Hydroxyapatite and Other Biomedical Coatings by Electrophoretic Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 89

A common method of altering the surface charge is by metal oxide or metal hydroxide
surface hydrolysis through particle surface ionization. This may occur either by protona-
tion or deprotonation of acid or base groups by changing the pH. For example, when oxide
particles are dispersed in an aqueous medium, the surfaces are coordinated by water mol-
ecules to form hydroxylated surfaces. The charge of the particle surface then depends on
the pH, as shown in Reactions 1 and 2 [19,30]:

pH < 7: M-OH + H + ↔ M-OH 2 + Reaction 1

pH > 7: M-OH + OH − ↔ M-O − + H 2 O Reaction 2

There are two terms used to define the state of the solid/medium interface of dispersed
particles. The isoelectric point corresponds to the condition where the zeta potential of a
particle surface is zero ( ζ = 0) [32]. This also is considered that point at which the surface
charge of the particle only is zero [33]. The pH at which this occurs is the pH iep . The other
related parameter is the point of zero charge , which corresponds to the condition where posi-
tive ([M-OH 2 + ]) and negative ([M-O − ]) surface electrolyte concentrations are equal (i.e., ψ =
0) [32]. This also is considered the point at which the net total of the external and internal
surface charges of the particle is zero [33]. The pH at which this occurs is the pH zpc .
The isoelectric point and point of zero charge often are used interchangeably. However,
they are equivalent only if the sorption of ions and dissociation of counterions are ignored
[32]. Whether this is the case depends principally on the method of powder fabrication,
stoichiometry, crystal structure, degree of surface hydration, leaching, and impurities.
Consequently, the differences between these two parameters can be significant. In general,
it can be observed that (1) the pH zpc tends to be close to the natural pH of a suspension [22]
but (2) the pH iep is strongly influenced and altered by the preceding factors. In this sense,
the zero point charge can be considered more of an intrinsic quantity while the isoelectric
point can be viewed pragmatically as an extrinsic quantity. In effect, the ease of altering
the surface isoelectric point explains why it is used to control the surface charge more fre-
quently than the point of zero charge.
Table 3.2 [25,34] provides a selection of pH values for isoelectric points (pH iep ) deter-
mined for some oxide ceramic powders.
The Australian authors of the present work recently have described the relation between
the pH, zeta potential, and rheology of suspensions in terms of charge saturation at the
particle surface [35]. This model differs from the conventional approach in three principal
ways:

• Conventionally, particles are considered to have only intrinsic positive or nega-
tive surface charge. In the present model, they possess both positive and negative
surface charges, which result from the anisotropic directional bonding in most
crystal structures, although only one typically dominates.
• Conventionally, the solid/liquid interface is viewed in terms of homogeneous and
complete saturation of the solid by oppositely charged species in the medium, as
shown in Figure 3.3. In the present model, while the surface charge is assumed to
be homogeneous, it is not assumed to be saturated.
• Conventionally, flocculation is viewed to result from compression of the repulsive
double layer with increasing electrolyte concentration [19]. In the present model,
anisotropic surface charges contribute to this compression.