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Gel

From Wikipedia, the free encyclopedia
Highly viscous liquid exhibiting a kind of semi-solid behavior
For other uses, seeGel (disambiguation).
Polymer science
Polyacetylene
An upturned vial ofhair gel
Silica gel

Agel is asemi-solid that can have properties ranging from soft and weak to hard and tough.[1][2] Gels are defined as a substantially dilutecross-linked system, which exhibits no flow when in the steady state, although the liquid phase may still diffuse through this system.[3]

IUPAC definition for a gel

Gels are mostly liquidby 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 wordgel was coined by 19th-century Scottish chemistThomas Graham byclipping fromgelatine.[4]

The process of forming a gel is calledgelation.

Composition

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Gels consist of a solid three-dimensional network that spans the volume of a liquid medium and ensnares it throughsurface tension effects. This internal network structure may result from physical bonds such as polymer chain entanglements (seepolymers) (physical gels) orchemical bonds such asdisulfide bonds (seethiomers) (chemical gels), as well ascrystallites 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

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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 toviscosity in their stretched state, because the stretched-out polymer takes up more space. This is also the reason gel hardens. Seepolyelectrolyte for more information.

Types

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Colloidal gels

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Acolloidal gel consists of apercolated network of particles in a fluid medium,[5] providingmechanical properties,[6] in particular the emergence of elastic behaviour.[7] The particles can show attractive interactions throughosmotic 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]

Hydrogels

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Main article:Hydrogel
See also:Superabsorbent polymer,Self-healing hydrogels, andHydrogel agriculture
Hydrogel of a superabsorbent polymer

Ahydrogel is a network of polymer chains that are hydrophilic, sometimes found as acolloidal gel in which water is the dispersion medium. A three-dimensional solid results from the hydrophilic polymer chains being held together by cross-links.[clarification needed] 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 highlyabsorbent (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 agel-sol transition to the liquid state.[15] Chemomechanical polymers are mostly also hydrogels, which upon stimulation change their volume and can serve asactuators orsensors. The first appearance of the term 'hydrogel' in the literature was in 1894.[16]

IUPAC definition for a polymer gel

Organogels

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See also:Organogels

Anorganogel is anon-crystalline,non-glassy thermoreversible (thermoplastic) solid material composed of aliquidorganic phase entrapped in a three-dimensionally cross-linked network. The liquid can be, for example, anorganic solvent,mineral oil, orvegetable oil. Thesolubility andparticle dimensions of the structurant are important characteristics for theelastic properties and firmness of the organogel. Often, these systems are based onself-assembly of the structurant molecules.[17][18] (An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization inpetroleum.[19])

Organogels have potential for use in a number of applications, such as inpharmaceuticals,[20] cosmetics, art conservation,[21] and food.[22]

Xerogels

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IUPAC definition for a xerogel
https://doi.org/10.1351/goldbook.X06700.

Axerogel/ˈzɪəroʊˌdʒɛl/ 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 smallpore size (1–10 nm). Whensolvent removal occurs undersupercritical conditions, the network does not shrink and a highly porous, low-density material known as anaerogel is produced. Heat treatment of a xerogel at elevated temperature produces viscoussintering (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

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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 metallicnanomaterials 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]

Properties

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Many gels displaythixotropy – they become fluid when agitated, but resolidify when resting.In general, gels are apparently solid, jelly-like materials. It is a type ofnon-Newtonian fluid.By replacing the liquid with gas it is possible to prepareaerogels, materials with exceptional properties including very low density,high specific surface areas, and excellent thermal insulation properties.

Thermodynamics of gel deformation

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A gel is in essence the mixture of a polymer network and asolvent phase. Upon stretching, the networkcrosslinks are moved further apart from each other. Due to the polymer strands between crosslinks acting asentropic springs, gels demonstrate elasticity likerubber (which is just a polymer network, without solvent). This is so because thefree energy penalty to stretch anideal polymer segmentN{\displaystyle N} monomers of sizeb{\displaystyle b} between crosslinks to anend-to-end distanceR{\displaystyle R} is approximately given by[27]

Fela∼kTR2Nb2.{\displaystyle F_{\text{ela}}\sim kT{\frac {R^{2}}{Nb^{2}}}.}

This is the origin of both gel andrubber elasticity. But one key difference is that gel contains an additional solvent phase and hence is capable of having significant volume changes underdeformation 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 compressivestress 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 volumeV0{\displaystyle V_{0}} that is stretched by factorsλ1{\displaystyle \lambda _{1}},λ2{\displaystyle \lambda _{2}} andλ3{\displaystyle \lambda _{3}} in the three orthogonal directions during swelling after being immersed in a solvent phase of initial volumeVs0{\displaystyle V_{s0}}. The final deformed volume of gel is thenλ1λ2λ3V0{\displaystyle \lambda _{1}\lambda _{2}\lambda _{3}V_{0}} and the total volume of the system isV0+Vs0{\displaystyle V_{0}+V_{s0}}, that is assumed constant during the swelling process for simplicity of treatment. The swollen state of the gel is now completely characterized by stretch factorsλ1{\displaystyle \lambda _{1}},λ2{\displaystyle \lambda _{2}} andλ3{\displaystyle \lambda _{3}} and hence it is of interest to derive thedeformation free energy as a function of them, denoted asfgel(λ1,λ2,λ3){\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})}. For analogy to the historical treatment ofrubber elasticity and mixing free energy,fgel(λ1,λ2,λ3){\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})} is most often defined as the free energy difference after and before the swelling normalized by the initial gel volumeV0{\displaystyle V_{0}}, that is, a free energy difference density. The form offgel(λ1,λ2,λ3){\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})} naturally assumes two contributions of radically different physical origins, one associated with theelastic deformation of the polymer network, and the other with themixing of the network with the solvent. Hence, we write[28]

fgel(λ1,λ2,λ3)=fnet(λ1,λ2,λ3)+fmix(λ1,λ2,λ3).{\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})=f_{\text{net}}(\lambda _{1},\lambda _{2},\lambda _{3})+f_{\text{mix}}(\lambda _{1},\lambda _{2},\lambda _{3}).}

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 ofrubber elasticity:

fnet(λ1,λ2,λ3)=G02(λ12+λ22+λ32−3),{\displaystyle f_{\text{net}}(\lambda _{1},\lambda _{2},\lambda _{3})={\frac {G_{0}}{2}}(\lambda _{1}^{2}+\lambda _{2}^{2}+\lambda _{3}^{2}-3),}

whereG0{\displaystyle G_{0}} denotes theshear modulus of the initial state. On the other hand, the mixing termfmix(λ1,λ2,λ3){\displaystyle f_{\text{mix}}(\lambda _{1},\lambda _{2},\lambda _{3})} is usually treated by theFlory-Huggins free energy ofconcentrated polymer solutionsf(ϕ){\displaystyle f(\phi )}, whereϕ{\displaystyle \phi } is polymer volume fraction. Suppose the initial gel has a polymer volume fraction ofϕ0{\displaystyle \phi _{0}}, the polymer volume fraction after swelling would beϕ=ϕ0/λ1λ2λ3{\displaystyle \phi =\phi _{0}/\lambda _{1}\lambda _{2}\lambda _{3}} since the number of monomers remains the same while the gel volume has increased by a factor ofλ1λ2λ3{\displaystyle \lambda _{1}\lambda _{2}\lambda _{3}}. As the polymer volume fraction decreases fromϕ0{\displaystyle \phi _{0}} toϕ{\displaystyle \phi }, a polymer solution of concentrationϕ0{\displaystyle \phi _{0}} and volumeV0{\displaystyle V_{0}} is mixed with a pure solvent of volume(λ1λ2λ3−1)V0{\displaystyle (\lambda _{1}\lambda _{2}\lambda _{3}-1)V_{0}} to become a solution with polymer concentrationϕ{\displaystyle \phi } and volumeλ1λ2λ3V0{\displaystyle \lambda _{1}\lambda _{2}\lambda _{3}V_{0}}. The free energy density change in this mixing step is given as

Vg0fmix(λ1λ2λ3)=λ1λ2λ3f(ϕ)−[V0f(ϕ0)+(λ1λ2λ3−1)f(0)],{\displaystyle V_{g0}f_{\text{mix}}(\lambda _{1}\lambda _{2}\lambda _{3})=\lambda _{1}\lambda _{2}\lambda _{3}f(\phi )-[V_{0}f(\phi _{0})+(\lambda _{1}\lambda _{2}\lambda _{3}-1)f(0)],}

where on the right-hand side, the first term is theFlory–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 ofϕ=ϕ0/λ1λ2λ3{\displaystyle \phi =\phi _{0}/\lambda _{1}\lambda _{2}\lambda _{3}} leads to

fmix(λ1,λ2,λ3)=ϕ0ϕ[f(ϕ)−f(0)]−[f(ϕ0)−f(0)].{\displaystyle f_{\text{mix}}(\lambda _{1},\lambda _{2},\lambda _{3})={\frac {\phi _{0}}{\phi }}[f(\phi )-f(0)]-[f(\phi _{0})-f(0)].}

Note that the second term is independent of the stretching factorsλ1{\displaystyle \lambda _{1}},λ2{\displaystyle \lambda _{2}} andλ3{\displaystyle \lambda _{3}} and hence can be dropped in subsequent analysis. Now we make use of theFlory-Huggins free energy for a polymer-solvent solution that reads[29]

f(ϕ)=kTvc[ϕNln⁡ϕ+(1−ϕ)ln⁡(1−ϕ)+χϕ(1−ϕ)],{\displaystyle f(\phi )={\frac {kT}{v_{c}}}[{\frac {\phi }{N}}\ln \phi +(1-\phi )\ln(1-\phi )+\chi \phi (1-\phi )],}

wherevc{\displaystyle v_{c}} is monomer volume,N{\displaystyle N} is polymer strand length andχ{\displaystyle \chi } is theFlory-Huggins energy parameter. Because in a network, the polymer length is effectively infinite, we can take the limitN→∞{\displaystyle N\to \infty } andf(ϕ){\displaystyle f(\phi )} reduces to

f(ϕ)=kTvc[(1−ϕ)ln⁡(1−ϕ)+χϕ(1−ϕ)].{\displaystyle f(\phi )={\frac {kT}{v_{c}}}[(1-\phi )\ln(1-\phi )+\chi \phi (1-\phi )].}

Substitution of this expression intofmix(λ1,λ2,λ3){\displaystyle f_{\text{mix}}(\lambda _{1},\lambda _{2},\lambda _{3})} and addition of the network contribution leads to[28]

fgel(λ1,λ2,λ3)=G02(λ12+λ22+λ32)+ϕ0ϕf(ϕ).{\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})={\frac {G_{0}}{2}}(\lambda _{1}^{2}+\lambda _{2}^{2}+\lambda _{3}^{2})+{\frac {\phi _{0}}{\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 theosmotic pressure of the polymer solution that favors the take in of solvent and expansion, the other is the restoring force of the polymer networkelasticity that favors shrinkage. At equilibrium, the two effects exactly cancel each other in principle and the associatedλ1{\displaystyle \lambda _{1}},λ2{\displaystyle \lambda _{2}} andλ3{\displaystyle \lambda _{3}} define the equilibrium gel volume. In solving the force balance equation, graphical solutions are often preferred.

In an alternative, scaling approach, suppose anisotropic gel is stretch by a factor ofλ{\displaystyle \lambda } in all three directions. Under theaffine network approximation, the mean-squareend-to-end distance in the gel increases from initialR02{\displaystyle R_{0}^{2}} to(λR0)2{\displaystyle (\lambda R_{0})^{2}} and the elastic energy of one stand can be written as

Fela∼kT(λR0)2Rref2,{\displaystyle F_{\text{ela}}\sim kT{\frac {(\lambda R_{0})^{2}}{R_{\text{ref}}^{2}}},}

whereRref{\displaystyle R_{\text{ref}}} is the mean-square fluctuation in end-to-end distance of one strand. The modulus of the gel is then this single-strand elastic energy multiplied by strand number densityν=ϕ/Nb3{\displaystyle \nu =\phi /Nb^{3}} to give[27]

G(ϕ)∼kTb3ϕN(λR0)2Rref2.{\displaystyle G(\phi )\sim {\frac {kT}{b^{3}}}{\frac {\phi }{N}}{\frac {(\lambda R_{0})^{2}}{R_{\text{ref}}^{2}}}.}

This modulus can then be equated toosmotic pressure (through differentiation of the free energy) to give the same equation as we found above.

Modified Donnan equilibrium of polyelectrolyte gels

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Consider ahydrogel made ofpolyelectrolytes decorated withweak acid groups that can ionize according to the reaction

HA⇌A−+H+{\displaystyle {\text{HA}}\rightleftharpoons {\text{A}}^{-}+{\text{H}}^{+}}

is immersed in a salt solution of physiological concentration. The degree ofionization of thepolyelectrolytes is then controlled by thepH{\displaystyle {\text{pH}}} and due to the charged nature ofH+{\displaystyle {\text{H}}^{+}} andA−{\displaystyle {\text{A}}^{-}},electrostatic interactions with other ions in the systems. This is effectively a reacting system governed byacid-base equilibrium modulated by electrostatic effects, and is relevant indrug delivery, sea waterdesalination anddialysis technologies. Due to the elastic nature of the gel, the dispersion ofA−{\displaystyle {\text{A}}^{-}} in the system is constrained and hence, there will be a partitioning of salts ions andH+{\displaystyle {\text{H}}^{+}} inside and outside the gel, which is intimately coupled to thepolyelectrolyte degree of ionization. This ion partitioning inside and outside the gel is analogous to the partitioning of ions across a semipemerable membrane in classicalDonnan theory, but a membrane is not needed here because the gel volume constraint imposed by network elasticity effectively acts its role, in preventing the macroions to pass through the fictitious membrane while allowing ions to pass.[30]

The coupling between the ion partitioning and polyelectrolyte ionization degree is only partially by the classicalDonnan 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 formA−{\displaystyle {\text{A}}^{-}}that electrostatically attracts positive chargedH+{\displaystyle {\text{H}}^{+}} and salt cations leading to a relatively high concentration ofH+{\displaystyle {\text{H}}^{+}} and salt cations inside the gel. But because the concentration ofH+{\displaystyle {\text{H}}^{+}} is locally higher, it suppresses the further ionization of the acid sites. This phenomenon is the prediction of the classical Donnan theory.[31] However, with electrostatic interactions, there are further complications to the picture. Consider the case of two adjacent, initially uncharged acid sitesHA{\displaystyle {\text{HA}}} are both dissociated to formA−{\displaystyle {\text{A}}^{-}}. Since the two sites are both negatively charged, there will be a charge-charge repulsion along the backbone of the polymer than tends to stretch the chain. This energy cost is high both elastically and electrostatically and hence suppress ionization. Even though this ionization suppression is qualitatively similar to that of Donnan prediction, it is absent without electrostatic consideration and present irrespective of ion partitioning. The combination of both effects as well as gel elasticity determines the volume of the gel at equilibrium.[30] Due to the complexity of the coupled acid-base equilibrium, electrostatics and network elasticity, only recently has such system been correctly recreated incomputer simulations.[30][32]

Animal-produced gels

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Some species secrete gels that are effective in parasite control. For example, thelong-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 includemucus, thevitreous humor of the eye,cartilage,tendons andblood 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 temporaryimplants (degradable) and permanent implants (non-degradable). A review article on the subject discusses the use of hydrogels fornucleus pulposus replacement, cartilage replacement, andsynthetic tissue models.[34]

Applications

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Many substances can form gels when a suitablethickener 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 resemblinghair 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

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References

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  1. ^Khademhosseini A.; Demirci U. (2016).Gels Handbook: Fundamentals, Properties and Applications. World Scientific Pub Co Inc.ISBN 978-981-4656-10-8.
  2. ^Seiffert S., ed. (2015).Supramolecular Polymer Networks and Gels. Springer.ASIN B00VR5CMW6.
  3. ^Ferry, John D. (1980).Viscoelastic Properties of Polymers. New York: Wiley.ISBN 0-471-04894-1.
  4. ^Harper D."Online Etymology Dictionary: gel".Online Etymology Dictionary. Retrieved2013-12-09.
  5. ^Zaccarelli, E. (15 August 2007). "Colloidal gels: equilibrium and non-equilibrium routes".Journal of Physics: Condensed Matter.19 (32) 323101.arXiv:0705.3418.Bibcode:2007JPCM...19F3101Z.doi:10.1088/0953-8984/19/32/323101.S2CID 17294391.
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  7. ^Whitaker, K., Varga, Z., Hsiao, L., Solomon, M., Swan, J., Furst, E. (May 2019)."Colloidal gel elasticity arises from the packing of locally glassy clusters".Nature Communications.10 (1) 2237.Bibcode:2019NatCo..10.2237W.doi:10.1038/s41467-019-10039-w.PMC 6527676.PMID 31110184.
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  13. ^Meidia, H., Irfachsyad, D., Gunawan, D. (12 November 2019)."Brownian Dynamics Simulation of Colloidal Gels as Matrix for Controlled Release Application".IOP Conference Series: Materials Science and Engineering.553 (1) 012011.Bibcode:2019MS&E..553a2011M.doi:10.1088/1757-899X/553/1/012011.S2CID 210251780.
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  27. ^abRubinstein, Michael; Colby, Ralph H. (2003).Polymer physics. Oxford: Oxford University Press.ISBN 0-19-852059-X.OCLC 50339757.
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External links

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Solids
Liquids
Respiratory tract (inhalation)
Solids


Liquids
Gas
Ophthalmic,
otic,nasal
Urogenital
Rectal (enteral)
Dermal (topical)
Parenterals,injections,
infusions
(into tissue/blood)
Skin (transdermal)
Organs
Central nervous system
Circulatory,
musculoskeletal
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