Malvern Zetasizer Nano Series, User Manual

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The magnitude of the zeta potential gives an indication of the potential stability of
the colloidal system. A colloidal system is when one of the three states of matter:
gas, liquid and solid, are finely dispersed in one of the others. For this technique
we are interested in the two states of: a solid dispersed in a liquid, and a liquid
dispersed in a liquid, i.e. an emulsion.
If all the particles in suspension have a large negative or positive zeta potential
then they will tend to repel each other and there is no tendency to flocculate.
However, if the particles have low zeta potential values then there is no force to
prevent the particles coming together and flocculating. The general dividing line
between stable and unstable suspensions is generally taken at 30mV or
-30mV. Particles with zeta potentials more positive than +30mV or more negative
than -30mV are normally considered stable.
The most important factor that affects zeta potential is pH . A zeta potential value
on its own without a quoted pH is a virtually meaningless number.
Imagine a particle in suspension with a negative zeta potential. If more
alkali is added to this suspension then the particles will tend to acquire a
more negative charge. If acid is then added to this suspension a point will be
reached where the negative charge is neutralised. Any further addition of
acid can cause a build up of positive charge. Therefore a zeta potential
versus pH curve will be positive at low pH and lower or negative at high
pH.
C H A P T E R 1 5 Z e t a s i z e r N a n o S e r i e s
P a g e 1 5 . 2
M A N 0 3 1 7
Electrical double
layer
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++ ++
Slipping plane
Diffuse layer Stern layer
Surface potential
Stern potential
Zeta potential
Distance from particle surface
0
-100
mV
Particle with negative
surface charge
ILL
6748