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 The
charge which develops at the interface between
a colloidal particle and the liquid medium in
which it is suspended may arise by any of several
mechanisms. Among these are the dissociation of
ionogenic groups in the particle surface and the
differential adsorption from solution of ions
of different charges into the surface region;
in clays, ion exchange mechanisms may also be
important.
The development of a nett charge
at the particle surface affects the distribution
of ions in the neighbouring interfacial region,
resulting in an increased concentration of counterions
- ions of charge opposite to that of the particle
- close to the surface. Thus an electrical double
layer is formed in the region of the particle-liquid
interface.
The double layer (see figure above) may be considered to consist of two parts:
an inner region which includes ions bound relatively
strongly to the surface (including specifically
adsorbed ions) and an outer, or diffuse, region
in which the ion distribution is determined by
a balance of electrostatic forces and random thermal
motion. The potential in this region, therefore,
decays as the distance from the surface increases
until, at sufficient distance, it reaches the
bulk solution value, conventionally taken to be
zero.
When subjected to an electric field as in microelectrophoresis,
each particle and its most closely associated
ions move through the solution as a unit and the
potential at the boundary between this unit i.e.
at the surface of shear between the particle with
its ion atmosphere and the surrounding medium,
is known as the zeta potential  .
When a layer of macromolecules, whether a polyelectrolyte
or an uncharged polymer, is adsorbed on the surface
of the particle, this can alter the zeta potential
simply because it shifts the location of the shear
plane further from the actual surface.
Zeta potential is therefore a function of the surface charge of the particle, any adsorbed layer at the interface and the nature and composition of the surrounding medium in which the particle is suspended. It is usually, but not necessarily, of the same sign as the potential actually at the particle surface but, unlike the surface potential, the zeta potential is readily accessible by experiment. Moreover, because it reflects the effective charge on the particles and is therefore related to the electrostatic repulsion between them, zeta potential has proven to be extremely relevant to the practical study and control of colloidal stability and flocculation processes.
The principal of determining zeta potential by microelectrophoresis is very simple. A controlled electric field is applied via electrodes immersed in the sample suspension and this causes the charged particles to move towards the electrode of opposite polarity. Viscous forces acting upon the moving particle tend to oppose this motion and an equilibrium is rapidly established between the effects of the electrostatic attraction and the viscous drag. The particles therefore reach a constant "terminal" velocity.
This velocity is dependent upon the electric
field strength or voltage gradient, the dielectric
constant and viscosity of the liquid - all of
which are known - and the zeta potential. It is
usually expressed as the particle mobility which
is the velocity under unit field strength. For
all practical purposes, the relationship between
mobility, µ, and zeta potential, ,
is quite simple and, for instance, in water at
25 o C can be expressed as:
= 12.85 µ
In practice, zeta potentials
are usually negative, i.e. the surface is negatively
charged, but they can lie anywhere in the range
from -100 to +100 mV.
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