Isoelectric Point
The isoelectric point (IEP) of a dispersion is the pH value where the zeta potential equals zero.
Determining the IEP
The IEP of a dispersion is the condition that is least stable and frequently leads to agglomeration for suspensions and phase separation for emulsions. Determining the IEP is a common zeta potential application for the Nicomp® DLS particle size and zeta potential analyzer.
Zeta potential is a measure of the charge on the surface of a particle. The zeta potential is a function of the specific surface chemistry of a dispersion and is affected by changes in pH, salt, and surfactant concentration. The IEP is the pH value at which the zeta potential value is zero, implying no electric charge on the surface of a particle (droplet, molecule, etc.). Determining the IEP of a dispersion can be helpful to predict stability/instability and to identify the predominant chemical species on the surface of an engineered particle.
IEP tests are helpful in the following situations:
- Predicting optimum conditions for dispersion stability
- Determining the predominant chemical species on the surface of complex particles
- The IEP of proteins is important for storage stability and in processes including gel electrophoresis
Featured Application Note
Featured Application Note
Featured Application Note
The IEP can be measured using the Nicomp Z3000 system by titrating the sample and measuring the zeta potential as a function of pH. Two examples of IEP measurements made on the Nicomp Z3000 system include an emulsion (Coffee mate® creamer) and a protein (bovine serum albumin).
IEP of Proteins
With the Nicomp Z3000 system it is possible to not only size proteins but also determine their IEP. Ampules of bovine serum albumin (BSA) were purchased from Sigma Aldrich and diluted 1:100 with deionized water. Figure A below shows the volume-weighted mean particle diameter to be 5.5 nm. Aggregates of ~25 nm were also detected. For determination of the IEP, this BSA solution was brought to pH 8 with 0.1M KOH and titrated with 0.01M HCl to a final pH of 3.75. Zeta potential readings were taken, and the results are shown in Figure B. The IEP for BSA was found to be at pH 5.07.
Figure A
Figure B
IEP of Coffee mate® Creamer
The isoelectric point (IEP) is the pH value where the zeta potential is zero. The IEP can be determined using the Nicomp Z3000 system by titrating the sample and recording zeta potential as a function of pH. Coffee mate creamer was prepared by mixing 0.1 g in 200 mL of DI water. The sample was placed on a magnetic stirring plate and mixed constantly during the study, with a newly calibrated pH probe immersed in the suspension. The pH was altered by adding drops of 0.1 N hydrochloric acid. After the pH stabilized a sample was removed by a syringe and injected into the zeta potential cell. Three zeta potential measurements were performed, and the average value were plotted vs pH. The results are shown in the graph.
The IEP of the Coffee mate suspension in this study was determined to be at pH 4.26.
