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LECTURE 9.

June 11, 2024

LECTURE 9. GELS. BIOPOLYMERS. GENERAL CHARACTERISTICS AND PROPERTIES OF GELS

Lyophilic colloids that is normally stable but may be induced to coagulate partially under certain conditions (e.g. lowering the temperature). This produces a pseudo-solid or easily deformable jelly-like mass enclosing the entire amount of the liquid within itself. Such a product is called a gel, in which entertaining particles endorse the whole dispersing medium. Gel is a liquid-solid system in which a liquid is dispersed in a solid.

Gels may be further subdivided into elastic gels (e.g. gelatin) and rigid gels/inelastic gels (e.g. silica gel). They are distinguished form each othe r by the process of dehydration and rehydration.

Elastic Gels: An elastic gel is obtained by cooling a lyop hlic colloid, such as as gelatin or agar solution prepared by warming these substance with water. Starch gelatin, agar-agar and pectin sols belong to this class. Solutions of soap also belong to elastic gels. The elastic gels are reversible. In these cases, dehydration and rehydr ation on exposure to water vapour are most reversible, even then the process is carried out more than one.

Inelastic gels: The best known example of inelastic gel is silicic acid or silica gel. This is prepared by the addition of hydrochloric acid to sodium silicate at an appropriate concentration. The system sets to gel almost immediately. In some cases the setting may be delayed. Other examples are Fe (OH) 3 , Al (OH ) 3 , and Cr (OH) 3 . The inelastic gels are irreversible. If silica gel is dehydrated, addition of water will not reset it into the gel.

Among the lyophilic sols the well known gels are gelatin, agar-agar, gum arabic, mastic and gamboge sol, etc. Amongst the lyophobic sols, the well known gels are silicic acid, ferric hydroxide, ferric phosphate sols. The sols should be in sufficiently high concentration to facilitate the gelation process. Both gels and gelation are very important in medicine and biology because the plants and animals are composed of naturally occurring gels. Gelatin made up of macromolecular solutions have got great importance in technological field. In food and technological field industries make use of gelatin.

Preparation of Gels

Gels may be prepared by the following process.

(1) Cooling of sols at Moderate Concentration: Agar-agar, gelatin gels are made by cooling their sols having moderate concentrations prepared in hot water. As we know that hydrophilic sols are extensively hydrated, when cooled, the hy drated particles agglomerate together to form larger aggregates and finally led to form a semi-solid network structure of gel.

(2) Double-Decomposition: Hydrophobic gel such as silicic ac ids (comonly known as silica gel) and aluminium hydroxide (commonly known as alumina gel) are prepared by double-decomposition method) HCl is added to sodium silicate solution. As a result, a highly hydrated silicic acid gets precipitated. When this is allowed to stand to set, it forms a gel.

By similar method alumina gel is prepared by adding NaOH on AlCl3 solution. A highly hydrated salt is obtained. On standing for few minutes, it changes to gel.

(3) Change of solvents: Certain hydrophobic gels are prepared by this method. When ethanol is added instantly to a solution of calcium acetate of high concentrations, the salts separates out to form a colloidal solution. On standing the solution undergoes gelation and finally form a semi-rigid gel of CaAc 2 .

The principal difference between elastic and inelastic gels consist in their behaviour upon dehydration and rehydration. Partial dehydration of an elastic gel (e.g., gelatin) leads to an elastic solid from which the elastic gel may be produced again by addition of water. An inelastic gel becomes glassy, falls to powder and loses its elasticity when dried. The essential difference between them is attributed to the rigidity of th e walls of the capillaries formed when the inelastic gels are dehydrated. With elastic gels the walls are flexible. Silica gel when dehydrated forms a honeycomb structure with the capillaries so that it is a valuable absorbing agent.

The most interesting property for both the elastic and inelastic gels is shrinkage in volume when allowed to stand. This phenomena is known as syneresis. At isoelectric point maximum syneresis is observed for protein. Syneresis decreases when pH decreased from the isoelectronic point (pI value). It is believed that the setting of gels is caused by formation of a structure into which the whole of the liquid is taken up. This is true for both elastic and inelastic gels. Usually the setting takes place by cooling specially in the case of elastic gels. But the setting may also take place isothermally. This isothermal sol-gel transformation is knwon as thixotropy. Thus a bentonite sol at the ordinary temperature kept in a test tube in about a minute time sets to a gel and does not flow out of the tube even though carefully inverted. A mild jerk given to the test tube however, destroys the gel structure resulting in the transformation to a sol and becoming mobile again. If the test tube at the time of imparting the jerk is held near the ear, a faint clicking sound may be heard, which evidently indicates the collapse of the structure. It is natural with thixotropic substances that they have high viscosity when they are in the gel state and do not yield to small shearing stress. When the stress is increased there is a certain value when the structure collapses or yields and the system flows. This value of the shearing stress is known as the yield value.

A few thioxotropic systems solidify rapidly if a slow circular to-and-from motion is given to the sol. This phenomenon has been termed rheopexy. According to Freundlich the particles in such systems are rod-like or plate-like structure and the slow circular motion serves to impart an orientation to the particles so that the particles being parallel to one another and solidification is facilitated. Such sols usually takes long time to set to a gel if left to themselves and becomes converted into a gel under strong agitation. It is only by mild circular agitation that they set.

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 (physical gels) or chemical bonds (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.

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. Gel is a colloid solution of dispersion phase as liquid and dispersion medium as solid.

Types of gels

Hydrogels

Hydrogel is a network of polymer chains that are hydrophilic, sometimes found as a colloidal gel in which water is the dispersion medium. Hydrogels are highly absorbent (they can contain over 99.9% 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 to repair tissue.

· hydrogel-coated wells have been used for cell culture

· environmentally sensitive hydrogels which are also known as ‘Smart Gels’ or ‘Intelligent Gels’. 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 drug delivery systems.

· provide absorption, desloughing and debriding of necrotic 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 absorb urine, or in sanitary napkins

· contact lenses (silicone hydrogels, polyacrylamides)

· EEG and ECG medical electrodes using hydrogels composed of cross-linked polymers (polyethylene oxide, polyAMPS and polyvinylpyrrolidone)

· water gel explosives

· rectal drug delivery and diagnosis

Other, less common uses include

· breast implants

· now used in glue.

· 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.

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

Xerogels

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 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.

Thixotropy is shear thinning property. Certain gels or fluids that are thick (viscous) under normal conditions flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed. They then take a fixed time to return to a more viscous state. In more technical language: some non-Newtonian pseudoplastic fluids show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite time to attain equilibrium viscosity when introduced to a step change in shear rate. Some thixotropic fluids return to a gel state almost instantly, such as ketchup, and are called pseudoplastic fluids. Others such as yogurt take much longer and can become nearly solid. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.

Some fluids are anti-thixotropic: constant shear stress for a time causes an increase in viscosity or even solidification. Constant shear stress can be applied by shaking or mixing. Fluids which exhibit this property are usually called rheopectic. They are much less common.

Natural examples

Some clays are thixotropic, with their behavior of great importance in structural and geotechnical engineering. Landslides, such as those common in the cliffs around Lyme Regis, Dorset and in the Aberfan spoil tip disaster in Wales are evidence of this phenomenon. Similarly, a lahar is a mass of earth liquefied by a volcanic event, which rapidly solidifies once coming to rest.

Drilling muds used in geotechnical applications can be thixotropic. Honey from honey bees may also exhibit this property under certain conditions.(heather honey