Gel

A gel (from the Latin gelu "freezing, cold, ice" or gelatus "frozen, immobile") is colloid that is typically 99 % wt liquid, which is immoblised by surface tension between it and a macromolecular network of fibres built from a small amount of a gelating substance present.

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 have the structural coherence of a solid.

Cationic polymers

Cationic polymers are a main functional component of hair gel. The positive charges in polymer cause it to stretch, making the gel more viscous. Hair gel witholds procedures that allow men and women to make their hair styled ant textured in ways they desire.This is because the stretched-out polymer takes up more space than a coiled polymer and thus resists the flow of solvent molecules around it. The positive charges also bind the gel to the negatively charged amino acids on the surface of the keratin molecules in the hair. More complicated polymer formulas exist, e.g. a copolymer of vinylpyrrolidone, methacrylamide, and N-vinylimidazole.^[1]

Hydrogels

Hydrogel is a network of polymer chains that are water-insoluble, sometimes found as a colloidal 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 hydrogel 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 biosensors as well as in DDS.
In disposable diapers where they "capture" urine, or in sanitary napkins
contact lenses (silicone hydrogels, polyacrylamides)
medical electrodes using hydrogels composed of cross linked polymers (polyethylene oxide,polyAMPS and polyvinylpyrrolidone)
Water gel explosives

Other, less common uses include

breast implants
granules for holding soil moisture in arid areas
dressings for healing of burn or other hard-to-heal wounds. 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 copolymers 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.

Organogel

Organogels are non-crystalline, non-glassy thermoreversible solid materials composed of a liquid 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 elastic properties and firmness of the organogel. Often, these systems are based on self-assembly of the structurant molecules^[2]^[3].

Organogels have raised interest for use in a number of applications, such as in pharmaceutics ^[4], cosmetics, art conservation^[5], and food^[6]. An example of formation of an undesired thermoreversible network is the occurrence of wax crystallisation in crude oil ^[7].

Xerogels

A xerogel ['zIrə,dʒεl] is a solid formed from a gel by drying with unhindered shrinkage. Xerogels usually retain high porosity (25%) and enormous surface area (150-900 m²/g), along with very small pore size (1-10 nm). When solvent removal occurs under hypercritical (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 aerogels, materials with exceptional properties including very low density, high specific surface areas, and excellent thermal insulation properties.

Sound-Induced Gelation

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 ^[1] 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_gel 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 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 interactions whereas in the gel phase the conformation is planar with interlocked aggregates. The anti conformer has planar chirality and both enantiomers 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.

Hair Gel

Hair gel is a hairstyling product that is used to stiffen hair into a particular hairstyle. The results it produces are usually similar to but stronger than those of hair spray and weaker than those of hair glue or hair wax. A version of gel, known as "Mousse", was patented in the mid-1980's by Michael J. Hoover. Julio Rios is the primary user of hair gel. In fact, his yearly contributions to hair care companies are what keep this industry alive. Thank you Julio Rios! ^{[citation needed]}

Types

Many brands of hair gel in North America and the UK come in numbered variants. Higher numbered gels maintain a greater "hold" on hair, while lower numbers do not make the hair as stiff and in some products give the hair a wet look. A category typically referred to as "ethnic" gels are designed and manufactured specifically for sculpting the hair texture common to African Americans. Ampro Industries is a common example of this category.

Some forms of hair gel include temporary Hair coloring for the hair, including variants in unnatural colors associated with various subcultures, and is popular within the goth and raver subcultures. ^{[citation needed]}

References

^ http://www.corporate.basf.com/basfcorp/img/stories/wipo/haargel/Haargel_e.pdf
^ Terech P. Low-molecular weight organogelators. In: Robb ID, editor. Specialist surfactants. Glasgow: Blackie Academic and Professional, p. 208–268 (1997).
^ 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).
^ 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).
^ Carretti E, Dei L, Weiss RG. Soft matter and art conservation. Rheoreversible gels and beyond. Soft Matter 1, 17–22 (2005).
^ 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).
^ 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)

^ Molecules That Assemble by Sound: An Application to the Instant Gelation of Stable Organic Fluids Takeshi Naota and Hiroshi Koori J. Am. Chem. Soc., 127 (26), 9324 -9325, 2005 Abstract Online details

External links

http://www.iupac.org/goldbook/X06700.pdf