What is the isoelectric point?

Why does the surface charge depend on the pH value?

The surface charge of a solid in an aqueous solution is created by deprotonation or protonation of functional chemical groups at the surface. Such groups include carboxyl groups (COOH) or amino groups (NH₂). These groups change when reactions such as deprotonation or protonation occur. During a deprotonation reaction, part of the functional group remains at the surface, while a detached proton (H⁺) combines with molecules of the aqueous solution, for example to form hydronium (H₃O⁺). During protonation, on the other hand, an additional proton (H⁺) is taken up by the functional group, while a hydroxide ion (OH^-) remains in the aqueous solution. Examples of these reactions are:

• Deprotonation of a carboxyl group (COOH)
  [surface]-COOH + H₂O ⇌ [surface]-COO^- + H₃O⁺_(aq)
• Protonation of an amino group (NH₂)
  [surface]-NH₂ + H₂O ⇌ [surface]-NH₃⁺ + OH^-_(aq)

Both deprotonation and protonation are reversible. Over time, an equilibrium of reactions is established. This equilibrium depends on how much hydronium or hydroxide ion is already contained in the aqueous solution, i.e., the pH value. If the pH value of the aqueous solution is changed, for example by adding an acid or base, the surface charge and the zeta potential of the surface change as well.

How the isoelectric point can be determined

The isoelectric point of solids in an aqueous solution can be determined experimentally by measuring the zeta potential as a function of the pH value. At a low pH value, the zeta potential is higher and decreases as the pH value increases. At the isoelectric point, the zeta potential is zero, as the surface is neither charged positively nor negatively.

The zeta potential can be determined quickly and easily with the ZPA 20 Zeta Potential Analyzer from DataPhysics Instruments. Together with an LDU 25 liquid dosing unit, acids and bases can be added to the aqueous solution during the measurement procedure. This allows the pH value of the solution to be changed and the zeta potential to be determined automatically as a function of the pH value in a series of measurements. The isoelectric point can then be determined from the series of measurements.

Applications for determining the isoelectric point

The measurement of the isoelectric point is relevant in many areas, particularly in colloid and interface research and in materials science. Some important areas of application are:

• One area of application is seawater desalination. Here, interactions between the components of the seawater and the filtration membranes are analysed using zeta potential measurements.
• In biology, the isoelectric point influences protein interactions, cell adhesion and the transport of biomolecules.
• In the textile industry, surfaces are also studied using the zeta potential and the isoelectric point. The focus here is on the kinetics of surface-active substances on textiles, particularly in textile cleaning.

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