Application Note
Application Note: Zeta Potential in Pharmaceutical Formulation
Prepared for Malvern Instruments by Clive Washington Dept. of Pharmacy, University of Nottingham, U.K.
Although particle size and its measurement are intuitively familiar to particle technologists, the concept of zeta potential is less widely understood and applied. This is unfortunate since it is at least as fundamentally important as particle size in determining the behaviour of particulate materials, especially those with sizes in the colloidal range below a micrometer. Zeta potential is related to the charge on the surface of the particle, and so influences a wide range of properties of colloidal materials, such as their stability, interaction with electrolytes, and suspension rheology.
What is zeta potential?
When a particle is immersed in a
fluid, a range of processes causes the
interface to become electrically
charged. Some of the most
commonly found charging
mechanisms include adsorption of
charged surfactants to the particle
surface (for example in an emulsion
stabilised by an ionic surfactant),
loss of ions from the solid crystal
lattice (silver halide particles used in
photographic emulsions) and
ionization of surface groups
(carboxylate in polymer
microspheres). These processes lead
to the production of a surface
charge density, expressed in
coulombs per square metre, which
is the fundamental measure of
charge at the interface. The charge
cannot be measured directly, but
only via the electrical field it
creates around the particle. Thus
the surface charge is normally
characterised in terms of a voltage
at the particle surface, the surface
potential, rather than a charge
density, although one can usually be
calculated from the other. The zeta
potential occurs at a distance from
the surface and this will be different
to the surface potential. In the
simplest approximation, the
potential decays exponentially with
distance from the surface of the
particle. As we will see, the
rate of decay is dependent on the
electrolyte content of the fluid.
Click here to download the complete application note in pdf format.
