Measuring Zeta Potential
The particle charge can be manipulated and controlled by adjusting the suspension pH and by using suitable dispersants, such as polyelectrolytes. However, optimizing the formulation of these materials requires a suitable method for measuring the particle charge, known as the zeta potential.
A new device* has been developed that provides rapid and accurate measurements of zeta potential without the need for sample preparation. The analyzer makes direct measurements on samples over a very wide concentration range, including the high solids contents used in ceramic slips. The instrument can handle samples ranging from free flowing fluids to flocculated pastes and gels. A key feature of the product is its automated titration capabilities for measuring zeta potential versus pH or as a function of dispersant concentration.
The Importance of Zeta PotentialThe charge on colloidal particles can arise from a number of different mechanisms, including dissociation of acidic or basic groups on the particle surface, or adsorption of a charged species from solution. The particle charge is balanced by an equal and opposite charge carried by ions in the surrounding liquid. These counter ions tend to cluster around the particles in diffuse clouds. This arrangement of particle surface charge surrounded by a diffuse cloud of countercharge is called the electrical double layer (see Figure 1).
The electrical potential drops off exponentially with distance from the particle and reaches a uniform value in the solvent outside the diffuse double layer. The zeta potential is the voltage difference between a plane a short distance from the particle surface and the solvent beyond the double layer.1
When two particles come so close that their double layers overlap, they repel each other. The strength of this electrostatic force depends on the zeta potential. If the zeta potential is too small (typically less than about 25 mV in magnitude), the repulsive force won’t be strong enough to overcome the Van der Waals attraction between the particles, and they will begin to agglomerate. The suspension is said to be unstable when this happens. If the suspension is concentrated and unstable, these agglomerates form networks, and the colloid turns into a paste.
A high zeta potential will prevent particle-particle agglomeration and keep the dispersion uniform and free flowing. Therefore, the goal in most formulations is to maximize the zeta potential. This is particularly important when trying to produce high strength ceramic materials.2 A review of ceramic dispersion stability and the importance of zeta potential is given by Pugh.3
The Isoelectric PointThe zeta potential depends on the surface charge density and the double layer thickness. The surface charge density, in turn, depends on the concentration of “potential-determining ions” in the solvent—ions that have a particular affinity for the surface. In many ceramic systems, the H+ ion is potential-determining, and so the zeta potential depends on pH.
A graph of zeta potential versus pH typically has the shape shown in Figure 2. This data was obtained on a concentrated alumina slurry using the new zeta potential instrument.
The zeta potential is positive for low pH values and negative for high pH values. The pH at which the zeta is zero is the isoelectric point (IEP) of the colloid. The IEP is a property of the particle surface. For alumina, the IEP is usually around 9.5. Thus, alumina slurries are usually stable below about pH 8.
Measuring Zeta—The Problem with DilutionLike most instruments for determining zeta potential, the new instrument measures how fast the particles move in an applied electric field and obtains zeta potential from the velocity per unit field. However, standard devices use optical techniques and are therefore limited to colloids that are diluted enough to allow light to pass through. Typically, this requires particle concentrations of less than 1⁄100th of a percent by volume.
Since ceramic slips are made at much higher concentrations, they require very substantial dilution in order to be measured on the conventional instruments. This dilution is time consuming, but worse, it can lead to large errors in the zeta potential due to:
- A change in the concentration of the background electrolyte. The suspension has to be diluted with an electrolyte that is exactly the same as the electrolyte beyond the double layers. In a concentrated colloid, that electrolyte concentration can be altered by the release of ions from the particle surface or by the partial dissolution of the colloidal particles—a dissolution that depends on particle concentration.4
- Contamination of the particle surface. The diluted sample has such a small total particle surface area that the zeta can be altered by trace amounts of surface-active impurities in the sample.
Measuring Zeta Potential in ConcentratesThe new zeta potential instrument avoids these problems by measuring directly on the concentrated colloid, so there is no need for dilution. The reason that the new instrument can measure in the opaque concentrates is that it determines the particle velocity by measuring sound rather than light. In this measurement, the suspension is subjected to a high frequency AC electric field. This causes the particles to jiggle back and forth at the applied frequency because of their electric charge. The particle motion generates ultrasound. The particle velocity is determined by measuring this ultrasound, and the particle zeta potential is determined from this velocity.
This electroacoustic measurement technique, involving the generation of ultrasound by an applied electric field, is called the electrokinetic sonic amplitude (ESA) method.
Note that at these MHz frequencies, the particle velocity depends on its size as well as its zeta potential because of inertia forces. Usually the particle size is not known apriori, but the new instrument accounts for these inertial effects by measuring how the particle velocity drops off as a function of frequency. Thus, there is no need to enter particle size in the new instrument, unlike earlier ESA devices.
Applying ESA Measurements to Ceramic MaterialsUsually the particle charge in ceramic slips is supplemented by the addition of polyelectrolyte stabilizers such as Dispex A40 or Darvan C. These polymers adsorb to the particle surface and boost the zeta potential. The new zeta potential instrument can be used to determine the optimum amount of dispersant to add.
This is illustrated in Figure 3, which shows the change in the zeta potential of alumina particles with the addition of the anionic polymer Darvan C. In this case, the zeta potential has leveled off after about 2.5 ml of polyelectrolyte, so there would be no point in adding more Darvan C.Note that the curve of zeta vs. polyelectrolyte addition depends on factors such as the pH and salt concentration in the solvent. Thus, to completely characterize an additive, it is necessary to make a large number of zeta measurements to examine additions over a broad range of pH and salt concentrations. This would be a daunting task in the standard microelectrophoresis devices, but in the new zeta potential instrument it is straightforward, since the titration process can be completely automated. Figure 4 shows the results of an ESA study of Darvan C addition to an 8 wt% alumina system.5
Controlling Slip RheologyAs mentioned above, the viscosity increases as the electrical repulsive forces are reduced. Thus, one would expect that the viscosity would be a maximum at the IEP for the material, and that the suspension should be free flowing for zeta potential values above about 30 mV in magnitude. This has been demonstrated by Johnson et. al.,6 who measured a plastic yield stress for an alumina in combination with ESA measurements, and by Greenwood and Bergstrom,4 who measured the zeta potential and low shear viscosity of a partially stabilized Ce-ZrO2 system at different levels of polyelectrolyte addition.
In these studies, it was found that the viscosity/yield strength directly correlates to the zeta potential. This is to be expected, since zeta is a direct measure of the inter-particle repulsive force.
Despite growing appreciation of the importance of the zeta potential in dispersion formulation and material characterization, it has not been used as a standard process parameter because of the inherent difficulties in measurement using the standard optical devices that require extremely diluted systems. The pH has been used as a defacto zeta potential indicator, since pH was a much easier parameter to measure. However the zeta potential can depend on factors other than pH, particularly in the presence of polyelectrolyte dispersing agents.
With the introduction of the new instrument, zeta potential measurements can now be made over a wide particle concentration range, without the need for assumptions of the particle size and particle size distribution. There is no longer any reason to fall back on defacto measurements. The zeta potential is the parameter that needs to be controlled, and it can now be directly and easily assessed by using the new instrument.
For More InformationFor more information about measuring zeta potential, contact Colloidal Dynamics, 11 Knight St., Building E18, Warwick, RI 02886; (401) 738-5515 ext. 11; fax (401) 738-5542; e-mail email@example.com; or visit .colloidal-dynamics.com. Colloidal Dynamics will also be at Pittcon 2002, booth 3741.
*The ZetaProbe from Colloidal Dynamics