Natural Applications of Colloids:

Blue Colour of Sky:

When the light is incident on particles whose size is smaller than the wavelength of light, it is scattered. The blue colour of the sky is due to the scattering of light by small particles (dust particles along with water) of the atmosphere. According to Rayleigh the intensity of scattered light is inversely proportional to the fourth power of wavelength. As the wavelength of blue colour is smallest and that of red light is longest, the blue light is scattered most and the red light is scattered the least. The scattered blue light reaching the eye gives the appearance of a blue sky.

When the aeroplane is flying high where there are no dust particles or water vapour then no scattering of any colour takes place and the sky looks black.

Red and Orange Colour of Sky in The Sunrise and Sunset:

When the sun is low on the horizon,  sunlight has to cover more distance through the atmosphere at sunset and sunrise than during the day, when the sun is higher in the sky. More distance through atmosphere means more molecules to scatter the violet and blue light away from our eyes. If the path is long enough, all of the blue and violet light scatters out of our line of sight. But the other colours continue on their way to our eyes. This is why sunrise and sunsets are often yellow, orange, and red.

Blue Colour of Sea:

The colloidal impurities in seawater due to their smaller size less than the wavelength of light scatter blue light. The Tyndall effect of scattering of light by colloids is responsible for the blue colour of the sea.

Fog Mist and Rain:

Fog, mist and rain are all colloidal in nature. In winters, at night, the moisture in the air condenses on the surface of dust particles, forming tiny droplets. These droplets, being colloidal in nature, float in the air and forms mist or fog.

Clouds are aerosols consisting of small droplets of water suspended in the air (aerosols). Clouds are the colloidal solution. On account of condensation in the upper atmosphere, the colloidal particles of water become bigger and bigger till they come down in the form of rain. They carry an electrical charge. The condensation occurs when the dust particles are cooled below its dew point. Sometimes rainfall occurs when oppositely charged clouds meet.

In the artificial rain, the clouds are sprayed by oppositely charged colloidal dust or sand particles or precipitates of silver iodide. This spraying neutralizes the charge on cloud resulting in coagulation of the water droplets which come down in the form of rain.

Industrial Applications of Colloids:

Thickening Agents:

Toothpaste, lotions, lubricants, coatings are the substances where the viscosity (degree of flow-ness) is very important. The substances added to them to change and maintain the viscosity are colloidal in nature. These colloid particles also provide stabilization of the colloidal solution and prevent the phase separation. They also act as fillers.

Example: various natural gums, microcrystalline cellulose, carboxymethyl cellulose, and fumed silica.

Paints:

Paints have been used since ancient times for both protective and decorative purposes. They consist basically of pigment particles dispersed in a liquid. The liquid capable of forming a stable solid film as the paint “dries”.on drying of the paint. On exposure to air, the pigments polymerize into the impervious film.

Inks:

The most critical properties of inks relate to their drying and surface properties. Inks must be able to flow properly and attach to the surface without penetrating it. It should dry very fast. Many inks consist of organic dyes dissolved in a water-based solvent and are not colloidal at all. The ink used in printing newspapers employs colloidal carbon black dispersed in an oil as the dispersion medium.

The inks employed in ball-point pens are gels, made in such a way that the ink will only flow over the ball and onto the paper when the shearing action of the ball (which rotates as it moves across the paper) “breaks” the gel into a liquid; the resulting liquid coats the ball and is transferred to the paper.

In conventional printing, the pigment particles remain on the paper surface, while the liquid gradually evaporates.

Rubber:

Latex obtained from rubber trees is an emulsion consisting of negatively charged rubber particles in water. Rubber is obtained by the coagulation of latex. This coagulated mass is later subjected to vulcanization and is solid as rubber with high abrasive strength. Vulcanized rubber is used in making tyres for vehicles.

Rubber plated articles can be prepared directly from latex by electrically depositing the negat6ively charged rubber particles over the article (made anode) which is to be rubber plated.

Leather Tanning Industry:

Raw skin hides of animals contain positively charged colloidal particles. These particles are coagulated by negatively charged tannin materials. After the tanning process, the leather becomes harder. Tanning material used are tannin and compounds of aluminium and chromium.

Cleansing Action of Soaps:

Soap solutions are colloidal in nature. They remove the dirt and oil particles either by adsorption or by emulsifying the greasy matter sticking to cloth.

Disinfectants:

Dettol and Lysol form an oil in water type colloidal solution which is used as the disinfecting agent.

Metallurgy:

The colloidal mixture of oil in water is used in the froth flotation process to separate sulphide ore particles.

Making of Photographic Plates:

Photographic plates and films are produced by coating an emulsion of the light-sensitive material like silver bromide in gelatin over class plates or celluloid films.

Sewage Precipitation:

Dirty and muddy water from gutters and drainages is called sewage is in colloidal form (colloidal solution).

Sewage water containing colloidal particles of mud, rubbish etc. is collected in a tank fitted with electrodes.

On applying an electric field, colloidal particles are attracted towards oppositely charged electrodes. As their charge gets neutralised, they settle as a precipitate. The precipitated or coagulated matter called sludge is used as manure while clear water is used for irrigation.

Smoke Precipitation:

Smoke is a colloidal solution of negatively charged carbon particles in the air (aerosol)

These carbon particles may condense water vapour on them and thus cities may have a thick cover of smog (smoke + fog). This smog causes air pollution.

Cottrel’s precipitator is a widely used smoke precipitator. Smoke is passed between metal electrodes at high voltage (about 50,000 V) The charged particles are neutralized at the oppositely charged electrode and get deposited there. The gases free from carbon particles are passed to a chimney or for further purification.

Other Applications of Colloids:

Synthetic plastics, rubber, graphite, lubricants, cement, etc. are colloidal solutions. Asphalt emulsion is used for road construction. The principles of colloids and interface science are used for the successful formulation and manufacture of photographic products.

Applications of Colloids in Food:

Food:

Most of the foods we eat are largely colloidal in nature. The function of food colloids generally has less to do with nutritional value than appearance, texture, and “mouthfeel”.  Mouthfeel is the ability to “melt” (transform from gel to liquid emulsion) on contact with the warmth of the mouth.

Dairy Products:

milk (oil in water), butter (water in oil), halva, icecreams, are in colloidal form.

Fruit Juices:

Fruit juices are colloidal solutions

Eggs:

The clear, viscous “white” of the egg can be transformed into a white, opaque semi-solid by brief heating, or rendered into more intricate forms by poaching, frying, scrambling, or baking.

The raw egg white is a colloidal sol of long-chain protein molecules, all curled up into compact folded forms due to hydrogen bonding between different parts of the same molecule. On heating, the hydrogen bonds are broken, and proteins unfold. The opened chains tangle and bind with each other, transforming the sol into a cross-linked hydrogel, and changes its appearance to opaque white.

Applications of Colloids in Agriculture:

Soil:

The four major components of soils are mineral sediments, organic matter, water, and air.  A fertile soil is colloidal in nature in which humus acts as a protective colloid.

Most soil colloids are negatively charged, and therefore attract cations such as Ca²⁺, Mg²⁺, and K⁺ into the outer parts of their double layers. As these ions are loosely bound, the plant roots can absorb these essential nutrients. Similarly, these ions are released into the soil again when the plant dies.

Formation of Delta:

River water is a colloidal solution of clay and seawater which mainly carry a negative charge. Seawater contains different electrolytes, mainly positive ions Na⁺, Mg²⁺, Ca²⁺. When the river meets the sea, the electrolytes in seawater bring about coagulation of clay particles in the river water. Thus the clay particles aggregate and settle down in course of water. Which results in the formation of a delta in due course.

Applications of Colloids in Medicine:

Colloidal medicines can act on a large surface area, hence they are more easily assimilated by the body. Therefore, they are more effective

Argyrol, a silver metal sol used as an eye lotion, colloidal antimony used for curing Kalaazar, colloidal gold used for inter-muscular injections.

Human Physiology:

Blood is a colloidal solution of albuminoid substances. Bleeding from a fresh cut can be stopped by applying a concentrated solution of ferric chloride or potash alum (this is known as the styptic action of alum or ferric chloride). In this case the cpagulation of blood takes place and a clot is formed which prevents further bleeding.

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In thisarticle, we shall study bout associated colloids :micelles).

Multimolecular Colloids:

Multimolecular colloids are those systems in which the dispersed phase particles are aggregates of many atoms or molecules. The particles in this colloidal solutions are held together by van der Wall’s forces. e.g. gold sol particles are an aggregation of many gold atoms. Other examples are silver sol and sulphur sol.

Macromolecular Colloids:

Macromolecular colloids are those systems in which the dispersed phase particles are a single macromolecule. They are lyophilic in character. e.g. sol of starch in water, Aqueous (Water) solution of proteins, enzymes.

Associated Colloids or Micelles

Colloids which behave as normal electrolytes at low concentrations, but exhibit colloidal properties at higher concentrations due to the formation of aggregated particles called associated colloids. The aggregated particles thus formed are called micelles.

The associated colloids are usually formed by surfactants (surface active agents) like soaps and synthetic detergents. The molecules of soaps and detergents are smaller than the colloidal particles. When dissolved in water soap and detergent molecules act as an electrolyte but if their concentration is increased then their molecules aggregate to form colloidal size particles called micelles. The formation of micelles takes place only above a certain concentration, this concentration is known as critical micellization concentration (CMC).

The Process of Formation of Micelles:

The formation of a micelle can be understood by taking the example of a soap solution. In general, soap can be represented as RCOONa, where R represents a long chain alkyl group. When dissolved in water, soap ionizes to give RCOO– and Na⁺ ions. The most commonly used washing soap is sodium stearate, C₁₇H₃₅COONa.

Clothes become dirty due to the deposition of dust and oily or greasy substances. Water is not capable of wetting oily or greasy substances. However, the hydrocarbon residue R of the soap anion (RCOO^–) can wet the oily or greasy substances. Soaps and detergents have two parts, the hydrophobic part and the hydrophilic part.

Hydrophobic or water repelling non-polar part (usually a long hydrocarbon chain) is soluble in oil and greases but insoluble in water. Hydrophilic or water attracting polar parts such as carboxylic group or sulphonate or sulphate is soluble in water and insoluble in oil and greases.

Molecules of soap and detergent form micelles in water. The hydrophobic part of soap dissolves in oil and grease while hydrophilic part of soap remains as free in soap solution. When the cloth is rubbed with hand or stirred mechanically, the big molecules of oil and soap break into small emulsified oil droplets.  These oil droplets repel each other due to the presence of anions of the hydrophilic part and do not precipitate. Thus they remain suspended in the soap solution without getting back on the cloth. These suspended oil and grease particles are then washed away by a stream of water.

In micelle formation, the long hydrocarbon chain which is insoluble in water is directed towards the centre while the soluble polar head is on the surface in contact with water.

The cleaning action of detergents such as sodium lauryl sulphate, CH₃ (CH₂)₁₁SO₄Na⁺ or sulphonates of long-chain hydrocarbons is similar to that of soaps. In case of detergents, the polar groups are sulphate (–SO₄) or sulphonate (-SO₃) groups.

Kraft’s Temperature:

The formation of micelles takes place only above a particular temperature is called the kraft’s temperature.

More Examples of Micelle System:

Sodium lauryl sulphate:

Sodium oleate:

Cetyltrimethyl ammonium bromide:

It forms micelle with a cationic terminal.

p-Dodecyl benzene sulphonate:

Surfactants:

Surfactants are compounds that lower the surface tension. They are preferentially adsorbed at the interfaces between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.

Cationic Surfactants:

The cationic surfactants are quaternary ammonium compounds with aryl or alkyl substituent groups, one of which is often a long hydrophobic carbon chain. Cationic surfactants are positively charged, and therefore are not as effective as detergents in cleansing systems. They can ideally be used as fabric softener.

Cetyl pyridinium chloride:

Cetyltrimethyl ammonium chloride:

Octadecyl ammonium chloride

Anionic Surfactants:

Anionic surfactant gives anion. Anionic surfactants are positively charged and are widely used.  Anionic surfactants possess a negative charge on their hydrophilic end. Generally, they make a lot of foam when agitated. They are free-flowing powdery when dry, not sticky like other surfactants.

Sodium Palmitate:

Sodium oleate:

Salts of sulphonic acid

Non-ionogenic or Non-ionic Surfactants:

Nonionic surfactants have no charge on their hydrophilic end, hence they are used as superior oily soil emulsifiers. They do not ionise or dissociate in an aqueous medium. Because of their lower foam profile and strong emulsifying potential, these surfactants are the preferred choice when formulating extraction cleaners and pre-sprays. Nonionics are thick liquids or syrups that are sticky. Nonionic surfactants include: ethoxylates, alkoxylates and cocamide

Characteristics of Micelles

• The formation of micelles takes place only above a certain concentration, this concentration is known as critical micellization concentration (CMC). As the concentration of a surfactant increases, adsorption takes place at the surface until it is fully overlaid, which corresponds to the minimum value of the surface tension.
• An increase in temperature usually increases CMC.
• Greater the chain length of the hydrocarbon chain smaller is the CMC.
• An increase in the hydrophobic part of the surfactant increases CMC.
• The addition of simple electrolyte in ionic micelles decreases their CMC.
• The formation of micelles takes place only above a particular temperature is called the Kraft’s temperature. Below Kraft’s temperature the solubility of surfactant not enough to form micelles.
• Kraft’s temperature increases with the increase in the number of carbon atoms.

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In this article, we shall study types of colloidal solutions (systems) on the basis of states of the dispersed phase and dispersion medium, the interaction between the dispersed phase and dispersion medium, and on the number of atoms and molecules in a colloidal particle.

Types of Colloidal Solutions on the Basis of States of Dispersed Phase and Dispersion Medium:

A colloidal system is made up of a dispersed phase and a dispersion medium. Because either the dispersed phase or the dispersion medium can be a gas, liquid or solid. There are eight types of the colloidal system possible since gases are miscible, the gas colloidal system is not possible. Gas-gas systems always form true solutions.

Classification of Sols On The Basis of  Interaction Between the Dispersed Phase and the Dispersion Medium:

A colloidal solution in which the dispersed phase is in solid-state and the dispersion medium is liquid is called a sol. e.g. Gum solution, the starch in water, Au, Ag, etc. in water, blood, etc.

Lyophilic Sols or Reversible Sols (Emulsoid):

The sols in which there is a strong affinity between the dispersed phase and dispersion medium are called as lyophilic sols. e.g. glue, gelatin, starch, proteins.

Characteristics of Lyophilic Sols:

• Lyophilic sols are readily formed by mixing together the substance forming disperse phase and solvent forming dispersion medium and heating the mixture if necessary.
• They are stable.
• There is a strong affinity between the dispersed phase and dispersion medium.
• The colloidal particles forming lyophilic sols are large single molecules or polymers like starch, proteins etc. of high molecular weight.
• If lyophilic sol is heated or dried we get solid but we get same sol if liquid (solvent or dispersion medium) is added to the solid. Thus lyophilic sols are reversible. After coagulation, they can again be converted into colloidal form.
• Lyophilic sols have lower surface tension than the dispersion medium.
• Lyophilic sols have a higher viscosity than dispersion medium.
• Stability of Lyophilic sols is due to high solvation due to the high affinity of particles towards dispersing medium.
• Lyophilic sols are stable and require a large quantity of electrolyte for coagulation. Thus they can not be coagulated easily.
• The particles cannot be detected easily under ultramicroscope.
• Lyophilic sols show weak Tyndall effect.

Stability of Lyophilic Sols:

In lyophilic sol, a thin film of the dispersion medium is formed around the dispersed phase colloidal particles due to the strong affinity between the dispersed phase and dispersion medium. The formation of this film around dispersed phase colloidal particles is called solvation. The stability of lyophilic sol is due to solvation.

Similarly, all the particles carry an electrical charge of the same nature, which results in mutual repulsion between the dispersed phase colloidal particles which also adds to the stability of lyophilic sol. But the charge on particles is very less or almost negligible.

Thus the stability of Lyophilic sols is due to solvation and charge on colloidal particles.

Lyophobic Sols or Irreversible Sols:

The sols in which there is no affinity between the dispersed phase and dispersion medium are called as lyophobic sols. e.g. sols of metals like Ag, Au, non-metals like sulphur, hydroxides like Al(OH)₃, Fe(OH)₃, sulphides like As₂S₃.

Characteristics of Lyophobic Sols:

• Lyophobic sols cannot be readily formed by mixing together the substance forming disperse phase and solvent forming dispersion medium.  Special methods like dispersion method or condensation method should be employed for making lyophobic sols.
• They are less stable.
• There is no or very little affinity between the dispersed phase and dispersion medium.
• The colloidal particles forming lyophobic sols are aggregates of a large number of atoms or molecules.
• If lyophilic sol is evaporated we get solid but we can not get the same sol if liquid (solvent or dispersion medium) is added to the solid. Thus lyophobic sols are irreversible. After coagulation, they cannot be converted into colloidal form again.
• Lyophobic sols have the same surface tension as the dispersion medium.
• Lyophobic sols have the nearly same viscosity as the dispersion medium.
• The stability of lyophobic sol is due to the charge on colloidal particles.
• Lyophobic sols are unstable and require a very small quantity of electrolyte for coagulation. Thus can be coagulated easily.
• The particles can be detected easily under an ultramicroscope.
• Lyophobic sols show a strong Tyndall effect.

Stability of Lyophobic Sols:

In lyophobic sols, all colloidal particles of the dispersed phase are either positively charged or negatively charged.

Colloidal particles remain suspended in the dispersion medium, without coagulation due to the repulsion between the particle having same nature of the charge,

Thus the stability of lyophobic sol is due to charge on colloidal particles.

Notes:

• If the dispersion medium is water then lyophilic and lyophobic sols are called hydrophilic and hydrophobic sols respectively.
• The colloidal solutions in alcohol and benzene are known as alcosols and benzosols respectively.
• The colloidal solutions in water are known as aquasols or hydrosols.

Classification of Colloidal Solutions on the Basis of the Number of Molecules or Atoms in the Colloidal Particle:

Multimolecular Colloids:

Multimolecular colloids are those systems in which the dispersed phase particles are aggregates of many atoms or molecules. The particles in these colloidal solutions are held together by van der Wall’s forces. e.g. gold sol particles are an aggregation of many gold atoms. other examples are silver sol and sulphur sol.

Macromolecular Colloids:

Macromolecular colloids are those systems in which the dispersed phase particles are a single macromolecule. They are lyophilic in character. e.g. sol of starch in water, Aqueous (Water) solution of proteins, enzymes.

Associated Colloids:

Colloids which behave as normal electrolytes at low concentrations, but exhibit colloidal properties at higher concentrations due to the formation of aggregated particles called associated colloids. The aggregated particles thus formed are called micelles.

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