Determining the Isoelectric Point (IEP) In Engineered Particles

Charges of the particles present in a colloidal dispersion can affect the stability of the suspension.

Each particle is covered by oppositely charged ions near the surface, forming a stern or a fixed layer. Both positive and negative ions form a charge cloud beyond the stern layer. The shear or slipping plane is the distance between the particle surfaces separating the ions in equilibrium from the double layer of ions moving along with the particle.

The zeta potential (z) is defined as the electric potential at the slipping plane, measure in mV. Figure 1 shows the colloidal dispersion of particles forming the stern layer.

Figure 1. Colloidal dispersion of particles forming stern layer

Stable dispersions may have high zeta potential, which helps in predicting the dispersion stability. The zeta potential can be easily altered by adjusting the pH of dispersion and therefore the dispersion stability. This article briefs about the isoelectric point (IEP) of dispersion, which is based on the zeta potential measurements.

What is Isoelectric Potential?

The IEP of dispersion is defined as the pH value at which the value of zeta potential is zero. The IEP is used for the determination of the conditions at which the dispersion is unstable and also the chemical species present at the surface of the engineered particles as zeta potential is specific to surface properties. For instance, the IEP of a silica particle covered with alumina will be similar to the IEP of alumina. Table 1 shows certain reference IEP values.

Table 1. Reference values of certain IEPs

Determination of Dairy Creamer IEP

Coffee-mate is a non-dairy creamer with a reproducible IEP value. The original powder product consists of water emulsion and oil, which is stabilized by emulsifiers such as mono and diglycerides.

The sample is prepared by dissolving 0.1g of powder in 200mL of DI water followed by stirring for three minutes. This sample was continuously stirred using a pH probe immersed in the sample. Titration of the sample is carried out by using 0.1N solution of HCl to change the pH from 6.7 to 3.5.

The steps given below explain the procedure of sample measurement:

• Place 3mL of sample in a disposable cell
• Insert the Nicomp dip cell electrodes into the disposable cell
• Perform 60s measurements for three times at each pH value
• Calculate the average zeta potential value
• Flush the dip cell electrodes after each measurement using DI water
• Lower the pH for the next measurement

The entire measurement settings of zeta potential were shown in the Figure 2. The results obtained after taking zeta potential measurements at seven pH values are shown in Figure 3. The value of IEP obtained in this study is 4.2 at 0mV zeta potential.

Figure 2. Zeta potential measurement settings

Figure 3. IEP data for Coffee-mate

Determination of IEP of Proteins

Protein samples were prepared by diluting bovine serum albumin (BSA) from Sigma Aldrich with DI water in 1:100 proportion. The BSA solution was mixed with 0.1M KOH to alter the pH to 8 and titrated using 0.1M HCl to achieve a final pH of 3.75. At each pH value, three zeta potential measurements were carried out, and the average value was obtained. Figure 4 shows the volume-weighted mean particle with a diameter of 5.5nm. Figure 5 shows the IEP titration data, where the IEP = 5.07.

Figure 4. Particle size of BSA protein - volume weighted

Figure 5. IEP data for BSA protein

Conclusion

Zeta potential measurements are carried out to nullify dispersion instability arising as a result of IEP under surface chemistry conditions. Methods of sample preparation and measurement of IEPs of proteins and dairy creamer are described in this article, and the graphs illustrating the zeta potential measurements of these substances are shown.

This information has been sourced, reviewed and adapted from materials provided by Entegris

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