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, although the liquid phase may still diffuse through this system.[3]
Gels are mostly liquid by mass, yet they behave like solids because of a three-dimensional cross-linked network within the liquid. It is the cross-linking 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.[4]
The process of forming a gel is called gelation.
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 such as polymer chain entanglements (see polymers) (physical gels) or chemical bonds such as disulfide bonds (see thiomers) (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 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.
A colloidal gel consists of a percolated network of particles in a fluid medium,[5] providing mechanical properties,[6] in particular the emergence of elastic behaviour.[7] The particles can show attractive interactions through osmotic depletion or through polymeric links.[8]
Colloidal gels have three phases in their lifespan: gelation, aging and collapse.[9] [10] The gel is initially formed by the assembly of particles into a space-spanning network, leading to a phase arrest. In the aging phase, the particles slowly rearrange to form thicker strands, increasing the elasticity of the material. Gels can also be collapsed and separated by external fields such as gravity.[11] Colloidal gels show linear response rheology at low amplitudes.[12] These materials have been explored as candidates for a drug release matrix.[13]
See main article: Hydrogel.
See also: Superabsorbent polymer, Self-healing hydrogels and Hydrogel agriculture.
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.[14] 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.[15] 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.[16]
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.[17] [18] (An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in petroleum.[19])
Organogels have potential for use in a number of applications, such as in pharmaceuticals,[20] cosmetics, art conservation,[21] and food.[22]
A xerogel 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) which results in a denser and more robust solid, the density and porosity achieved depend on the sintering conditions.
Nanocomposite hydrogels[23] [24] or hybrid hydrogels, are highly hydrated polymeric networks, either physically or covalently crosslinked with each other and/or with nanoparticles or nanostructures.[25] 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.[23] [26]
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.
A gel is in essence the mixture of a polymer network and a solvent phase. Upon stretching, the network crosslinks are moved further apart from each other. Due to the polymer strands between crosslinks acting as entropic springs, gels demonstrate elasticity like rubber (which is just a polymer network, without solvent). This is so because the free energy penalty to stretch an ideal polymer segment
N
b
R
Fela\simkT
R2 | |
Nb2 |
.
This is the origin of both gel and rubber elasticity. But one key difference is that gel contains an additional solvent phase and hence is capable of having significant volume changes under deformation by taking in and out solvent. For example, a gel could swell to several times its initial volume after being immersed in a solvent after equilibrium is reached. This is the phenomenon of gel swelling. On the contrary, if we take the swollen gel out and allow the solvent to evaporate, the gel would shrink to roughly its original size. This gel volume change can alternatively be introduced by applying external forces. If a uniaxial compressive stress is applied to a gel, some solvent contained in the gel would be squeezed out and the gel shrinks in the applied-stress direction.
To study the gel mechanical state in equilibrium, a good starting point is to consider a cubic gel of volume
V0
λ1
λ2
λ3
Vs0
λ1λ2λ3V0
V0+Vs0
λ1
λ2
λ3
fgel(λ1,λ2,λ3)
fgel(λ1,λ2,λ3)
V0
fgel(λ1,λ2,λ3)
fgel(λ1,λ2,λ3)=fnet(λ1,λ2,λ3)+fmix(λ1,λ2,λ3).
We now consider the two contributions separately. The polymer elastic deformation term is independent of the solvent phase and has the same expression as a rubber, as derived in the Kuhn's theory of rubber elasticity:
fnet(λ1,λ2,λ3)=
G0 | |
2 |
2 | |
(λ | |
1 |
+
2 | |
λ | |
2 |
+
2 | |
λ | |
3 |
-3),
where
G0
fmix(λ1,λ2,λ3)
f(\phi)
\phi
\phi0
\phi=\phi0/λ1λ2λ3
λ1λ2λ3
\phi0
\phi
\phi0
V0
(λ1λ2λ3-1)V0
\phi
λ1λ2λ3V0
Vg0fmix(λ1λ2λ3)=λ1λ2λ3f(\phi)-[V0f(\phi0)+(λ1λ2λ3-1)f(0)],
where on the right-hand side, the first term is the Flory–Huggins energy density of the final swollen gel, the second is associated with the initial gel and the third is of the pure solvent prior to mixing. Substitution of
\phi=\phi0/λ1λ2λ3
fmix(λ1,λ2,λ3)=
\phi0 | |
\phi |
[f(\phi)-f(0)]-[f(\phi0)-f(0)].
Note that the second term is independent of the stretching factors
λ1
λ2
λ3
f(\phi)=
kT | [ | |
vc |
\phi | |
N |
ln\phi+(1-\phi)ln(1-\phi)+\chi\phi(1-\phi)],
where
vc
N
\chi
N\toinfty
f(\phi)
f(\phi)=
kT | |
vc |
[(1-\phi)ln(1-\phi)+\chi\phi(1-\phi)].
Substitution of this expression into
fmix(λ1,λ2,λ3)
fgel(λ1,λ2,λ3)=
G0 | |
2 |
2 | |
(λ | |
1 |
+
2 | |
λ | |
2 |
+
2) | |
λ | |
3 |
+
\phi0 | |
\phi |
f(\phi).
This provides the starting point to examining the swelling equilibrium of a gel network immersed in solvent. It can be shown that gel swelling is the competition between two forces, one is the osmotic pressure of the polymer solution that favors the take in of solvent and expansion, the other is the restoring force of the polymer network elasticity that favors shrinkage. At equilibrium, the two effects exactly cancel each other in principle and the associated
λ1
λ2
λ3
In an alternative, scaling approach, suppose an isotropic gel is stretch by a factor of
λ
2 | |
R | |
0 |
(λ
2 | |
R | |
0) |
Fela\simkT
| |||||||||
|
,
where
Rref
\nu=\phi/Nb3
G(\phi)\sim
kT | |
b3 |
\phi | |
N |
| |||||||||
|
.
This modulus can then be equated to osmotic pressure (through differentiation of the free energy) to give the same equation as we found above.
Consider a hydrogel made of polyelectrolytes decorated with weak acid groups that can ionize according to the reaction
HA\rightleftharpoonsA-+H+
is immersed in a salt solution of physiological concentration. The degree of ionization of the polyelectrolytes is then controlled by the
pH
H+
A-
A-
H+
The coupling between the ion partitioning and polyelectrolyte ionization degree is only partially by the classical Donnan theory. As a starting point we can neglect the electrostatic interactions among ions. Then at equilibrium, some of the weak acid sites in the gel would dissociate to form
A-
H+
H+
H+
HA
A-
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.[33]
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.[34]
Many substances can form gels when a suitable thickener or gelling agent is added to their formula. This approach is common in the manufacture of a wide range of products, from foods to paints and adhesives.
In fiber optic 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.