Malvern Zetasizer Nano Series, User Manual

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Introduction
The aim of this chapter is to describe the basic Zeta potential measurement
principles behind the Zetasizer Nano. This will help in understanding the
meaning of the results achieved.
The chapter is divided into two major sections. What is Zeta Potential? and
Operation of the Zetasizer Nano - Zeta potential measurements . The first
section describes the zeta potential theory, while the second describes the physical
operation of how a zeta potential measurement is performed.
What is Zeta Potential?
The Zetasizer Nano series calculates the zeta potential by determining the
Electrophoretic Mobility and then applying the Henry equation . The
electrophoretic mobility is obtained by performing an electrophoresis experiment
on the sample and measuring the velocity of the particles using
Laser Doppler
Velocimetry ( LDV ).
These techniques are described in the following sections.
Zeta potential and the Electrical double layer
The development of a nett charge at the particle surface affects the distribution of
ions in the surrounding interfacial region, resulting in an increased concentration
of counter ions (ions of opposite charge to that of the particle) close to the
surface. Thus an
electrical double layer exists around each particle.
The liquid layer surrounding the particle exists as two parts; an inner region,
called the Stern layer, where the ions are strongly bound and an outer , diffuse,
region where they are less firmly attached. Within the diffuse layer there is a
notional boundary inside which the ions and particles form a stable entity. When
a particle moves (e.g. due to gravity), ions
within the boundary move with it, but
any ions beyond the boundary do not travel with the particle. This boundary is
called the surface of hydrodynamic shear or slipping plane .
The potential that exists at this boundary is known as the Zeta potential .
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 . 1