Polyelectrolytes

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Example: polystyrene sulfonate

Polyelectrolytes are water-soluble compounds with a large chain length ( polymers ) that carry anionic ( polyacids ) or cationic ( polybases ) dissociable groups.

A distinction can be made between strong polyelectrolytes, which carry a permanent charge that is independent of the pH of the solution, and weak polyelectrolytes, the degree of dissociation of which depends on the pH of the solution. Examples of strong polyelectrolytes are sodium polystyrene sulfonate (anionic) or poly-diallyldimethylammonium chloride (cationic), representatives of the weak polyelectrolytes are polyacrylic acid (acidic) or polyethyleneimine (basic). The properties of a polyelectrolyte solution are largely determined by the repulsive interactions of the groups of the same charge on the polymer chain.

The charge intensity of polyelectrolytes can be precisely determined by titration (polyelectrolyte titration).

A strong polyelectrolyte belonging to the group of biopolymers is also the DNA molecule.

Polyelectrolyte effect

A typical example of this is the viscosity behavior of salt-free polyelectrolyte solutions. While the viscosity of neutral polymer solutions decreases linearly with increasing dilution, polyelectrolyte solutions show an increase in viscosity. This is explained by the increasing stiffening of the polymer due to the repulsion of the groups with the same charge , since the ionic strength of the solution decreases with decreasing concentration and thus the charges are more poorly shielded and weak polyelectrolytes dissociate more strongly with increasing dilution ( Ostwald's law of dilution ). This stiffening effect is known as the polyelectrolyte effect. The viscosity as a macroscopically accessible property of polymer solutions is thus determined by the conformation of the dissolved polyelectrolyte molecules. Strongly charged polyelectrolytes stretch out in the solution, whereas polyelectrolytes with reduced charge or shielded charges tend to tangle. This entanglement can be controlled with weak polyelectrolytes by the pH value and the salt concentration and with strong polyelectrolytes exclusively by the salt concentration. An important application of these properties is the polyelectrolyte adsorption on solid surfaces, as it is e.g. B. is used in the layer-by-layer process.

Polyelectrolyte adsorption

Dissolved polyelectrolytes can adsorb on oppositely charged surfaces. The adsorption is driven, among other things, by the electrostatic attraction between the charged monomer units and oppositely charged dissociated surface groups (e.g. SiO groups on silicon dioxide surfaces). However, the release of counterions or the formation of hydrogen bonds also enable adsorption. The conformation of the polyelectrolyte in the dissolved state determines the amount of substance adsorbed. Stretched polyelectrolyte molecules adsorb as thin films (0.2–1 nm) on the surface, whereas coiled polyelectrolyte molecules form thicker layers (1–8 nm).

Degree of polymerization

In addition to their charge activity, polyelectrolytes differ primarily in terms of their molar mass . Low molecular weight polyelectrolytes have a molar mass of up to 100,000 g · mol −1 , high molecular weight polyelectrolytes up to over 10 million. With a theoretical CC distance in the molecular chain of 1.53 Angstroms , the molecular chain can reach a length of over 15 µm. Low molecular weight polyelectrolytes are mainly used as dispersants , high molecular weight polyelectrolytes as flocculation aids . Depending on the molar mass, the viscosity of the polyelectrolyte solutions increases. Low molecular solutions are aqueous to slightly viscous, high molecular solutions are extremely viscous at a concentration of 0.1%.

Because of their extreme viscosity, high molecular weight polyelectrolytes cannot be produced as aqueous solutions, but only by block polymerization , with powders being formed as end products, or by emulsion polymerization with liquid emulsions as end products.

Examples of important polyelectrolytes

The most important representatives of the anionic polyelectrolytes ( polyanions ) are:

The cationic polyelectrolytes include:

literature

Individual evidence

  1. ↑ Stability of Aqueous Al203 Suspensions with PMAA Polyelectrolyte (PDF; 729 kB), J. Am. Ceram. Soc., 1988, 71 (4), 250-55.
  2. ^ PH-Dependent Thickness Behavior of Sequentially Adsorbed Layers of Weak Polyelectrolytes, Macromolecules, 2000, 33 (11), 4213-4219; doi : 10.1021 / ma991645q .