gel

An upturned vial of hair gel

A gel (from the lat. gelu—freezing, cold, ice or gelatus—frozen, immobile) 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 crosslinked system, which exhibits no flow when in the steady-state.^[1] By weight, gels are mostly liquid, yet they behave like solids due to a three-dimensional crosslinked network within the liquid. It is the crosslinks within the fluid that give a gel its structure (hardness) and contribute to stickiness (tack).

Composition

A solid three-dimensional network spans the volume of a liquid medium. This internal network structure may result from physical or chemical bonds, 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 liquid 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

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 than a coiled polymer and this 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 acids 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.^[2] Introduction--- In certain concentrations, dispersions of lyophilic colloids tend to form solid masses, particularly when the solubility of the colloidal material is reduced by change in temperature. These semi-solids are known as gels. The settings of solutions of gelatin and agar on cooling are well known examples of gel formation

Types of gels

Hydrogels

Hydrogel (also called Aquagel) 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 highly absorbent (they can contain over 99% water) natural or synthetic polymers. Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content.

Common uses for hydrogels include

• 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 systems
• 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.
• used 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 gels are excellent for helping to create or maintain a moist 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, hyaluronan, and other naturally derived polymers.

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.^[3][4]

Organogels have potential for use in a number of applications, such as in pharmaceuticals,^[5] cosmetics, art conservation,^[6] and food.^[7] An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in petroleum.^[8]

Xerogels

A xerogel (pronounced /ˈzɪərɵdʒɛl/ zero jell) 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^[9] 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 whilst 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)
• Food rheology

References

Further reading

• Ajayaghosh, A., Praveen, V.K. & Vijayakumar, C. Organogels as scaffolds for excitation energy transfer and light harvesting. Chem Soc Rev 37, 109-22(2008).
• Ajayaghosh, A. & Praveen, V.K. p-Organogels of Self-Assembled p-Phenylenevinylenes: Soft Materials with Distinct Size, Shape, and Functions. Acc. Chem. Res. 40, 644-656(2007).
• Estroff, L.A. & Hamilton, A.D. Water gelation by small organic molecules. Chem Rev 104, 1201-18(2004).
• Fairclough, J.P.A. & Norman, A.I. Structure and rheology of aqueous gels. Annu. Rep. Prog. Chem., Sect. C 99, 243-276(2003).
• Pich, A.Z. & Adler, H.P. Composite aqueous microgels: an overview of recent advances in synthesis, characterization and application. Polymer International 56, 291-307(2007).

External links

• Xerogel (definition)