Hydroxyapatite and Other Biomedical Coatings by Electrophoretic Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 105

Organic Suspensions

Conversely to the case of aqueous suspensions, a general perception is that organic liquids
are associated with the following advantages [24]:

• No risk of electrolysis
• Little risk of Joule heating, so high electrical field strengths can be applied
• No risk of chemical attack of the substrate
• No risk of chemical attack of the deposit

However, there also are some disadvantages [24]:

• Low dielectric constants, which limit the amount of surface charge
• High costs of media
• Necessity of proper disposal of waste media

With organic media, it appears that electron transfer between solid and liquid is the means
by which the surface charge changes [20,24]. There is evidence that it is possible to use the
pH and the zeta potential to assess the suspensions, analogously to the behavior in water.
However, in this case, the relevant phenomenon is the electron donicity , which is defined
as a measure of the tendency of the medium to donate electrons. If the medium has a
higher electron donicity than the particle, then the former will donate electrons to the lat-
ter, thereby facilitating a particle of negative surface charge. Conversely, if the donicity of
the medium is lower than that of the particle, then the latter will donate electrons to the
former, thereby forming a positive surface charge.
Alcohols are regarded as neutral amphiprotic media and so they can donate or accept H +
and OH − ions [24]. Hence, they are similar to water in their effect on the surface charge and
so the preceding comments concerning aqueous media are relevant to alcohols. In the case
of proton donation, the mechanism of particle charging in alcohol is [86]:

Pure alcohol: RCH 2 OH + RCH 2 O-OH ↔ RCH 2 O − + RCH 2 OH 2 + Reaction 3

Aqueous alcohol: RCH 2 O-H + H 2 O ↔ RCH 2 O − + H 3 O + Reaction 4

For the pure alcohol, the adsorbed alcohol ionizes into a protonated alcohol (RCH 2 OH 2 + )
and an alkoxide ion (RCH 2 O − ), followed by the dissociation of the protonated alcohol [86].
The dissociated alcohol and the alkoxide ion then desorb into the solution, leaving the
proton on the particle surface.
On the other hand, ketones and ethers are aprotic media and so they cannot donate
or accept a proton (i.e., the hydrogen ion is nondissociable) [20]. They have relatively low
dielectric constants and it has been observed that relatively high dielectric constants, in
the limited range 12 to 25, are necessary for electrophoresis. Equation 3.1 shows that the
Debye length is proportional to the dielectric constant, so low values are equivalent to
compression of the repulsive double layer. It is apparent that the ionization of these media
is insufficient to donate charge to the particle surface.
Table 3.3 [37-85] provides a representative example of reports of the organic electropho-
retic deposition of a range of bioceramics.