Characterizing Stability

May 1, 2006
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Zeta potential and particle size analysis techniques can accurately characterize the stability of ceramic dispersions.

Ceramics in the form of colloidal dispersions are used in a wide variety of products and applications, from polishing semiconductor wafers to forming corrosion-resistant coatings. The dispersions are expected to retain their original properties for a specified period of time, which is often referred to as the product's shelf life or stability. A product that remains free from change or variation under acceptable environmental conditions for a specified period of time can be said to be stable.

Stability is important because if the size distribution of the dispersed particles changes, the resulting product will not meet the expected specifications. For example, if a dispersion used to make a coating that increases the hardness of a tool undergoes a shift to larger particle sizes, the resulting coating will have defects and thus won't completely protect the softer surface underneath.

Evaluating Stability

All dispersed systems are thermodynamically unstable. When a dispersed system is called stable, we are really referring to the rate of change. A stable emulsion in this context is one that resists change over the course of its specified shelf life. A product that changes slowly can have a long shelf life, while a product that changes quickly will have a short shelf life. The required shelf life depends on the particular product and how quickly it is expected to be used.

During the time a product is on the shelf, the dispersed particles are in motion. If they are denser than the suspending medium, the particles tend to settle to the bottom of the container, and if they are less dense than the suspending medium they migrate to the top. The rate at which this occurs depends on the size of the particles and the interactions between them. Furthermore, those interactions influence whether the particles aggregate or coalesce into larger particles prior to migrating to the bottom or top of the container.

Stability often varies from product to product and even lot to lot. Consequently, the stability of these dispersions must be evaluated on a routine basis by the user. Most often, shelf life tests are used for this purpose. Shelf life testing has the advantage of being a direct measure of stability under the conditions that the product would normally be stored. However, it has the disadvantage of taking a long time to complete.

Instead of measuring the stability directly, other methods can be used to predict stability. Particle size analysis has the benefit of generating results quickly. However, it is not a direct measurement of stability. Additionally, since most commercial particle size analyzers are based on laser light scattering, the correlation between particle size and stability is often poor.1 Certain specialized particle sizing techniques, like single particle optical sizing (SPOS),2 that count oversized particles can often provide a more accurate assessment of stability. However, the best technique would measure the particle-particle interactions that are directly responsible for a dispersion's stability.

Figure 1. Representative energy curves detailing the repulsive forces between two particles.

Understanding Zeta Potential

The repulsive forces between particles that act as a barrier to agglomeration can be measured through a physiochemical parameter called zeta potential, which is a characteristic not of the particle itself but of the particle in the colloidally dispersed state. It is important to understand that in the dispersed state, all particles are charged. This charge has two effects-it repels particles of like charge over long distances, and it attracts dissolved ions of opposite charge that may be present. The repulsive energy that exists between two particles is a function of the surface charges on both particles and the extent that oppositely charged ions attracted to the particles screen or otherwise neutralize those surface charges. This repulsive energy is measured as the zeta potential.

Thus, the zeta potential is not only a function of the actual charge at the surface of a particle but also of its ionic environment. Change the environment (i.e., alter the pH or the surfactant used to wet the particles), and the zeta potential will change accordingly. The red curve in Figure 1 represents the repulsive energy between particles as a function of distance between the particles. The repulsive energy increases slowly as the charged particles approach each other and more quickly as the electrical double layers of the particles overlap. The sharp increase in repulsive energy forms a barrier and prevents the particles from coming closer together. For them to coalesce, their kinetic energy must exceed this barrier.

Zeta potential is related to the height of the barrier. The blue curve represents the repulsive energy between the same particles, except the ionic environment was changed such that the barrier and thus the zeta potential were reduced. Less energy is needed to overcome the reduced barrier. Coalescence is easier to achieve, and the dispersion is less stable.

Figure 2. A particle in an electric field. The velocity of the moving particle is proportional to the charge on the particle, as well as the magnitude of the electric field.

Electrophoretic Light Scattering

One way to measure zeta potential is through electrophoretic light scattering (ELS). This method involves subjecting the dispersion to an electric field, which causes the particles to move to the oppositely charged electrodes (see Figure 2). The velocity of the moving particles is proportional to the charge on the particles, as well as to the magnitude of the electric field. The velocity is measured by scattering laser light from the particles. By measuring either the Dobbler shift (frequency mode) or the rate of change of the phase (phase mode) of this scattered light, the particle velocity can be determined. This velocity is then used to calculate the zeta potential.

Figure 3. Frequency power spectra obtained from an ELS analysis of a ceramic dispersion at pH 10.
Figure 3 contains a frequency power spectrum of a silicon nitride (Si3N4) ceramic dispersion. This power spectrum was obtained by Fourier analysis of the intensity vs. time profile of the scattered light mixed with unscattered light. The power spectra before and after the electric field is applied can be used to determine the Dobbler shift caused by the charged Si3N4 particles moving in an electric field of 15 volts/cm. The pH was measured to be 10. As can be seen, the zeta potential is -32.6 mV. This is considered a relatively high valve, meaning that at pH 10, this Si3N4 dispersion is most likely stable because the repulsive energy between particles is too high to overcome.

This dispersion was titrated, and the resulting measured zeta potentials as a function of pH can be seen in Table 1. When the pH is lowered to 8, the zeta potential remains negative but drops in magnitude by almost 30%. As the pH is lowered into the acidic region, the zeta potential increases again but it switches sign; the particles are now positively charged. The particles should be as stable as they were at pH 10 but with the opposite charge. Finally, at a very acidic pH, the zeta potential drops to +7.0 mV, low enough to assume that the particles will begin to coalesce.

These data illustrate that the ionic environments that produce the most stable dispersions are at either very basic or slightly acidic pH levels. The pH at which the particles have a zeta potential of 0 must be between pH 6 and 8; this point is known as the iso-electric point. The iso-electric point can tell us whether the particles that make up the dispersion are chemically acidic or basic. Colloidal silicon oxide (SiO2) has an iso-electric point at pH 2, which means that dispersed SiO2 particles act as acids. With an iso-electric point near the neutral range, the Si3N4 particles are most likely to act as neither an acid nor a base.

Combining Techniques

By measuring zeta potential, ELS can provide crucial information about the repulsive forces between particles that contribute to colloidal stability and which, in turn, can be used to predict shelf life. This method is experimentally similar to dynamic light scattering (DLS), which is a technique used to measure the particle size distributions of dispersed particles.

Ceramic dispersions can be more completely characterized by using these methods in conjunction with each other. In ceramic polishing slurries, for example, the mean diameter as measured by DLS can be used to determine the removal rate, while the zeta potential as measured by ELS can predict the appearance of aggregates that will scratch the working surface. These methods become even more powerful when combined in one package to create a multi-functional dispersion analyzer. Clearly, both DLS and ELS provide powerful tools to measure important features of the dispersed state.

For more information about particle sizing or zeta potential analysis, contact Patrick O'Hagan at 8203 Krystel Circle, Port Richey, FL 34668; (727) 846-0866; (727) 846-0865; e-mail ; or visit .


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