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Electrophoretically Deposited Polymers for Organic Electronics 367

Flocculation by Particle Accumulation
17
According to Hamaker and Verwey, EPD is akin to sedimentation,
and the primary function of the applied field is to move the particles
toward the electrode. Accumulated particles then deposit due to the
pressure exerted by those incoming and in the outer layers. This mech-
anism is feasible when deposition does not occur at the electrode, e.g.,
deposition on a dialysis membrane. It explains deposits on porous
membranes that are not electrodes.
10.2.3 Theory of EPD
A successful EPD relies on the formation of well-stabilized, unagglomer-
ated, and homogeneous colloidal suspension. A distinguishing feature of
a colloidal system is that the contact area between the particle surface and
the dispersing liquid is large. As a result, the interparticle forces strongly
influence the suspension behavior. The dominating interparticle forces in
most colloidal systems are (1) the van der Waals attractive force, (2) double-
layer (electrostatic) repulsive force, and (3) steric (polymeric) forces. To
obtain a well-stabilized suspension, particles dispersed in the suspend-
ing medium must exhibit sufficient repulsive forces to offset the van der
Waals attraction. These forces are better understood in terms of electrical
double layer and their interactions are discussed in the next section.

The Electrical Double Layer and Electrophoretic Mobility
EPD relies on the capability of powder to acquire an electric charge in
the liquid in which it is dispersed. In general, when solid powder is
dispersed in a polar liquid such as water, usually it results in the
buildup of a charge at the solid-liquid interface. Sarkar and Nicholson 16
inserted a dialysis membrane between EPD electrodes in an Al O sus-
2 3
pension. The membrane is permeable to ions, but a dense deposit is
formed thereon and the current is passed via ionic discharge at the
cathode. They concluded that the majority of the charge is carried by
ions that result in the passage of current. It is now well recognized that
development of the electric charge on colloid particles dispersed in
water is due to (1) surface group ionization (controlled by the pH of the
dispersion media), (2) differential solubility of ions (e.g., silver iodide
crystals are sparingly soluble in water and silver ions dissolve prefer-
entially to leave a negatively charged surface), (3) isomorphous
4+
replacement/lattice substitution (e.g., in kaolinite, Si is replaced by
3+
Al to give negative charges), (4) charged crystal surface fracturing
(crystals can reveal surfaces with different properties), and (5) specific
ion adsorption (surfactant ions may be specifically adsorbed). This surface
charge influences the distribution of nearby ions in the polar medium.
The ions, which establish the surface charge, are termed potential deter-
mining ions (PDIs). These normally include ions of which the solid is
composed; hydrogen and hydroxyl ions; and ions capable of forming
complex or insoluble salts with the solid surface species. Ions of oppo-
site charge (counter-ions) are attracted toward the surface, and ions of