Page 201 - Introduction to Colloid and Surface Chemistry

190 Charged interfaces

The four electrokinetic phenomena are as follows:

1. Electrophoresis - the movement of a charged surface plus attached
material (i.e. dissolved or suspended material) relative to station-
ary liquid by an applied electric field.
2. Electro-osmosis - the movement of liquid relative to a stationary
charged surface (e.g. a capillary or porous plug) by an applied
electric field (i.e. the complement of electrophoresis). The
pressure necessary to counterbalance electro-osmotic flow is
termed the electro-osmotic pressure.
3. Streaming potential - the electric field which is created when liquid
is made to flow along a stationary charged surface (i.e. the
opposite of electro-osmosis).
4. Sedimentation potential - the electric field which is created when
charged particles move relative to stationary liquid (i.e. the
opposite of electrophoresis).

Electrophoresis has the greatest practical applicability of these
electrokinetic phenomena and has been studied extensively in its
various forms, whereas electro-osmosis and streaming potential have
been studied to a moderate extent and sedimentation potential
rarely, owing to experimental difficulties.

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Electrophoresis
A number of techniques have been developed for studying the
migration of colloidal material in an electric field.

Particle (microscope) electrophoresis

If the material under investigation is in the form of a reasonably
stable suspension or emulsion containing microscopically visible
particles or droplets, then electrophoretic behaviour can be observed
and measured directly. Information relevant to soluble material can
also be obtained in this way if the substance is adsorbed on to the
surface of a carrier, such as oil droplets or silica particles.
The electrophoresis cell usually consists of a horizontal glass tube,
of either rectangular or circular cross-section, with an electrode at
each end and sometimes with inlet and outlet taps for cleaning and
filling (Figures 7.5 and 7.6). Platinum black electrodes are adequate