Polyelectrolytes are
polymers whose repeating units bear an
electrolyte
An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...
group.
Polycations and polyanions are polyelectrolytes. These groups
dissociate
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an aci ...
in
aqueous
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in wat ...
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 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, and
DNA are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.
Charge
Acid
An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
s 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 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 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's
ionic strength, and therefore the
Debye length. This, in turn, affects other properties, such as
electrical conductivity
Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...
.
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.
Conformation
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 Double layer forces occur between charged objects across liquids, typically water. This force acts over distances that are comparable to the Debye length, which is on the order of one to a few tenths of Nanometre, nanometers. The strength of these ...
, 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 good
solvent
A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...
).
Polymer
conformation affects many bulk properties (such as
viscosity
Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
,
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.
Polyampholytes
Polyelectrolytes that 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 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 All
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s are polyampholytes, as some
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
s tend to be acidic, while others are basic.
Applications
Polyelectrolytes have many applications, mostly related to modifying flow and stability properties of aqueous solutions and
gels. For instance, they can be used to destabilize a
colloidal suspension and to initiate
flocculation
In colloidal chemistry, flocculation is a process by which colloidal particles come out of Suspension (chemistry), suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The actio ...
(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,
emulsifier
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althou ...
s,
conditioners,
clarifying agents, and even
drag reducers. They are used in
water treatment
Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, ...
and for
oil recovery. Many
soap
Soap is a salt (chemistry), salt of a fatty acid (sometimes other carboxylic acids) used for cleaning and lubricating products as well as other applications. In a domestic setting, soaps, specifically "toilet soaps", are surfactants usually u ...
s,
shampoos, and
cosmetics
Cosmetics are substances that are intended for application to the body for cleansing, beautifying, promoting attractiveness, or altering appearance. They are mixtures of chemical compounds derived from either Natural product, natural source ...
incorporate polyelectrolytes. Furthermore, they are added to many foods and to
concrete
Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...
mixtures (
superplasticizer). Some of the polyelectrolytes that appear on food labels are
pectin,
carrageenan,
alginates, and
carboxymethyl cellulose. All but the last are of natural origin. Finally, they are used in various materials, including
cement.
Because some of them are water-soluble, they are also investigated for biochemical and medical applications. There is currently much research on using
biocompatible polyelectrolytes for
implant 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 and muscle like soft actuators.
Multilayers
Polyelectrolytes have been used in the formation of new types of materials known as polyelectrolyte multilayers (PEMs). These thin films are constructed using a layer-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 electrostatically
cross-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 as
nanoparticles or
clay platelets in place of or in addition to one of the polyelectrolytes. LbL deposition has also been accomplished using
hydrogen bonding instead of
electrostatics. For more information on multilayer creation, please see
polyelectrolyte 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 using
multi-parametric surface plasmon resonance to determine adsorption kinetics, layer thickness, and optical density.
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 of
metal
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
or
semiconductor
A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
nanoparticles, or
porosity phase transitions to create
anti-reflective coatings, optical
shutters, and
superhydrophobic coatings.
Bridging
If polyelectrolyte chains are added to a system of charged macroions (i.e., an array of DNA molecules), an interesting phenomenon called the polyelectrolyte bridging might occur. The term bridging interactions is usually applied to the situation where a single polyelectrolyte chain can
adsorb 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 by
steric 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's
rubber elasticity.
Because of its connectivity, the behavior of the polyelectrolyte chain bears almost no resemblance to that of confined, unconnected ions.
Polyacid
In
polymer
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
terminology, a polyacid is a polyelectrolyte composed of
macromolecules containing
acid
An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
groups on a substantial fraction of the
constitutional units. Most commonly, the acid groups are , , or .
See also
*
Dispersity
In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsi ...
*
Ion-exchange resin
*
Polypyridinium salts
References
External links
Max Planck Institute for Polymer Research, Mainz, Germany*
ttps://angelchemindia.com/ Polyelectrolytes: Vadodara, Gujarat, India
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Colloidal chemistry
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