Polyelectrolyte

Polyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups will dissociate in aqueous 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 describing their statistical properties differ profoundly from those of their electrically neutral counterparts, while their unique properties are being exploited in a wide range of technological and industrial fields. One of their major roles, however, seems to be the one played in biology and biochemistry. Many biological molecules are polyelectrolytes. For instance, polypeptides (thus all proteins) and DNA are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.

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Polyelectrolyte charge

Acids 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 is one which 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 solution, and moreover their fractional charge can be modified by changing the solution pH, counterion concentration, or ionic strength.

The physical properties of polyelectrolyte solutions are usually strongly affected by this degree of charging. 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.

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 irreversibly bind together.

Polyelectrolyte 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 (Coulomb repulsion), which causes the chain to adopt a more expanded, rigid-rod-like conformation. If the solution contains a great deal of added salt, the charges will be screened and consequently the polyelectrolyte chain will collapse to a more conventional conformation (essentially identical to a neutral chain in good solvent).

Polymer conformation of course affects many bulk properties (such as viscosity, 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 Coulomb interaction.

Polyampholytes

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.

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 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, flocculents, and even drag reducers. They are used in water treatment and for oil recovery. Many soaps, shampoos, and cosmetics incorporate polyelectrolytes. Additionally, they are added to many foods. 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.

Polyelectrolyte multilayers

Recently, polyelectrolytes have been utilized 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.

The main benefits to 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 or semiconductor nanoparticles, or porosity phase transitions to create anti-reflective coatings, optical shutters, and superhydrophobic coatings..

Polyelectrolyte bridging

If polyelectrolyte chains are added to a system of charged macroions (ie. 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 (see adsorption) 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 chain's rubber elasticity.

Due to its connectivity the behaviour of the polyelectrolyte chain bears almost no resemblance to the case of confined unconnected ions.

References

Gero Decher, Science, Vol 277, Issue 5330, 1232-1237 , 29 August 1997. DOI: 10.1126/science.277.5330.1232

Chaoyang Jiang, Dinesh S. Kommireddy, Vladimir V. Tsukruk, "Gradient Array of Freely Suspended Nanomembranes", Advanced Functional Materials,2006, 16, 27-32.

Ulrich, S.; Seijo, M.; Stoll, S. Current Opinion in Colloid & Interface Science 2006, 11, 268.

De Gennes, P.-G., Scaling Concepts in Polymer Physics, Cornell University Press, 1979

S.A. Baeurle and E.A. Nogovitsin, 'Challenging scaling laws of flexible polyelectrolyte solutions with effective renormalization concepts', Polymer 48, 4883-4899 (2007).