Polyelectrolyte

polyelectrolyte

[¦päl·ē·ə′lek·trə‚līt] (organic chemistry) A natural or synthetic electrolyte with high molecular weight, such as proteins, polysaccharides, and alkyl addition products of polyvinyl pyridine; can be a weak or strong electrolyte; when dissociated in solution, it does not give uniform distribution of positive and negative ions (the ions of one sign are bound to the polymer chain while the ions of the other sign diffuse through the solution).

Polyelectrolyte

a polymer electrolyte, that is, a polymer that dissociates into ions in solution. A large number of periodically repeating charges arises in each macromolecule during dissociation. Polyelectrolytes are divided into polyacids (for example, polyacrylic acids), polybases (such as polyvinyl pyridinium), and polyampholytes (copolymers containing both basic and acidic groups). Most polyelectrolytes contain weak acidic or basic groups and therefore are ionized only in the presence of a strong base (for a polyacid) or strong acid (for a polybase).

Primary biopolymers such as proteins and nucleic acids are among the polyelectrolytes. Cross-linked polyelectrolytes, which are prepared by the introduction of readily dissociating groups (for example, sulfo and amino groups) into various cross-linked polymers, are of great importance to industry and laboratory practice. The most valuable cross-linked polyelectrolytes are ion-exchange resins.

The dissociating groups in the polymer molecules determine the solubility of polyelectrolytes in water and other polar liquids. For example, a sulfonated linear polystyrene dissolves freely in water, although polystyrene itself is one of the most water-resistant polymers known. Cross-linked polyelectrolytes of three-dimensional structure swell in water rather than dissolving. The properties of polyelectrolyte molecules in solution are determined by the electrostatic interaction of charged groups in the chain with one another and with low-molecular-weight ions in the solution. The strong electrostatic field generated by the charges in the polyelectrolyte molecule holds a large number of oppositely charged ions close to the molecule. The electrostatic repulsion of groups of like charge leads to a substantial alteration of the macromolecular conformations in solutions: the effective size of the molecules increases, and the coiled chains straighten out, assuming an approximately linear form as the degree of polyelectrolyte dissociation increases. The physicochemical properties of solutions also undergo considerable alteration (for example, solution viscosity increases by hundreds and thousands; the higher the concentration, the greater the viscosity). The theory that was developed for solutions of low-molecular-weight electrolytes ceases to be valid for polyelectrolyte solutions. The low-molecular-weight ions that appear during the dissociation of polar groups of these polyelectrolytes create a diffuse shell around the oppositely charged surface of the polymer and may to some degree be replaced by other ions of like sign.

REFERENCES

Tager, A. A. Fiziko-khimiia polimerov, 2nd ed. Moscow, 1968.
Rice, S. A., and M. Nagasawa. Polyelectrolyte Solutions: A Theoretical Introduction. London-New York, 1961.

M. E. ERLYKINA

polyelectrolyte