Gel

A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough.[1][2] Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state.[3] A gel has been defined phenomenologically as a soft, solid or solid-like material consisting of two or more components, one of which is a liquid, present in substantial quantity.[4]

For other uses, see Gel (disambiguation).
An upturned vial of hair gel
Silica gel

By weight, gels are mostly liquid, yet they behave like solids due to a three-dimensional cross-linked network within the liquid. It is the crosslinking within the fluid that gives a gel its structure (hardness) and contributes to the adhesive stick (tack). In this way, gels are a dispersion of molecules of a liquid within a solid medium. The word gel was coined by 19th-century Scottish chemist Thomas Graham by clipping from gelatine.[5]

IUPAC definition
See also: International Union of Pure and Applied Chemistry
Gel
Nonfluid colloidal network or polymer network that is expanded throughout its whole volume by a fluid.[6][7]
Note 1: A gel has a finite, usually rather small, yield stress.
Note 2: A gel can contain:
  1. a covalent polymer network, e.g., a network formed by crosslinking polymer chains or by nonlinear polymerization;
  2. a polymer network formed through the physical aggregation of polymer chains, caused by hydrogen bonds, crystallization, helix formation, complexation, etc., that results in regions of local order acting as the network junction points. The resulting swollen network may be termed a "thermoreversible gel" if the regions of local order are thermally reversible;
  3. a polymer network formed through glassy junction points, e.g., one based on block copolymers. If the junction points are thermally reversible glassy domains, the resulting swollen network may also be termed a thermoreversible gel;
  4. lamellar structures including mesophases {Sing et al.[8] defines lamellar crystal and mesophase}, e.g., soap gels, phospholipids, and clays;
  5. particulate disordered structures, e.g., a flocculent precipitate usually consisting of particles with large geometrical anisotropy, such as in V2O5 gels and globular or fibrillar protein gels.
Note 3: Corrected from the Gold Book[9] where the definition is via the property identified in Note 1 (above) rather than of the structural characteristics that describe a gel.
Hydrogel
Gel in which the swelling agent is water.
Note 1: The network component of a hydrogel is usually a polymer network.
Note 2: A hydrogel in which the network component is a colloidal network may be referred to as an aquagel.[7]
Xerogel
Open network formed by the removal of all swelling agents from a gel.
Note: Examples of xerogels include silica gel and dried out, compact macromolecular structures such as gelatin or rubber.
Modified from the Gold Book.[10] The definition proposed here is recommended as being more explicit.[11]

Composition

Gels consist of a solid three-dimensional network that spans the volume of a liquid medium and ensnares it through surface tension effects. This internal network structure may result from physical bonds (physical gels) or chemical bonds (chemical gels), as well as crystallites or other junctions that remain intact within the extending fluid. Virtually any fluid can be used as an extender including water (hydrogels), oil, and air (aerogel). Both by weight and volume, gels are mostly fluid in composition and thus exhibit densities similar to those of their constituent liquids. Edible jelly is a common example of a hydrogel and has approximately the density of water.

Polyionic polymers

Polyionic polymers are polymers with an ionic functional group. The ionic charges prevent the formation of tightly coiled polymer chains. This allows them to contribute more to viscosity in their stretched state, because the stretched-out polymer takes up more space. This is also the reason gel hardens. See polyelectrolyte for more information.

Types

Hydrogels
Main article: Hydrogel
See also: Superabsorbent polymer, Self-healing hydrogels, and Hydrogel agriculture
Hydrogel of a superabsorbent polymer

A hydrogel is a network of polymer chains that are hydrophilic, sometimes found as a colloidal gel in which water is the dispersion medium. A three-dimensional solid results from the hydrophilic polymer chains being held together by cross-links. Because of the inherent cross-links, the structural integrity of the hydrogel network does not dissolve from the high concentration of water.[12] Hydrogels are highly absorbent (they can contain over 90% water) natural or synthetic polymeric networks. Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content. As responsive "smart materials," hydrogels can encapsulate chemical systems which upon stimulation by external factors such as a change of pH may cause specific compounds such as glucose to be liberated to the environment, in most cases by a gel-sol transition to the liquid state. Chemomechanical polymers are mostly also hydrogels, which upon stimulation change their volume and can serve as actuators or sensors. The first appearance of the term 'hydrogel' in the literature was in 1894.[13]

Organogels
See also: Organogels

An organogel is a non-crystalline, non-glassy thermoreversible (thermoplastic) solid material composed of a liquid organic phase entrapped in a three-dimensionally cross-linked network. The liquid can be, for example, an organic solvent, mineral oil, or vegetable oil. The solubility and particle dimensions of the structurant are important characteristics for the elastic properties and firmness of the organogel. Often, these systems are based on self-assembly of the structurant molecules.[14][15] (An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in petroleum.[16])

Organogels have potential for use in a number of applications, such as in pharmaceuticals,[17] cosmetics, art conservation,[18] and food.[19]

Xerogels

A xerogel /ˈzɪəroʊˌdʒɛl/ is a solid formed from a gel by drying with unhindered shrinkage. Xerogels usually retain high porosity (15–50%) and enormous surface area (150–900 m2/g), along with very small pore size (1–10 nm). When solvent removal occurs under supercritical conditions, the network does not shrink and a highly porous, low-density material known as an aerogel is produced. Heat treatment of a xerogel at elevated temperature produces viscous sintering (shrinkage of the xerogel due to a small amount of viscous flow) and effectively transforms the porous gel into a dense glass.

Nanocomposite hydrogels

Nanocomposite hydrogels[20][21] or hybrid hydrogels, are highly hydrated polymeric networks, either physically or covalently crosslinked with each other and/or with nanoparticles or nanostructures.[22] Nanocomposite hydrogels can mimic native tissue properties, structure and microenvironment due to their hydrated and interconnected porous structure. A wide range of nanoparticles, such as carbon-based, polymeric, ceramic, and metallic nanomaterials can be incorporated within the hydrogel structure to obtain nanocomposites with tailored functionality. Nanocomposite hydrogels can be engineered to possess superior physical, chemical, electrical, thermal, and biological properties.[20][23]

Properties

Many gels display thixotropy – they become fluid when agitated, but resolidify when resting. In general, gels are apparently solid, jelly-like materials. It is a type of non-Newtonian fluid. By replacing the liquid with gas it is possible to prepare aerogels, materials with exceptional properties including very low density, high specific surface areas, and excellent thermal insulation properties.

Animal-produced gels

Some species secrete gels that are effective in parasite control. For example, the long-finned pilot whale secretes an enzymatic gel that rests on the outer surface of this animal and helps prevent other organisms from establishing colonies on the surface of these whales' bodies.[24]

Hydrogels existing naturally in the body include mucus, the vitreous humor of the eye, cartilage, tendons and blood clots. Their viscoelastic nature results in the soft tissue component of the body, disparate from the mineral-based hard tissue of the skeletal system. Researchers are actively developing synthetically derived tissue replacement technologies derived from hydrogels, for both temporary implants (degradable) and permanent implants (non-degradable). A review article on the subject discusses the use of hydrogels for nucleus pulposus replacement, cartilage replacement, and synthetic tissue models.[25]

Applications

Many substances can form gels when a suitable thickener or gelling agent is added to their formula. This approach is common in manufacture of wide range of products, from foods to paints and adhesives.

In fiber optics communications, a soft gel resembling hair gel in viscosity is used to fill the plastic tubes containing the fibers. The main purpose of the gel is to prevent water intrusion if the buffer tube is breached, but the gel also buffers the fibers against mechanical damage when the tube is bent around corners during installation, or flexed. Additionally, the gel acts as a processing aid when the cable is being constructed, keeping the fibers central whilst the tube material is extruded around it.

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See also

References
  1. Khademhosseini A, Demirci U (2016). Gels Handbook: Fundamentals, Properties and Applications. World Scientific Pub Co Inc. ISBN 9789814656108.
  2. Seiffert S, ed. (2015). Supramolecular Polymer Networks and Gels. Springer. ASIN B00VR5CMW6.
  3. Ferry JD (1980). Viscoelastic Properties of Polymers. New York: Wiley. ISBN 0471048941.
  4. Almdal, K.; Dyre, J.; Hvidt, S.; Kramer, O. (1993). "Towards a phenomenological definition of the term 'gel'". Polymer Gels and Networks. 1 (1): 5–17. doi:10.1016/0966-7822(93)90020-I.
  5. Harper D. "Online Etymology Dictionary: gel". Online Etymology Dictionary. Retrieved 2013-12-09.
  6. Jones RG, Kahovec J, Stepto R, Wilks ES, Hess M, Kitayama T, Metanomski WV (2008). IUPAC. Compendium of Polymer Terminology and Nomenclature, IUPAC Recommendations 2008 (the "Purple Book") (PDF). RSC Publishing, Cambridge, UK.
  7. Slomkowski S, Alemán JV, Gilbert RG, Hess M, Horie K, Jones RG, et al. (2011). "Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)" (PDF). Pure and Applied Chemistry. 83 (12): 2229–2259. doi:10.1351/PAC-REC-10-06-03. S2CID 96812603.
  8. Sing KS, Everett DH, Haul RA, Moscou L, Pierotti RA, Rouquérol J, Siemieniewska T (1985). "Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity". Pure Appl. Chem. 57: 603. doi:10.1351/pac198557040603. S2CID 14894781.
  9. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "gel". doi:10.1351/goldbook.G02600
  10. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "xerogel". doi:10.1351/goldbook.X06700
  11. Alemán JV, Chadwick AV, He J, Hess M, Horie K, Jones RG, et al. (2007). "Definitions of terms relating to the structure and processing of sols, gels, networks, and inorganic-organic hybrid materials (IUPAC Recommendations 2007)" (PDF). Pure Appl Chem. 79 (10): 1801. doi:10.1351/pac200779101801. S2CID 97620232.
  12. Warren DS, Sutherland SP, Kao JY, Weal GR, Mackay SM (2017-04-20). "The Preparation and Simple Analysis of a Clay Nanoparticle Composite Hydrogel". Journal of Chemical Education. 94 (11): 1772–1779. Bibcode:2017JChEd..94.1772W. doi:10.1021/acs.jchemed.6b00389. ISSN 0021-9584.
  13. Bemmelen JM (1907). "Der Hydrogel und das kristallinische Hydrat des Kupferoxydes". Zeitschrift für Chemie und Industrie der Kolloide. 1 (7): 213–214. doi:10.1007/BF01830147. S2CID 197928622.
  14. Terech P. (1997) "Low-molecular weight organogelators", pp. 208–268 in: Robb I.D. (ed.) Specialist surfactants. Glasgow: Blackie Academic and Professional, ISBN 0751403407.
  15. Van Esch J, Schoonbeek F, De Loos M, Veen EM, Kellogg RM, Feringa BL (1999). "Low molecular weight gelators for organic solvents". In Ungaro R, Dalcanale E (eds.). Supramolecular science: where it is and where it is going. Kluwer Academic Publishers. pp. 233–259. ISBN 079235656X.
  16. Visintin RF, Lapasin R, Vignati E, D'Antona P, Lockhart TP (July 2005). "Rheological behavior and structural interpretation of waxy crude oil gels". Langmuir. 21 (14): 6240–9. doi:10.1021/la050705k. PMID 15982026.
  17. Kumar R, Katare OP (October 2005). "Lecithin organogels as a potential phospholipid-structured system for topical drug delivery: a review". AAPS PharmSciTech. 6 (2): E298-310. doi:10.1208/pt060240. PMC 2750543. PMID 16353989.
  18. Carretti E, Dei L, Weiss RG (2005). "Soft matter and art conservation. Rheoreversible gels and beyond". Soft Matter. 1 (1): 17. Bibcode:2005SMat....1...17C. doi:10.1039/B501033K.
  19. Pernetti M, van Malssen KF, Flöter E, Bot A (2007). "Structuring of edible oils by alternatives to crystalline fat". Current Opinion in Colloid & Interface Science. 12 (4–5): 221–231. doi:10.1016/j.cocis.2007.07.002.
  20. Gaharwar AK, Peppas NA, Khademhosseini A (March 2014). "Nanocomposite hydrogels for biomedical applications". Biotechnology and Bioengineering. 111 (3): 441–53. doi:10.1002/bit.25160. PMC 3924876. PMID 24264728.
  21. Carrow JK, Gaharwar AK (November 2014). "Bioinspired Polymeric Nanocomposites for Regenerative Medicine". Macromolecular Chemistry and Physics. 216 (3): 248–264. doi:10.1002/macp.201400427.
  22. Kutvonen A, Rossi G, Puisto SR, Rostedt NK, Ala-Nissila T (December 2012). "Influence of nanoparticle size, loading, and shape on the mechanical properties of polymer nanocomposites". The Journal of Chemical Physics. 137 (21): 214901. arXiv:1212.4335. Bibcode:2012JChPh.137u4901K. doi:10.1063/1.4767517. PMID 23231257. S2CID 26096794.
  23. Zaragoza J, Babhadiashar N, O'Brien V, Chang A, Blanco M, Zabalegui A, et al. (2015-08-24). "Experimental Investigation of Mechanical and Thermal Properties of Silica Nanoparticle-Reinforced Poly(acrylamide) Nanocomposite Hydrogels". PLOS ONE. 10 (8): e0136293. Bibcode:2015PLoSO..1036293Z. doi:10.1371/journal.pone.0136293. PMC 4547727. PMID 26301505.
  24. Dee EM, McGinley M, Hogan CM (2010). "Long-finned pilot whale". In Saundry P, Cleveland C (eds.). Encyclopedia of Earth. Washington DC: National Council for Science and the Environment.
  25. "Injectable Hydrogel-based Medical Devices: "There's always room for Jell-O"1". Orthoworld.com. September 15, 2010. Retrieved 2013-05-19.
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