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Eliminate the uncertainties in colloid stabilization
By Richard O'Brien, PhD
Pioneer in the field of zeta potential
From foods to inks and from pharmaceuticals to cements, stabilization of colloids is critical to the performance of many materials. Now, the Agilent 7020 ZetaProbe analyzer makes this job straightforward by enabling rapid, reliable zeta potential measurements that avoid the errors associated with sample dilution.
A colloidal sol is a suspension of tiny particles in a liquid. The particles must be stabilized to stop them from flocculating under the Van der Waals force of attraction. There are two common ways to create a repulsive force to overcome this attraction. One is by adjusting either pH or electrolyte concentration to boost the electric charge on the particles, and the other is by adding a polymeric dispersant that adsorbs on the particles and creates steric repulsion.
The Agilent 7020 ZetaProbe is the ideal tool for stabilizing a colloid, because you can use it to monitor and control the electrical repulsive force or to determine the optimum dispersant dose.
Zeta potential indicates stability
The 7020 ZetaProbe measures the zeta potential, which is the voltage difference between the particle and the electrolyte.1 This voltage difference arises because of electric charge on the particle surface. An aqueous colloid is usually stable if its zeta is more than about 25 mV in magnitude.
The 7020 ZetaProbe measures zeta while adding titrant doses, and provides curves of zeta as a function of electrolyte parameters or dispersant level.2 In this way, you can quickly find the conditions for colloid stability.
Better way to measure zeta
Commercial zeta devices apply an electric field to the colloid and measure the resulting particle velocity. Most zeta instruments use an optical technique to measure the particle velocity, but this has the severe drawback that it works only on very dilute suspensions. The dilution is time-consuming, but worse, it can alter the very thing you seek to measure. The Agilent 7020 ZetaProbe overcomes this problem by using ultrasound instead of light to measure the particle velocity.
Plots of zeta potential show the effects of electrolytes and dispersants
Figure 1 shows the graphs of zeta versus pH for a sample of alumina particles in water at three different electrolyte concentrations. At the lowest salt concentration (0.01 M KCl), zeta is more than 25 mV in magnitude if the pH is less than 8.5 or more than 11, so the colloid should be stable in this range. At higher salt levels, the allowable pH range for stability is reduced and at 1 M salt the suspension is unstable at all pH values.
To achieve stability at higher salt levels, it is necessary to use a polymeric dispersant. Figure 2 shows how zeta potential is used to determine the optimum dispersant doses for a cement sample. The zeta potential levels off when the particles are completely covered with polymer, so any polymer that is added beyond that point is wasted. Plots of zeta potential help you to optimize dispersant dosage, and to choose the most cost-effective dispersant.
The titrant in this case is called a superplasticizer. These molecules are used to increase the repulsion between cement particles as the cement mix is setting. The repulsion reduces the suspension viscosity and allows it to flow readily into the formwork. This allows the cement to be prepared with a lower water fraction, resulting in a much stronger final product. In the figure we show the effect of adding non-ionic (PCE-M4) and anionic (S200) superplasticizers to a fresh Portland cement mix. For both additives, the optimum dose is around 0.5 mL.
Note that these measurements are made under very difficult conditions. The pH is 13, which is too high for the optical devices. The Agilent 7020 ZetaProbe not only measures zeta under these conditions, it measures it with unprecedented precision, as shown by the smooth curves in this figure.
Faster, worry-free measurements
The Agilent 7020 ZetaProbe measures zeta potential in undiluted samples, which avoids experimental errors that can compromise the quality of your product or research. If you need to stabilize colloids, and you want an unequivocal method to measure zeta potential, learn more on our product page.
References
- “Basics of Zeta Potential,” Application Note 5989-8124EN.
- “Use of the Agilent 7020 ZetaProbe to control suspension stability,” Application Note 5989-8123EN.
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