Gel

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

A gel (coined by 19th-century Scottish chemist Thomas Graham, by clipping from gelatine[1]) is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state.[2] 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 give a gel its structure (hardness) and contribute to the adhesive stick (tack).[3] In this way gels are a dispersion of molecules of a liquid within a solid in which the solid is the continuous phase and the liquid is the discontinuous phase.

IUPAC definition

Gel: Nonfluid colloidal network or polymer network that is expanded throughout its whole volume by a fluid.[4]

Note 1: A gel has a finite, usually rather small, yield stress.

Note 2: A gel can contain:

(i) a covalent polymer network, e.g., a network formed by crosslinking polymer chains or by nonlinear polymerization;
(ii) 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;
(iii) 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;
(iv) lamellar structures including mesophases {Ref.[5] defines lamellar crystal and mesophase}, e.g., soap gels, phospholipids, and clays;
(v) 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 ref.,[6] where the definition is via the property identified in Note 1 (above) rather than of the structural characteristics that describe a gel.[7]

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.

Note 3: Definition quoted from refs.[7][8][9]

Composition[edit]

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.

Cationic polymers[edit]

Cationic polymers are positively charged polymers. Their positive charges prevent the formation of coiled polymers. This allows them to contribute more to viscosity in their stretched state, because the stretched-out polymer takes up more space. Gel is a colloid solution of dispersion phase as liquid and dispersion medium as solid.

Types[edit]

Hydrogels[edit]

Hydrogel is a network of polymer chains that are hydrophilic, sometimes found as a colloidal gel in which water is the dispersion medium. 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. Common uses for hydrogels include:

Other, less common uses include

Common ingredients are e.g. polyvinyl alcohol, sodium polyacrylate, acrylate polymers and copolymers with an abundance of hydrophilic groups.

Natural hydrogel materials are being investigated for tissue engineering; these materials include agarose, methylcellulose, hyaluronan, and other naturally derived polymers.

Organogels[edit]

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.[11][12]

Organogels have potential for use in a number of applications, such as in pharmaceuticals,[13] cosmetics, art conservation,[14] and food.[15] An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in petroleum.[16]

Xerogels[edit]

A xerogel /ˈzɪərɵɛl/ 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.

Properties[edit]

Many gels display thixotropy – they become fluid when agitated, but resolidify when resting. In general, gels are apparently solid, jelly-like materials. 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[edit]

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.[17]

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.[18]

Applications[edit]

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.

See also[edit]

References[edit]

  1. ^ Harper, Douglas. "Online Etymology Dictionary: gel". Online Etymology Dictionary. Retrieved 2013-12-09. 
  2. ^ Ferry, John D. (1980) Viscoelastic Properties of Polymers. New York: Wiley, ISBN 0471048941.
  3. ^ "Gel". Princeton.edu. Retrieved 2012-01-07. "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 crosslinks within the fluid that give a gel its structure (hardness) and contribute to stickiness (tack)." 
  4. ^ Richard G. Jones, Edward S. Wilks, W. Val Metanomski, Jaroslav Kahovec, Michael Hess, Robert Stepto, Tatsuki Kitayama, ed. (2009). Compendium of Polymer Terminology and Nomenclature (IUPAC Recommendations 2008) ("The Purple Book") (2nd ed.). RSC. p. 464. ISBN 978-0-85404-491-7. 
  5. ^ "Reporting physisorption data for gas/solid systems with Special Reference to the Determination of Surface Area and Porosity". Pure and Applied Chemistry 57 (4): 603–619. 1985. doi:10.1351/pac198557040603. 
  6. ^ Alan D. MacNaught, Andrew R. Wilkinson, ed. (1997). Compendium of Chemical Terminology: IUPAC Recommendations (the "Gold Book") (2 ed.). Oxford: Blackwell Science. ISBN 0865426848. 
  7. ^ a b "Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)". Pure and Applied Chemistry 83 (12): 2229–2259. 2011. doi:10.1351/PAC-REC-10-06-03. 
  8. ^ Richard G. Jones, Edward S. Wilks, W. Val Metanomski, Jaroslav Kahovec, Michael Hess, Robert Stepto, Tatsuki Kitayama, ed. (2009). Compendium of Polymer Terminology and Nomenclature (IUPAC Recommendations 2008) ("The Purple Book"). RSC. ISBN 978-0-85404-491-7. 
  9. ^ Alan D. MacNaught, Andrew R. Wilkinson, ed. (1997). Compendium of Chemical Terminology: IUPAC Recommendations (the "Gold Book") (2nd ed.). Blackwell Science. ISBN 0865426848. 
  10. ^ Discher, D. E.; Janmey, P; Wang, YL (2005). "Tissue Cells Feel and Respond to the Stiffness of Their Substrate". Science 310 (5751): 1139–43. Bibcode:2005Sci...310.1139D. doi:10.1126/science.1116995. PMID 16293750. 
  11. ^ Terech P. (1997) "Low-molecular weight organogelators", pp. 208–268 in: Robb I.D. (ed.) Specialist surfactants. Glasgow: Blackie Academic and Professional, ISBN 0751403407.
  12. ^ van Esch J., Schoonbeek F., De Loos M., Veen E.M., Kellog R.M., Feringa B.L. (1999) "Low molecular weight gelators for organic solvents", pp. 233–259 in: Ungaro R., Dalcanale E. (eds.) Supramolecular science: where it is and where it is going. Kluwer Academic Publishers, ISBN 079235656X.
  13. ^ Kumar, R; Katare, OP (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. 
  14. ^ Carretti E, Dei L, Weiss RG (2005). "Soft matter and art conservation. Rheoreversible gels and beyond". Soft Matter 1: 17. Bibcode:2005SMat....1...17C. doi:10.1039/B501033K. 
  15. ^ 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. doi:10.1016/j.cocis.2007.07.002. 
  16. ^ Visintin RFG, Lapasin R, Vignati E, D'Antona P, Lockhart TP (2005). "Rheological behavior and structural interpretation of waxy crude oil gels". Langmuir 21 (14): 6240–9. doi:10.1021/la050705k. PMID 15982026. 
  17. ^ Dee, Eileen May; McGinley, Mark and Hogan, C. Michael (2010). "Long-finned pilot whale" in Saundry, Peter and Cleveland, Cutler (eds.) Encyclopedia of Earth. National Council for Science and the Environment. Washington DC.
  18. ^ "Injectable Hydrogel-based Medical Devices: "There's always room for Jell-O"1". Orthoworld.com. September 15, 2010. Retrieved 2013-05-19. 

External links[edit]