Polyelectrolytes which bear both cationic and anionic repeat groups are called polyampholytes. The competition between the acid-base equilibria of these groups leads to additional complications in their physical behavior. These polymers usually only dissolve when there is sufficient added salt, which screens the interactions between oppositely charged segments. Many proteins are polyampholytes, as some amino acids tend to be acidic while others are basic.

Polyelectrolytes have many applications, mostly related to modifying flow and stability properties of aqueous solutions and gels. For instance, they can be used to either stabilize colloidal suspensions, or to initiate flocculation (precipitation). They can also be used to impart a surface charge to neutral particles, enabling them to be dispersed in aqueous solution. They are thus often used as thickeners, emulsifiers, conditioners, flocculants, and even drag reducers. They are used in water treatment and for oil recovery. Many soaps, shampoos, and cosmetics incorporate polyelectrolytes. Furtheremore they are added to many foods and to concrete mixtures (superplasticizer). Some of the polyelectrolytes that appear on food labels are pectin, carrageenan, alginates, polyvinylpyrrolidone and carboxymethyl cellulose. All but the last two are of natural origin. Finally, they are used in a variety of materials, including cement.

Because some of them are water-soluble, they are also investigated for biochemical and medical applications. There is currently much research in using biocompatible polyelectrolytes for implant coatings, for controlled drug release, and other applications.

http://en.wikipedia.org/wiki/Polyelectrolyte