Gel
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224657656
2008-07-09T21:06:22Z
69.134.122.144
/* Organogels */
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[[Image:Hairgel.JPG|thumb|right|An upturned vial of hair gel]]
A '''gel''' (from the [[Latin|lat.]] ''gelu''—freezing, cold, ice or ''gelatus''—frozen, immobile) is an apparently solid, jelly-like material formed from a [[colloid]]al solution. By weight, gels are mostly liquid, yet they behave like solids due to the addition of a [[gelling agent]].
==Composition==
A solid network spans the volume of a liquid medium. Both by weight and volume, gels are mostly liquid in composition and thus exhibit densities similar to liquids. However, they have the structural coherence of a solid. The network can be composed of a wide variety of materials, including particles, polymers and proteins.
===Cationic polymers===
[[Cationic polymers]] are positively charged polymers. Their positive charges prevent the formation of coiled polymers. This allows them to contribute more to [[viscosity|viscosity]] in their stretched state, because the stretched-out polymer takes up more space than a coiled polymer and thus resists the flow of solvent molecules around it.
Cationic polymers are a main functional component of hair gel, because the positive charged polymers also bind the negatively charged [[amino acid]]s on the surface of the [[keratin]] molecules in the hair. More complicated polymer formulas exist, e.g., a [[copolymer]] of [[vinylpyrrolidone]], [[methacrylamide]], and hydrogel [[N-vinylimidazole]].<ref>http://www.corporate.basf.com/basfcorp/img/stories/wipo/haargel/Haargel_e.pdf</ref>
==Types of gels==
===Hydrogels===
'''Hydrogel''' is a network of polymer chains that are water-insoluble, sometimes found as a [[colloid]]al [[gel]] in which [[water]] is the dispersion medium. Hydrogels are [[superabsorbent]] (they can contain over 99% [[water]]) natural or synthetic [[polymers]].
Hydrogels possess also a degree of flexibility very similar to natural tissue, due to their significant water content.
Common uses for hydrogels are
*currently used as scaffolds in tissue engineering. When used as scaffolds, hydrogels may contain human cells in order to repair tissue.
*environmentally sensitive hydrogels. These hydrogels have the ability to sense changes of pH, temperature, or the concentration of metabolite and release their load as result of such a change.
*as sustained-release delivery system
*provide absorption, desloughing and debriding capacities of necrotics and fibrotic tissue.
*hydrogels that are responsive to specific molecules, such as glucose or antigens can be used as [[biosensor]]s as well as in DDS.
*In disposable [[diaper]]s where they "capture" [[urine]], or in [[sanitary napkin]]s
*[[contact lens]]es ([[silicone]] hydrogels, [[polyacrylamide]]s)
*[[medical electrodes]] using hydrogels composed of [[cross link]]ed polymers ([[polyethylene oxide]],[[polyAMPS]] and [[polyvinylpyrrolidone]])
* [[Water Gel Explosives|Water gel explosives]]
Other, less common uses include
*[[breast implant]]s
*granules for holding [[soil]] moisture in arid areas
*dressings for healing of [[burn (injury)|burn]] or other hard-to-heal [[wound]]s. Wound GEL are excellent for helping to create or maintain environment.
*reservoirs in [[topical drug delivery]]; particularly ionic drugs, delivered by [[iontophoresis]] (see [[ion exchange resin]])
Common ingredients are e.g. [[polyvinyl alcohol]], [[sodium polyacrylate]], [[acrylate]] polymers and [[copolymer]]s with an abundance of [[hydrophilic]] groups.
Natural hydrogel materials are being investigated for tissue engineering, these materials include agarose, methylcellulose, hylaronan, and other naturally derived polymers.
===Organogels===
An '''organogel''' is a [[crystallinity|non-crystalline]], [[glass|non-glassy]] thermoreversible solid [[material]]s composed of a [[liquid]] [[organic compound|organic]] phase entrapped in a structuring network. The liquid can be e.g. an [[organic solvent]], a mineral [[oil]] or a [[vegetable oil]]. The [[solubility]] and [[particle]] dimensions of the structurant are important characteristics for the [[elasticity|elastic]] properties and firmness of the organogel. Often, these systems are based on [[self-assembly]] of the structurant molecules<ref>Terech P. Low-molecular weight organogelators. In: Robb ID, editor. Specialist surfactants. Glasgow: Blackie Academic and Professional, p. 208–268 (1997).</ref><ref>van Esch J, Schoonbeek F, De Loos M, Veen EM, Kellog RM, Feringa BL. Low molecular weight gelators for organic solvents. In: Ungaro R, Dalcanale E, editors. Supramolecular science: where it is and where it is going. Kluwer Academic Publishers, p. 233–259 (1999).</ref>.
Organogels have potential for use in a number of applications, such as in [[pharmaceutics|pharmaceuticals]] <ref>Kumar R, Katare OP. Lecithin organogels as a potential phospholipid-structured system for topical drug delivery: A review. American Association of Pharmaceutical Scientists PharmSciTech 6, E298–E310 (2005).</ref>, cosmetics, art conservation<ref>Carretti E, Dei L, Weiss RG. Soft matter and art conservation. Rheoreversible gels and beyond. Soft Matter 1, 17–22 (2005).</ref>, and food<ref>Pernetti M, van Malssen KF, Flöter E, Bot A. Structuring of edible oil by alternatives to crystalline fat. Current Opinion in Colloid and Interface Science 12, 221–231 (2007).</ref>. An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in [[crude oil]] <ref>Visintin RFG, Lapasin R, Vignati E, D'Antona P, Lockhart TP. Rheological behavior and structural interpretation of waxy crude oil gels. Langmuir 21, 6240–6249 (2005)</ref>.
===Xerogels===
A '''xerogel''' ['z<font face="Times New Roman"><small>I</small></font>rə,d{{IPA|ʒ}}εl] is a solid formed from a [[gel]] by drying with unhindered shrinkage. Xerogels usually retain high porosity (25%) and enormous surface area (150-900 m<sup>2</sup>/g), along with very small [[pore]] size (1-10 nm). When [[solvent]] removal occurs under hypercritical ([[supercritical fluid|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==
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 [[aerogel]]s, materials with exceptional properties including very low density, high specific surface areas, and excellent thermal insulation properties.
===Sound-induced gelation===
[[Image:Soundinducedgelation.gif|frame|The palladium complex is synthesised from palladium acetate and N,N'-Bis(salicylidene)pentamethylenediamine in boiling benzene and forms the anti conformer (left) and the syn conformer (right)]]
Sound induced gelation is described in 2005 <ref>Naota T, Koori H. Molecules That Assemble by Sound: An Application to the Instant Gelation of Stable Organic Fluids. [[J. Am. Chem. Soc.]], 127 (26), 9324-9325 (2005) [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/2005/127/i26/abs/ja050809h.html Abstract] [http://pubs.acs.org/subscribe/journals/jacsat/suppinfo/ja050809h/ja050809hsi20050525_073625.pdf Online details]</ref> in an [[organopalladium]] compound that in solution transforms from a transparent liquid to an opaque gel upon application of a short burst (seconds) of [[ultrasound]]. Heating to above the so-called ''gelation temperature'' T<sub>gel</sub> takes the gel back to the solution. The compound is a dinuclear [[palladium]] complex made from [[palladium acetate]] and a N,N'-Bis-salicylidene diamine. Both compounds react to form an [[Geometric isomerism|anti conformer]] (gelling) and a syn conformer (non-gelling) which are separated by [[column chromatography]]. In the solution phase the dimer molecules are bent and self-locked by [[aromatic stacking interaction]]s whereas in the gel phase the conformation is planar with interlocked aggregates.
The anti conformer has [[planar chirality]] and both [[enantiomer]]s were separated by [[chiral column chromatography]]. The (-) anti conformer has a [[specific rotation]] of -375° but is unable to gelate by itself. In the gel phase the dimer molecules form stacks of alternating (+) and (-) components. This process starts at the onset of the sonication and proceeds even without further sonication.
==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, 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 whist the tube material is extruded around it.
==See also==
*[[Aerogel]]
*[[Hydrocolloid]]
*[[Gel electrophoresis]], [[Agarose gel electrophoresis]], [[2-D electrophoresis]], [[SDS-PAGE]]
*[[Gel filtration chromatography]], [[Gel permeation chromatography]]
*[[Paste (rheology)]]
== References ==
{{Reflist}}
==External links==
* [http://www.iupac.org/goldbook/X06700.pdf Xerogel (definition)]
[[Category:Physical chemistry]]
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