Download Zeta Potential: A New Approach

Zeta Potential:
A New Approach
by
Bruce B. Weiner, Walther W. Tscharnuter, David Fairhurst
Brookhaven Instruments Corporation Holtsville, NY 11742 US
A paper presented at the Canadian Mineral Analysts Meeting,
September 8-12, 1993 held in Winnipeg, Manitoba, Canada.
Bruce B. Weiner, Walther W. Tscharnuter, David
Fairhurst Brookhaven Instruments Corporation, Holtsville,
NY 11742
The zeta potential (ZP) is a function of the surface charge which develops when any material is placed
in a liquid. It is a very good index of the magnitude of the electrostatic repulsive interaction between
particles. The ZP is commonly used to predict and control dispersion stability. The characteristics of
the solid-liquid interface may also influence, inter alia, adhesion, flotation and, in more concentrated
systems, rheological behavior. Reliable ZP determinations are thus of practical concern to may
industries including ceramics preparation, processing and application.
A group of phenomena, known collectively as Electrokinetic Effects, can be exploited as the basis for
determining the zeta potential (ZP). The most widely studied, and of most practical relevance to
colloidal-sized paniculate suspensions, is particle electrophoresis, or microelectrophoresis, the
movement of charged particles suspended in a liquid under the influence of an applied electric field.
This offers the possibility of measuring the complete mobility spectrum. Over the past decade, the
technique of Electrophoretic Light Scattering (ELS) has been used extensively to characterize the ZP of
colloidal sized particles. ELS measurements are many times faster than conventional manual
techniques. Although the basic ELS illuminating optics and signal processing appear deceptively
simple, in practice, careful optimization is needed to obtain usable signals. Aside from the optical
problems in either the cross beam or the reference beam configurations, an inherent limitation to obtain
precise and repeatable results is the electro-osmotic effect, which up to now could only be accounted
for by careful and time consuming scans across the sample cell.
The transformation from electrophoretic mobility to ZP is not a trivial matter; the assumption of a
simple, constant, relationship (commonly the Smoluchowski limit of the Henry equation) is not
generally applicable. Of prime importance is the need to know the particle size, which can be
determined rapidly by Dynamic Light Scattering (DLS) using a newly developed single board real time
correlator with a dynamic range exceeding 10 decades.
The present paper will address these questions and discuss recent advances in instrument design to
reduce or even eliminate (in the important case of electro-osmosis) the various instrument limitations.
The importance of the ZP to ceramic processes, and the factors which effect the magnitude of the ZP,
are illustrated using examples from measurements of various oxides and non-oxides commonly used in
ceramics formulations.
Attraction of colloidal particles is fundamental. It arises from a variety of dispersion forces, and it is a
relatively long range force. The energy of attraction varies as 1/r2, where r is the distance between
particles. All colloidal dispersions will eventually aggregate unless there are sufficient forces to
prevent adherence of particles. For particles in water or other polar liquids, the most common force
of repulsion arises from charges on the particles.
As a result of the charge at the surface of the particle in the liquid an electrical double layer is formed.
One part of the layer consists of the charges on the particle's surface. The other is the increased
concentration of counterions near the surface and the corresponding decrease of coions. These changes
of ion concentrations near the surface decrease further from the surface due to the thermally driven
motion of ions
ELECTROKINETIC PHENOMENA
ARISES WHEN TWO PHASES MOVE WITH RESPECT TO EACH OTHER
WITH AN ELECTRIC DOUBLE LAYER AT THE INTERFACE
FOUR RELATED PHENOMENA:
ALL INVOLVE RELATIVE MOVEMENT BETWEEN RIGID AND MOBILE
PARTS OF THE DOUBLE LAYER
CHARGE SURFACE. IONS -------------Æ
Å-------------------- AND SOLVENT
1.
ELECTROPHORESiS
MOVEMENT OF CHARGED SURFACE STATIONARY
LIQUID APPLIED ELECTRIC FIELD
2.
ELECTRO-OSMOSIS
MOVEMENT OF LIQUID STATIONARY CHARGED
SURFACE APPLIED ELECTRIC FIELD
(i.e., the complement of electrophoresis)
3.
STREAMING POTENTIAL
MEASURED ELECTRICAL FIELD MOVEMENT OF LIQUID
- MECHANICAL STATIONARY CHARGED SURFACE (i.e.,
the opposite of electro-osmosis)
4.
SEDIMENTATION POTENTIAL
MEASURED ELECTRICAL FIELD MOVEMENT OF
CHARGED PARTICLES STATIONARY LIQUID
(i.e., the opposite of electrophoresis)
When charged electrodes are placed in a suspension of charged particles, the particles move toward the
electrode of opposite charge. The simplest arrangement for observing the movement is a Van Gils cell.
The electrophoretic velocity is proportional to the electric field, with the proportionality constant
called the electrophoretic mobility.
ELECTROPHORETIC MOBILITY
Zeta potential is proportional to the electrophoretic mobility, at least in the simplest case. Zeta
potential is the parameter which determines the strength of the repulsion between particles. As
such, it is this quantity, not mobility, that is important.
ZETA POTENTIAL
In the Van Gils' cell, and all other commercially available cells, except one, the electric field lines cut
across the charged walls of the cell. Electroosmotic movement of the liquid occurs. In a closed cell
this forces a flow in the cell that results in two, equal counterflow patterns. The measured velocity of
particles anywhere in the cell is the sum of the electroosmotic and electrophoretic velocities. The
electroosmotic velocity is zero at two special planes, called stationary planes, in the cell. If the
velocity is measured at these special positions, it is equal to the desired electrophoretic velocity
The stationary planes are very close to the cell walls. Unfortunately, electroosmotic velocity changes
most rapidly around the stationary planes. The depth of field of most optical systems is large enough
that averaging occurs. Thus, it is difficult to ignore the effect on the measured velocity of the
electroosmotic part
For accurate measurements it is necessary to verify, each time, the positions of the stationary planes (also
called stationary levels). Prof. Bob Pelton at the Univeristy of Toronto has shown that large errors ensue
if the levels are not verified.
Due to the rapid change around the stationary planes of the electroosmotic velocity, it is particularly
difficult to measure an accurate width of the electrophoretic mobility distribution. The distribution width
is an indication of the distribution of zeta potential. It is an important parameter for determining the longterm stability of the dispersion.
Theory shows that the observed velocity distribution across the cell is parabolic, except in the case
of very low mobility. In this special case, near the walls, the velocity may change direction. In any
case, where electroosmosis exists, it is essential, for accurate measurements, to measure at several
positions across the cell. The data are then fit to a parabola. If the fit is good, then the average
electrophoretic velocity, at the stationary planes can be determined by interpolation.
In recent years the technique of Electrophoretic Light Scattering (ELS) has been applied to the
measurement of the velocity of charged particles in a liquid. This has the advantage of measuring a
large number of particles simultaneously without the subjective and time-consuming character
imposed by classical, visual observation. Unfortunately, all commercially available instruments,
except the ZetaPlus, a new instrument, also suffer from the problem of electroosmosis. In an ELS
instrument a focused beam of laser light passes through the cell. A part of the laser beam is diverted
before it reaches the cell. This beam is combined with the scattered light. This beam is variously
called the reference beam or local oscillator. Its purpose, as shown later, is to determine the sign of
the charge on the particle, and, hence, the sign of the zeta potential.
In the ZetaPlus the electrodes are entirely within the cell. The electric field does not cut across the
cell walls. As a result no electroosmosis occurs, and the velocity measured between the electrodes is
only due to electrophoresis. There is no electroosmotic effect, nor any stationary planes to verify.
A large number of advantages arise as a result of using this configuration.
Since there are no standards to which any particle electrophoresis measurements can be compared, it is
only possible to compare different techniques. Here the electrophoretic mobility of TiO, is compared
as a function of pH using a classical Van Gils' cell ( Rank Mk II instrument) with visual
measurements and the ZetaPlus cell with ELS measurements. The agreement is excellent, indicating
the essential accuracy of the new design. It is worthwhile pointing out that each point plotted using
the Van Gils' cell is the result of interpolation at the stationary plane after measurements across the
cell were made. Such measurements take a long time. In contrast the ELS measurements are much
faster, and no interpolation is required.
Measurement of the Zeta Potential of Degussa P25 Titaniom Oxide in
aqueous suspension: Comparison of the BI-ZetaPlus and the Rank Mk II
When particles move perpendicular to the laser beam the frequency of the scattered light is shifted. This
Doppler shifted frequency contains the information on the electrophoretic mobility. Since frequency is
inherently a positive quantity, it is necesssary to add a local oscillator (reference beam) in order to
separate positive and negative shifts. Doppler shifts greater than the frequency of the local oscillator arise
from positively charged particles. Doppler shifts less than the frequency of the local oscillator arise from
negatively charged particles. The local oscillator frequency in the ZetaPlus is 250 Hz. Doppler shifts are
generally less than 100 Hz
The measured Doppler shift frequency is proportional to the electrophoretic velocity, the proportionality
constant is a well-known function of laser wavelength, liquid refractive index, and scattering angle.
In addition to motion of the particle due to electrophoresis, Brownian motion is also important. The
resulting diffusion of the particles broadens the measured mobility distribution.
The diffusional broadening is inversely proportional to particle size and is a strong function of scattering
angle: the smaller the angle, the less the diffusional broadening. In the ZetaPlus the scattering angle is
only 15°.
Zeta potential is a parameter that characterizes the stability of an electrostatically stabilized
dispersion.
Although zeta potential is not a direct measure of surface charge density—most surfaces are too
complicated for such a direct proportionality to exist—it changes when the surface charge changes. Any
added species which changes the surface charge will cause some change in the zeta potential. A common
example occurs when the pH is changed in dispersions of oxides or other surfaces that are either acidic or
basic. The pH at which the zeta potential is zero is called the isoelectric point (I.E.P.). At this pH the
repulsive forces are zero, and aggregation will occur.
Such a list, while instructive, can be misleading. The zeta potential is determined by the surface
chemistry. Even a small percent of a component, if preferentially adsorbed at the surface of the
particle, will largely determine the surface charge density, the resulting zeta potential, and the
stability, or lack thereof, of the dispersion. In the following examples, a commercially available TiO2
sample, one sold as pure titanium dioxide, acts like either alumina or silica depending on which of
these components is at the surface. The lesson is this. It is not the bulk properties of the material
which determine stability; it is the surface properties.
Since small changes in surface chemistry have a large effect on dispersion stability, it is also a
mistake to assume that batch-to-batch and lot-to-lot variations in will not affect dispersion. In the
example shown here the I.E.P. varies by nearly two pH units, and this was the same bulk material
from the same supplier tested over a five year period. It is not possible to measure zeta potential
once, or rely on average properties found in the literature. It is necessary to make measurements on
a continuing basis