Low pH
+
-
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
-
-
-
- -
-
-
-
-
-
+
-
+
-
+
-
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
High pH
Low pH
High pH
Saturation of surface charge
Low
Moderate
Complete
State of agglomeration
Flocculated
Moderately deflocculated
Strongly deflocculated
FIGURE 3.5
Effects of surface charge anisotropy on suspensions. (Reprinted from Taib, H., Synthesis and Electrophoretic
Deposition of Tin Oxide (SnO
2
). Ph.D. Thesis, University of New South Wales, 2009. With permission.)
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