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) 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.
