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

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).