Editing Polyelectrolyte

{{Short description|Polymers whose repeating units bear an electrolyte group}}
[[File:polyelectrolyte examples.png|thumb|200px|Chemical structures of two synthetic polyelectrolytes, as examples. To the left is [[sodium polystyrene sulfonate|poly(sodium styrene sulfonate)]] (PSS), and to the right is [[polyacrylic acid]] (PAA). Both are negatively charged polyelectrolytes when dissociated. PSS is a 'strong' polyelectrolyte (fully charged in solution), whereas PAA is 'weak' (partially charged).]]

'''Polyelectrolytes''' are [[polymers]] whose repeating units bear an [[electrolyte]] group. [[Ion#Anions and cations|Polycations and polyanions]] are polyelectrolytes. These groups [[dissociation (chemistry)|dissociate]] in [[aqueous]] solutions (water), making the polymers [[charge (physics)|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 [[viscosity|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<ref name="Gennes1979">{{cite book|first=Pierre-Gilles |last=de Gennes|title=Scaling Concepts in Polymer Physics|url={{google books |plainurl=y |id=ApzfJ2LYwGUC}}|year=1979|publisher=Cornell University Press|isbn=0-8014-1203-X}}</ref><ref>{{cite journal | last1=Chremos | first1=A. | last2=Horkay | first2=F. | title=Disappearance of the polyelectrolyte peak in salt-free solutions | journal=Phys. Rev. E | publisher=American Physical Society (APS) | volume=102 | date=2020-07-27 | issue=1 | doi=10.1103/PhysRevE.102.012611 | pages=012611| pmid=32794995 | pmc=8243406 | bibcode=2020PhRvE.102a2611C }}</ref> 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]], [[glycosaminoglycan]]s, and [[DNA]] are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.
 {{Quote box|width = 35%
 |title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
 |quote = '''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.)
<ref name='Gold Book "polyelectrolyte"'>{{cite journal |title=polyelectrolyte |url=https://goldbook.iupac.org/terms/view/P04728 |website=Gold Book |date=2014 |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book P04728 |doi=10.1351/goldbook.P04728|url-access=subscription }}</ref>
}}

== Charge ==

[[Acid]]s are classified as either [[weak acid|weak]] or [[strong acid|strong]] (and [[Base (chemistry)|bases]] similarly may be either [[weak base|weak]] or [[strong base|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 [[Conductivity (electrolytic)|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 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]], 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]]).

Polymer [[Chemical structure|conformation]] 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 electrostatic interaction.
Techniques such as [[static light scattering]] can be used to study polyelectrolyte conformation and conformational changes.

== Polyampholytes ==
 {{Quote box|width = 35%
 |title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
 |quote = '''ampholytic polymer''': Polyelectrolyte composed of macromolecules containing both cationic and anionic groups, or corresponding ionizable groups. (See Gold Book entry for note.)<ref name='Gold Book "ampholytic polymer"'>{{cite journal |title=ampholytic polymer |url=https://goldbook.iupac.org/terms/view/AT07196 |website=Gold Book |date=2008 |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book AT07196 |doi=10.1351/goldbook.AT07196|doi-access=free }}</ref>
}}
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<ref>{{Cite journal | doi = 10.3144/expresspolymlett.2012.38| title = Novel macroporous amphoteric gels: Preparation and characterization| journal = Express Polymer Letters| volume = 6| issue = 5| pages = 346–353| year = 2012| last1 = Kudaibergenov | first1 = S.| doi-access = free}}</ref><ref>{{Cite journal | doi = 10.1002/masy.201100065| title = Metal Complexes of Amphoteric Cryogels Based on Allylamine and Methacrylic Acid| journal = Macromolecular Symposia| volume = 317–318| pages = 18–27| year = 2012| last1 = Tatykhanova | first1 = G. S. | last2 = Sadakbayeva | first2 = Z. K. | last3 = Berillo | first3 = D. | last4 = Galaev | first4 = I. | last5 = Abdullin | first5 = K. A. | last6 = Adilov | first6 = Z. | last7 = Kudaibergenov | first7 = S. E. }}</ref> All [[protein]]s are polyampholytes, as some [[amino acid]]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 [[gel]]s. For instance, they can be used to destabilize a [[colloidal suspension]] and 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 [[Thickening agent|thickeners]], [[emulsifier]]s, [[Conditioner (chemistry)|conditioners]], [[clarifying agent]]s, and even [[Drag (physics)|drag]] reducers. They are used in [[water treatment]] and for [[Petroleum extraction|oil recovery]]. Many [[soap]]s, [[shampoo]]s, and [[cosmetics]] incorporate polyelectrolytes. Furthermore, they are added to many foods and to [[concrete]] mixtures ([[superplasticizer]]). Some of the polyelectrolytes that appear on food labels are [[pectin]], [[carrageenan]], [[alginate]]s, 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 (medicine)|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<ref>{{Cite journal |doi = 10.1002/mabi.201200023 |title = Oxidized Dextran as Crosslinker for Chitosan Cryogel Scaffolds and Formation of Polyelectrolyte Complexes between Chitosan and Gelatin| journal = Macromolecular Bioscience |volume = 12| issue = 8 |pages = 1090–9 |year = 2012| last1 = Berillo |first1 = D. |last2 = Elowsson |first2 = L. |last3 = Kirsebom |first3 = H. |pmid = 22674878 |doi-access = free}}</ref> 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-link]]ed 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 [[nanoparticle]]s or [[Zeolite|clay platelet]]s<ref>{{cite journal|title=Layer-By-Layer Assembly Of Zeolite Crystals On Glass With Polyelectrolytes As Ionic Inkers|doi=10.1021/ja010517q|year=2001|last1=Lee|first1=Goo Soo|last2=Lee|first2=Yun-Jo|last3=Yoon|first3=Kyung Byung|journal=Journal of the American Chemical Society|volume=123|issue=40|pages=9769–79|pmid=11583538}}</ref> in place of or in addition to one of the polyelectrolytes. LbL deposition has also been accomplished using [[hydrogen bond]]ing instead of [[electrostatic]]s. For more information on multilayer creation, please see [[polyelectrolyte adsorption]]. 
[[File:Polyelectrolyte multilayer formation.jpg|thumbnail|right|Formation of 20 layers of PSS-PAH polyelectrolyte multilayer measured by multi-parametric surface plasmon resonance]]
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.<ref>{{cite journal|title=Characterizing Ultrathin and Thick Organic Layers by Surface Plasmon Resonance Three-Wavelength and Waveguide Mode Analysis|doi=10.1021/la401084w|year=2013|last1=Granqvist|first1=Niko|last2=Liang|first2=Huamin|last3=Laurila|first3=Terhi|last4=Sadowski|first4=Janusz|last5=Yliperttula|first5=Marjo|last6=Viitala|first6=Tapani|journal=Langmuir|volume=29|issue=27|pages=8561–71|pmid=23758623}}</ref>

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]] or [[semiconductor]] nanoparticles, or [[porosity]] phase transitions to create [[anti-reflective coating]]s, optical [[shutter (photography)|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.<ref>{{cite journal|title=Polyelectrolyte bridging interactions between charged macromolecules|doi=10.1016/j.cocis.2006.08.001|year=2006|last1=Podgornik|first1=R.|last2=Ličer|first2=M.|journal=Current Opinion in Colloid & Interface Science|volume=11|issue=5|pages=273}}</ref> The term bridging interactions is usually applied to the situation where a single polyelectrolyte chain can [[adsorption|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]] terminology, a '''polyacid''' is a polyelectrolyte composed of [[macromolecules]] containing [[acid]] groups on a substantial fraction of the [[monomer|constitutional units]]. Most commonly, the acid groups are {{nowrap|–COOH}}, {{nowrap|–SO<sub>3</sub>H}}, or {{nowrap|–PO<sub>3</sub>H<sub>2</sub>}}.<ref>{{cite journal|doi=10.1351/pac200678112067 |url=http://old.iupac.org/publications/pac/2006/pdf/7811x2067.pdf|title=Terminology of polymers containing ionizable or ionic groups and of polymers containing ions (IUPAC Recommendations 2006)|year=2006|last1=Hess|first1=M.|last2=Jones|first2=R. G.|last3=Kahovec|first3=J.|last4=Kitayama|first4=T.|last5=Kratochvíl|first5=P.|last6=Kubisa|first6=P.|last7=Mormann|first7=W.|last8=Stepto|first8=R. F. T.|last9=Tabak|first9=D.|last10=Vohlídal|first10=J.|last11=Wilks|first11=E. S.|journal=Pure and Applied Chemistry|volume=78|issue=11|pages=2067|s2cid=98243251|display-authors=8}}</ref>

==See also==
* [[Dispersity]]
* [[Ion-exchange resin]]
* [[Polypyridinium salts]]

== References ==
{{reflist}}

== External links ==
* [http://www.mpip-mainz.mpg.de/ Max Planck Institute for Polymer Research, Mainz, Germany]
* [http://www-dick.chemie.uni-regensburg.de/group/stephan_baeurle/30,0,polyelectrolytes,index,0.html Polyelectrolytes: Institute of Physical & Theoretical Chemistry, University of Regensburg, Regensburg, Germany]
* [https://angelchemindia.com/ Polyelectrolytes: Vadodara, Gujarat, India]

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