Polyelectrolytes arepolymers whose repeating units bear anelectrolyte group.Polycations and polyanions are polyelectrolytes. These groupsdissociate inaqueous solutions (water), making the polymerscharged. Polyelectrolyte properties are thus similar to both electrolytes (salts) and polymers (highmolecular weight compounds) and are sometimes calledpolysalts. Like salts, their solutions are electrically conductive. Like polymers, their solutions are oftenviscous. 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[1][2] to describe 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, andDNA are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.
polyelectrolyte: Polymer composed of macromolecules in which a substantial portion of the constitutional units contains ionic or ionizable groups, or both. (See Gold Book entry for note.)[3]
Acids are classified as eitherweak orstrong (andbases similarly may be eitherweak orstrong). Similarly, polyelectrolytes can be divided into "weak" and "strong" types. A "strong" polyelectrolyte dissociates completely in solution for most reasonablepH values. A "weak" polyelectrolyte, by contrast, has adissociation 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 the 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 ionization. Since the polyelectrolyte dissociation releases counter-ions, this necessarily affects the solution'sionic strength, and therefore theDebye length. This, in turn, affects other properties, such aselectrical conductivity.
When solutions of two oppositely charged polymers (that is, a solution ofpolycation and one ofpolyanion) 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-dimensionalrandom walk), the charges on a linear polyelectrolyte chain will repel each other viadouble layer forces, which causes the chain to adopt a more expanded, rigid-rod-like conformation. The charges will be screened if the solution contains a great deal of added salt. Consequently, the polyelectrolyte chain will collapse to a more conventional conformation (essentially identical to a neutral chain in goodsolvent).
Polymerconformation affects many bulk properties (such asviscosity,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 asstatic light scattering can be used to study polyelectrolyte conformation and conformational changes.
ampholytic polymer: Polyelectrolyte composed of macromolecules containing both cationic and anionic groups, or corresponding ionizable groups. (See Gold Book entry for note.)[4]
Polyelectrolytes that bear both cationic and anionic repeat groups are calledpolyampholytes. The competition between the acid-base equilibria of these groups leads to additional complications in their physical behavior. These polymers usually only dissolve when sufficient added salt screens the interactions between oppositely charged segments. In the case of amphoteric macroporous hydrogels, the action of concentrated salt solution does not lead to the dissolution of polyampholyte material due to the covalent cross-linking of macromolecules. Synthetic 3-D macroporous hydrogels shows the excellent ability to adsorb heavy-metal ions in a wide range of pH from extremely diluted aqueous solutions, which can be later used as an adsorbent for purification of salty water[5][6] Allproteins are polyampholytes, as someamino acids tend to be acidic, while others are basic.
Polyelectrolytes have many applications, mostly related to modifying flow and stability properties of aqueous solutions andgels. For instance, they can be used to destabilize acolloidal suspension and to initiateflocculation (precipitation). They can also be used to impart asurface charge to neutral particles, enabling them to be dispersed in aqueous solution. They are thus often used asthickeners,emulsifiers,conditioners,clarifying agents, and evendrag reducers. They are used inwater treatment and foroil recovery. Manysoaps,shampoos, andcosmetics incorporate polyelectrolytes. Furthermore, they are added to many foods and toconcrete mixtures (superplasticizer). Some of the polyelectrolytes that appear on food labels arepectin,carrageenan,alginates, andcarboxymethyl cellulose. All but the last are of natural origin. Finally, they are used in various materials, includingcement.
Because some of them are water-soluble, they are also investigated for biochemical and medical applications. There is currently much research on usingbiocompatible polyelectrolytes forimplant coatings, controlled drug release, and other applications. Thus, recently, the biocompatible and biodegradable macroporous material composed of polyelectrolyte complex was described, where the material exhibited excellent proliferation of mammalian cells[7] and muscle like soft actuators.
Polyelectrolytes have been used in the formation of new types of materials known aspolyelectrolyte multilayers ('PEMs). These thin films are constructed using alayer-by-layer (LbL) deposition technique. During LbL deposition, a suitable growth substrate (usually charged) is dipped back and forth between dilute baths of positively and negatively charged polyelectrolyte solutions. During each dip, a small amount of polyelectrolyte is adsorbed, and the surface charge is reversed, allowing the gradual and controlled build-up of electrostaticallycross-linked films of polycation-polyanion layers. Scientists have demonstrated thickness control of such films down to the single-nanometer scale. LbL films can also be constructed by substituting charged species such asnanoparticles orclay platelets[8] in place of or in addition to one of the polyelectrolytes. LbL deposition has also been accomplished usinghydrogen bonding instead ofelectrostatics. For more information on multilayer creation, please seepolyelectrolyte adsorption.
An LbL formation of PEM (PSS-PAH (poly(allylamine) hydrochloride)) on a gold substrate can be seen in the Figure. The formation is measured usingmulti-parametric surface plasmon resonance to determine adsorption kinetics, layer thickness, and optical density.[9]
The main benefits of PEM coatings are the ability to conformably coat objects (that is, the technique is not limited to coating flat objects), the environmental benefits of using water-based processes, reasonable costs, and the utilization of the particular chemical properties of the film for further modification, such as the synthesis ofmetal orsemiconductor nanoparticles, orporosity phase transitions to createanti-reflective coatings, opticalshutters, andsuperhydrophobic coatings.
If polyelectrolyte chains are added to a system of charged macroions (i.e., an array of DNA molecules), an interesting phenomenon called thepolyelectrolyte bridging might occur.[10] The term bridging interactions is usually applied to the situation where a single polyelectrolyte chain canadsorb to two (or more) oppositely charged macroions (e.g. DNA molecule) thus establishing molecular bridges and, via its connectivity, mediate attractive interactions between them.
At small macroion separations, the chain is squeezed in between the macroions and electrostatic effects in the system are completely dominated bysteric effects – the system is effectively discharged. As we increase the macroion separation, we simultaneously stretch the polyelectrolyte chain adsorbed to them. The stretching of the chain gives rise to the above-mentioned attractive interactions due to the chain'srubber elasticity.
Because of its connectivity, the behavior of the polyelectrolyte chain bears almost no resemblance to that of confined, unconnected ions.
Inpolymer terminology, apolyacid is a polyelectrolyte composed ofmacromolecules containingacid groups on a substantial fraction of theconstitutional units. Most commonly, the acid groups are–COOH,–SO3H, or–PO3H2.[11]
