Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes (salts) and polymers (high molecular weight compounds) and are sometimes called polysalts. Like salts, their solutions are electrically conductive. Like polymers, their solutions are often viscous. Charged molecular chains, commonly present in soft matter systems, play a fundamental role in determining structure, stability and the interactions of various molecular assemblies. Theoretical approaches to describing their statistical properties differ profoundly from those of their electrically neutral counterparts, while technological and industrial fields exploit their unique properties. Many biological molecules are polyelectrolytes. For instance, polypeptides, glycosaminoglycans, and DNA are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.
Acids are classified as either weak or strong (and bases similarly may be either weak or strong). Similarly, polyelectrolytes can be divided into "weak" and "strong" types. A "strong" polyelectrolyte is one that dissociates completely in solution for most reasonable pH values. A "weak" polyelectrolyte, by contrast, has a dissociation constant (pKa or pKb) in the range of ~2 to ~10, meaning that it will be partially dissociated at intermediate pH. Thus, weak polyelectrolytes are not fully charged in solution, and moreover their fractional charge can be modified by changing the solution pH, counter-ion concentration, or ionic strength.
The physical properties of polyelectrolyte solutions are usually strongly affected by this degree of charging. Since the polyelectrolyte dissociation releases counter-ions, this necessarily affects the solution's ionic strength, and therefore the Debye length. This in turn affects other properties, such as electrical conductivity.
When solutions of two oppositely charged polymers (that is, a solution of polycation and one of polyanion) are mixed, a bulk complex (precipitate) is usually formed. This occurs because the oppositely-charged polymers attract one another and bind together.
The conformation of any polymer is affected by a number of factors: notably the polymer architecture and the solvent affinity. In the case of polyelectrolytes, charge also has an effect. Whereas an uncharged linear polymer chain is usually found in a random conformation in solution (closely approximating a self-avoiding three-dimensional random walk), the charges on a linear polyelectrolyte chain will repel each other via double layer forces, which causes the chain to adopt a more expanded, rigid-rod-like conformation. If the solution contains a great deal of added salt, the charges will be screened and consequently the polyelectrolyte chain will collapse to a more conventional conformation (essentially identical to a neutral chain in good solvent).
Polymer conformation of course affects many bulk properties (such as viscosity, turbidity, etc.). Although the statistical conformation of polyelectrolytes can be captured using variants of conventional polymer theory, it is in general quite computationally intensive to properly model polyelectrolyte chains, owing to the long-range nature of the electrostatic interaction. Techniques such as static light scattering can be used to study polyelectrolyte conformation and conformational changes.
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