Polyelectrolytes, also known as polymer electrolytes and polymeric electrolytes, are water-soluble polymers that contain positively or negatively charged ionic groups; that is, cations (+) or anions (–). They may be natural materials, such as alginates, carrageenan, carbohydrates, chitosan, pectin, and nucleic acids, or synthetics, such as poly(acrylic acid), polyvinylamine, poly(vinylsulfonic acid), and polyvinylpyridine. Polyelectrolytes that carry both positive and negative charges, such as proteins, are known as polyampholytes. When polyelectrolytes dissociate in water, one type of charge remains on the polymers (polyions) and their small counterions diffuse throughout the solution. In contrast, the positive and negative ions of electrolytes such as salt (NaCl) and hydrogen chloride (HCl) dissociate uniformly in water. See also: Alginate; Carbohydrate; Carrageenan; Electrolyte; Ion; Isoelectric point; Nucleic acid; Pectin; Polyacrylate resin; Polymer; Polysaccharide; Protein; Salt (chemistry)

Because polyions interact with other charged particles, they have been used in applications such as removing (precipitating) small suspended particles from liquids in water treatment as well as in thickening, stabilizing, dispersing, and emulsifying cosmetics, foods, and other formulated products. Polyelectrolyte gels, which are charged polymer networks, have potential applications as scaffolds for tissue engineering and for the replacement of biological tissues. Layer-by-layer deposition of alternating positively and negatively charged polymers on charged substrates has been used as a technique for producing multilayer coatings or complexes for the controlled release of drugs as well as for depositing coatings on films and fibers and building supports for catalysts. See also: Biodegradable materials for tissue engineering; Catalysis; Emulsion; Gel; Microengineered hydrogels; Regenerative medicine; Water treatment