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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A colloidal system in which both the dispersed phase as well as dispersion medium are immiscible or partially miscible liquids is called an emulsion. e.g. Milk, cod liver oil, oil paints, vanishing cream, cold creams, etc. are emulsions.

Generally, one of the two liquids is water and the other which is immiscible with water is designated as oil. The liquid in excess is the dispersion medium and the liquid which forms droplets or globules is the dispersed phase.

Emulsifying Agent or Emulsifier:

Emulsions of oil and water are unstable and sometimes they separate into two layers on standing. To obtain a stable emulsion a small quantity of third substance is added along with two immiscible liquids. This third substance is called Emulsifying agent or Emulsifier. Emulsifier forms a protective layer around disperse phase droplets and prevent coagulation. e.g. Soaps, Detergents, Lyophilic substances like gelatin, gum etc.

Types of Emulsions and Their Preparation:

Oil in Water Type (O/W):

In this type of emulsion, oil is the dispersed phase and water is the dispersion medium. e.g. Milk is (fats) oil in water type of emulsion, vanishing creams.

Preparation: 

The emulsifier is dissolved in water and oil is added to it drop by drop with continuous agitation.

Characteristics of Oil in Water Emulsions:

• Oil is the dispersed phase and water is a dispersion medium.
• If water is added it is miscible with the emulsion.
• If oil is added it is not miscible with the emulsion.
• Addition of small amount of electrolyte makes emulsion conducting.
• Water is a continuous phase.
• Basic metal sulphates, water-soluble alkali metal soaps are used as emulsifiers.

Water in Oil Type (W/O):

In this type of emulsions, water is the dispersed phase and oil is the dispersion medium. e.g. Cod liver oil in which particles of water are dispersed in oil, Cold creams.

Preparation: 

The emulsifier is dissolved in oil and water is added to it drop by drop with continuous agitation.

Characteristics of Water in Oil Emulsions:

• Water is the dispersed phase and oil is the dispersion medium.
• If oil is added it is miscible with the emulsion.
• If water is added it is not miscible with the emulsion.
• Addition of small amount of electrolyte does not make emulsion conducting.
• Oil is a continuous phase.
• Water-insoluble soaps such as those of Zn, Al, Fe, alkaline earth metals are used as emulsifiers.

Methods of Identification of Type of Emulsions:

Dye Solubility Test:

In this test, an emulsion is mixed with a water-soluble dye (amaranth) and observed under the microscope. If the continuous phase appears red, it means that the emulsion is o/w type as water is in the external phase and the dye will dissolve in it to give colour. If the scattered globules appear red and continuous phase colourless, then it is w/o type. Similarly, if an oil-soluble dye (Scarlet red C or Sudan III) is added to an emulsion and the continuous phase appears red, then it is w/o emulsion.

Viscosity Test:

The viscosity of water in oil type emulsion is more than the viscosity of oil in water type emulsion.

Electrical Conductive Test:

The basic principle of this test is that water is a good conductor of electricity. In the case of o/w emulsion, this test will be positive as water is the continuous phase. If electrolyte like NaCl is added to oil in water type emulsion, its conductivity increases greatly.

Spreading Test:

Water in oil type emulsion spread on the surface of oil but not on the surface of the water.

Dilution Test:

In this test, the emulsion is diluted either with oil or water. If the emulsion is o/w type and it is diluted with water, it will remain stable as water is the dispersion medium. For example, milk which is oil in water emulsion can be diluted with water, but butter which is water in oil emulsion cannot be diluted with water.

If emulsion o/w type is diluted with oil, the emulsion will break as oil and water are not miscible with each other. Oil in water emulsion can easily be diluted with an aqueous solvent whereas water in oil emulsion can be diluted with an oily liquid.

Cobalt Chloride Test:

When a filter paper soaked in cobalt chloride solution is dipped into an emulsion and dried, it turns from blue to pink, indicating that the emulsion is o/w type.

Fluorescence Test:

If an emulsion on exposure to ultra-violet radiations shows continuous fluorescence under a microscope, then it is w/o type and if it shows only spotty fluorescence, then it is o/w type.

Properties of Emulsions:

Precipitation:

In many emulsions, the size of the dispersed droplets is larger than the particles found in sols. These droplets can be precipitated by adding suitable electrolytes.

De-emulsification:

Separation of two liquid phases (two distinct layers) from emulsions due to the removal of emulsifier by any means is called as de-emulsification or de-emulsification. It can be done by heating, freezing, centrifuging or by addition of electrolytes.

Dilution:

Emulsions can be diluted with any amount of dispersion medium. But it is to be noted that any increase in the concentration of dispersed phase results in demulsification.

Coalescence:

It is the phenomenon of disappearance of the boundary between two particles in contact (as in case of droplets and bubbles). The formation of aggregates may be followed by coalescence. In excessive conditions, the coalescence leads to breaking of the emulsion. Coalescence of solid particles is called sintering.

Note:

Emulsions show similar properties as that of sols. When the droplets are very small they show Tyndall effect, Brownian movement and electrophoresis.

Characteristics of Emulsions:

• It is the colloidal system in which the dispersed phase and the dispersion medium both are liquid.
• It is a mobile liquid.
• Droplets of one liquid dispersed in another liquid.
• No tendency to absorb a liquid or to swell.
• The emulsifying agent is needed.
• It is classified as oil in water and water in oil.
• They show Tyndall effect, Brownian movement and electrophoresis.

Uses of Emulsions:

• Phenyl, when poured in water, gives oil in water emulsion.  It is used as a disinfectant.
• Cleansing action of soap or detergent is due to emulsion formation of oil with water.
• Emulsions are used in medicines such as emulsions of cod liver oil, malt and yeast.
• They are used in oil paints, plastic emulsion paints.
• In the concentration of sulphide ore by forth floatation method, oil in water type of emulsion is formed.
• Milk is an important household emulsion.
• Asphalt emulsified in water is used for the construction of roads.

Method of Breaking of Emulsions:

• The two liquids in the emulsion can be separated by heating, freezing or centrifuging.
• The addition of a large quantity of electrolyte causes coagulation of the dispersed phase.
• The chemical destruction of the emulsifier causes the separation of the two liquids.

Disadvantages of Emulsion:

• To refine the emulsion of petroleum with water is expensive.
• The emulsion of oil in water may be coming as water supply, which is unfit for drinking.

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A gel is a colloidal system in which the dispersed phase is liquid and the dispersion medium is solid. e.g. when warm sol of gelatin is cooled, it sets to a semi-solid mass which is a gel. Jellies, jams, curd, butter, shoe polish, etc. are gels.

The interior (the cytoplasm) of each cell in the soft tissues of our bodies consists of a variety of inclusions (organelles) suspended in a gel-like liquid phase called the cytosol. Dissolved in the cytosol are a variety of ions and molecules varying from the small to the large; among the latter, proteins and carbohydrates make up the “solid” portion of the gel structure. The nature of gel depends on the coexistence between the solid network and the liquid medium. A gel is a soft material that can be easily cut with a knife. Removal of the liquid phase leads to the xerogel or aerogel which depends upon the drying conditions of the gel.

Gelation:

When a colloidal solution is coagulated, a precipitate is formed which may or may not be gelatinous. Under certain conditions, it may be possible to obtain the dispersed phase as more or less rigid mass enclosing within it all of the liquid. The product formed is called a gel and the process is known as gelation.

Characteristics of Gels:

• It is the colloidal system in which the dispersed phase is liquid and the dispersion medium is solid.
• It is an immobile semi-solid.
• It has a honeycomb-like structure.
• Many gels have a tendency to absorb liquid and swell.
• No such agent is required for its formation.
• It is classified as an elastic gel and non-elastic gel.
• They do not show the Tyndall effect, Brownian movement, and electrophoresis.

Types of gels:

Elastic Gels:

Elastic gels or reversible gels are gels which when heated carefully, form a dry mass and if this dried mass, when placed in contact with liquid, absorb liquid, swell up and regain their original form. e.g. agar-agar, gelatin, fruit jams, etc.

Characteristics of Elastic Gels:

• On heating (dehydration) they give elastic solid.
• The original gel can be obtained by addition of water or liquid to elastic solid.
• They are reversible
• They are lyophilic
• They show imbibition
• Gels, made up of organic substances are elastic.

Non Elastic Gels:

Non-elastic gels or irreversible gels are gels which when on heating loose liquid and gets converted into dry mass but cannot absorb liquid and regain their original form when they are placed in contact with the liquid. e.g. Silica gel, solid alcohol, hydroxides of Fe, Al, Cr, etc. are non-elastic gels.

Characteristics of Non Elastic Gels:

• On heating (dehydration) they give a powder.
• The original gel cannot be obtained by addition of water or liquid to powder.
• They are irreversible.
• They are lyophobic
• They do not show imbibition
• Gels, made up of inorganic substances are non-elastic.

Properties of Gels:

Swelling or Imbibition:

The tendency of a gel to take up a large quantity of water or liquid and go on increasing in volume is called swelling of gel or the smiling of gel or imbibition of gel. Only elastic gels show this property.

Syneresis:

The decrease in the volume of a gel due to the loss of liquid on standing is called syneresis or weeping of gel. Many inorganic gels on standing, undergo shrinkage which is accompanied by exudation of liquid. This process is the reverse of imbibition.

Thixotropy:

Some gels turn into a sol on shaking and reset to the gel on standing. This reversible gel-sol transformation is called thixotropy. Iron oxide and silver oxide gels exhibit this property.

Fragile Nature:

In a newly-opened container of yogurt or sour cream (gel) appears to be smooth and firm, but once some of the material has been spooned out, little puddles of liquid appear in the hollowed- portion.

It can be explained as follows. As the spoon is plunged into the gel, it pulls the nearby layers of the gel along with it, creating a shearing action that breaks it apart, releasing the liquid entrapped inside.

Hofmeister or Lyotropic Series:

The tendency of a gel to take up a large quantity of water or liquid and go on increasing in volume is called as swelling of gel or smiling of gel or imbibition of gel. Only elastic gel shows this property. Hofmeister studied the effect of a salt on swelling (imbibition) of gel. Different ions are found to have a different effect on swelling (imbibition). For example, in presence of iodide ions, the process of imbibition is so fast that the imbibition takes place even at room temperature. For other ions heating is required.

The series of cations Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and of anions citrate^3-, tartrate^2-, SO₄^2-, acetate^–, NO₃^–, CIO₃^–, I^–, CNS^–   (among others) is called lyotropic series or Hofmeister series. The ions of the Hofmeister series are ordered from the most (top) to least (bottom) effective in precipitating proteins out of solution.

Uses of Gels:

• Alcohol jellied with calcium acetate is used as solid fuel for military field services.
• Silica gel is most valuable adsorbing and drying (desiccating) agent which is used in industry and laboratory.
• Many articles of common use are gels. e.g. curd, fruit jams, butter, cheese, jellies, shoe polish, various foodstuffs etc.

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The process of precipitation of colloidal particles due to aggregation of the particles is called coagulation or flocculation.

Explanation:

The presence of the same type of electric charge on colloidal particles causes repulsion and keep them in a suspended state. If by some means, the charge on the colloidal particles is removed, these particles come together, aggregated, become large enough to settle down in the form of a precipitate. Thus the charge on the colloidal particles is neutralized. In this process, the dispersion medium and dispersed phase are separated from each other.

If the precipitate formed instead of settling down, floats on the surface of the dispersion medium then the phenomenon is called as flocculation.

Methods of Coagulation of Lyophobic Sols: 

By Mutual Coagulation or Meteral Coagulation:

If two sols of opposite charge are mixed together, their charges are mutually neutralised and both sols get coagulated.

Example:

Fe(OH)₃   (+ve sol)  +   As₂S₃ (- ve sol)   →     Precipitate

By heating:

On the heating thermal energy of colloidal particles increases so much that it overcomes the repulsive forces between them, and particles unite to form larger particles.

The charge on colloidal particles is due to preferential adsorption. The phenomenon of adsorption is inversely proportional to temperature. Due to heating desorption of adsorbed ions from the surface of colloidal particles takes place. Similarly, the dispersion medium particles due to increase in kinetic energy disturb the surface layer of adsorbed charge.

Thus due to the above two reasons, the neutral sol particles aggregate to form a precipitate.

Example :

If an egg is boiled egg albumin gets coagulated.

Egg albumin    →      Precipitate.

By Electrophoresis:

Under the influence of an electric field, sol particles move to the electrode where they get discharged as a neutral particle. Neutral sol particles aggregate to form a precipitate.

Example: 

Rubber from latex is coagulated by electrophoresis on cloth or metal.

By Persistent Dialysis:

On prolonged dialysis almost all the electrolyte in the colloidal solution is removed making ythe sol unstable. It leads to coagulation.

By Addition of Electrolyte:

When an excess of an electrolyte is added to a sol, oppositely charged ions of electrolyte removes the charged on sol particles. Neutral sol particles aggregate to form a precipitate. Any negatively charged sol is coagulated by the cations of electrolyte and vice-versa. The ion carrying the charge responsible for coagulation is called a flocculating ion.

Example: If BaCl₂ is added to the As₂S₃ sol which is negative sol, Ba²⁺ ions causes coagulation of sol.

Flocculation Value:

The minimum concentration of an electrolyte in millimoles per litre required to cause precipitation of sol in 2 hours is called flocculation value. The smaller the flocculation value, the higher will be the coagulation power of the ion.

Hardy- Schulze Rule:

Statement: (Part – I)

The ions of electrolyte having opposite charge with respect to as that of the sol particles are responsible for the coagulation.

Example:

For positively charged sol like Fe(OH)₃, the anions cause coagulation. Similarly, for negatively chargedsol like As₂S₃ the cations cause coagulation.

Statement: (Part – II)

Coagulating power of ions of electrolyte which cause coagulation increases with the valency (charge) of ions. More the valency of the effective ions the greater is its precipitating power.

Example :

Coagulating power of cations, coagulating any negative sol is in the order,  Al³⁺   >   Ba²⁺   > Na⁺

Coagulating power of anions, coagulating any +ve sol is in the order, PO₄^3-  > SO₄^2-  > Cl^–

Coagulation of Lyophilic Sols:

Stability of lyophilic sols is due to charge and solvation of colloidal particles. Hence if these two factors are removed their coagulation can be achieved. It can be done by adding electrolyte and by adding suitable solvent like alcohol or acetone.

Protective Colloids:

Lyophilic sols are more stable than the lyophobic sols. The stability of lyophilic sols is due to the solvation i.e. a protective layer is created around the colloidal particles by the dispersion medium in which they are dispersed.

If lyophilic sols are added to lyophobic sols they form a layer around the lyophobic particles and protect them from the action of an electrolyte. Thus the lyophilic sols on the addition to lyophobic sols give extra stability to the lyophobic sol. Hence, in this case, lyophilic sols are called protective colloids.

The power of protection of lyophilic sols is measured in terms of Gold number. The number of milligrams of lyophilic colloid that will just prevent the precipitation of 10 ml of gold sol on the addition of 1 ml of 10% sodium chloride solution is a gold number. This number was introduced by Zsigmondy.

The gold numbers of some commonly used protective colloid are given below.

• Higher is the gold number, lower will be the protective power.
• Gelatin is added in the preparation of ice cream as a protective colloid for colloidal particles of ice.
• Argyrol is used as eye drops is a silver sol protected by organic material.
• Proteins in blood like haemoglobin protect the blood from coagulation due to electrolytes.
• Source of mil are human milk, cow milk, and buffalo milk. But human milk is considered the best because it is better protected than other types of milk.

Ageing:

It is a spontaneous process of destabilization in colloids by which the dispersed phase get separated from the dispersion medium on its own. (i.e. no artificial methods are used).

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The colloidal particles carry an electric charge. The most important property of colloidal solution is that all suspended particles possess either positive or a negative charge. i.e. they carry the same nature of the charge. The mutual forces of repulsion between similarly charged particles prevent them from aggregating and settling under the action of gravity.  This gives stability to the sol.

The dispersion medium carries the opposite charge, hence as a whole, the colloidal solution is electrically neutral.

Origin of the Charge on Colloidal Particles:

A small quantity of electrolyte is always present in the colloidal dispersion. Its presence is necessary for the stability of the sol, as complete removal of the sol causes coagulation of the sol.

Presence of Some Acidic or Basic Groups in Colloidal Solution: 

Colloidal particles may acquire electric charge due to the presence of certain acidic or basic groups in colloidal solution.

For example, protein molecules give rise to the formation of colloidal solutions. Thus the particles of protein sol either have a positive charge or a negative charge depending upon the pH of the medium. A molecule of protein contains a carboxylic acid (COOH) group and also a basic amino (–NH₂) group, it will form a positively charged particle in the acidic medium and a negatively charged particle in the basic or alkaline medium.

In the case of the colloidal solution of proteins, the nature of charge on colloidal particles depends on the pH of the solution. The isoelectric point of a colloid is a pH at which net charge on colloidal particles is zero. Above this pH, the particles are negatively charged and below this pH, particles are positively charged. At isoelectric point, the particles exist in the form of Zwitter ion. Hence they do not migrate under the influence of the electric field.

The isoelectric pH for some proteins is Haemoglobin (pH 4.3-5.3), Casein from human milk (pH 4.1 –  4.7), and Gelatin (pH 4.7).

Due to Self Dissociation:

The dissociation of surface molecules leads to electric charge on colloidal particles of the sol.

For example, Consider an aqueous solution of soap (sodium palmitate) which undergoes dissociation into ions.

The cations (Na⁺) pass into the solvent. Due to the weak attractive forces present in the long hydrocarbon chains, the anions (C₁₅H₃₁ COO^–) have a tendency to form negatively charged aggregates of colloidal dimensions. This type of development of charge is only possible with electrolytes. This is not possible in colloidal solutions of non-electrolytes such as clay, smoke etc.

Electron Capture by Colloidal Particles: 

It is believed that the colloidal solutions prepared by Bredig’s Arc Dispersion Method acquire a charge by electron capture.

Selective or Preferential Adsorption of Ions:

When two or more ions are present in the dispersion medium, then the colloidal particles adsorb preferentially positive or negative ions present in the dispersion medium.

Example: Positively charged Ferric hydroxide sol: 

If FeCl₃ solution is added to the Ferric hydroxide, preferentially Fe³⁺ ions adsorb on Fe(OH)₃ molecules, that is why colloidal particles of ferric hydroxide are positively charged.

FeCl₃ ⇌ Fe³⁺ + 3 Cl^–

Fe(OH)₃ + Fe³⁺ → Fe(OH)₃ / Fe³⁺

Example: Positive charged Siver iodide sol:

When dilute KI is added in excess dilute AgNO₃, the Ag⁺ ions are adsorbed on Agl, and [AgI]Ag⁺ is formed. Thus the colloidal particles have a positive charge.

AgNO₃ + KI  → AgI + KNO₃

AgNO₃ ⇌ Ag⁺ + NO₃^–

AgI + Ag⁺ → AgI / Ag⁺

Example: Negative charged Siver iodide sol: 

When dilute AgNO₃ is added in excess dilute KI, the I^– ions are adsorbed on Agl, and [AgI]I^– is formed. Thus colloidal particles have a negative charge.

AgNO₃ + KI  → AgI + KNO₃

KI ⇌ K⁺ + I^–

AgI + Ag⁺ → AgI / I^–

Example: Negatively charged Arsenious Sulphide Sol: 

It is prepared by passing H₂S gas slowly through the solution of AS₂O₃.

AgNO₃ + KI  → AgI + KNO₃

H2S ⇌ 2H⁺ + S^2-

AS₂S₃ + S^2- → AS₂S₃ / S^2-

Frictional Electrification:

The origin of charge on the colloidal particles may be due to frictional electrification. By mutual rubbing of colloidal particles with molecules of the dispersion medium, the charge is developed on the sol. This view is not satisfactory.

Electrical Double Layer:

Colloidal particles are charged. This charge on the particle is balanced by an opposite charge in the dispersion medium. The charge in the fluid dispersion medium is in the form of free ions. There is a region around each colloidal particle where the charge on particle attracts the free ions from the dispersion medium to form an electrical cloud around it and is called the electrical double layer (Helmholtz electrical double layer).

An electric double layer consists of three regions

• Surface charge – charged ions adsorbed on the particle surface.
• Stern layer – counterions (charged opposite to the surface charge) attracted to the particle surface and closely attached to it by the electrostatic force.
• Diffuse layer – a film of the dispersion medium (solvent) adjacent to the particle. The diffuse layer contains free ions with a higher concentration of the counterions. The ions of the diffuse layer are affected by the electrostatic force of the charged particle. The boundary of this layer is called the slipping plane (shear plane).

The value of the electric potential at the slipping plane is called Zeta potential. The zeta potential is an important parameter for a colloid. Zeta potential depends on the properties of the colloid. For example, adding salt to a colloid shrinks the electrical double layer, and reduces the zeta potential. Zeta potential is given by the relation

Where, ξ = zeta potential

η = coefficient of viscosity

u = velocity of colloidal particles

D = Dielectric constant of the medium = K

Zeta potential and particle size are key indicators of the way colloids behave both in storage and in use. Zeta potential influences the effective size of the particles in the colloid.

Importance of Charge on Colloidal Particles:

• The mutual forces of repulsion between similarly charged colloidal particles prevent them from aggregating and settling under the action of gravity.
• This gives stability to the sol. In the case of lyophobic sols, charge on colloidal particles is fully responsible for its stability.
• In the case of a lyophilic sol, the stability is due to the charge on colloidal particles and solvation.

Electrophoresis or Cataphoresis:

The unidirectional migration of sol particles or dispersed phase particles or colloidal particles towards the oppositely charged electrode under the influence of the applied electric field is called electrophoresis or cataphoresis.

Cause of Electrophoresis:

All sol particle (colloidal particles) carry the same electric charge either positive or negative.  If an electric potential is applied across two platinum electrodes dipping in a sol, the sol particles move towards oppositely charged electrodes.

Illustration:

Consider a sol of As₂S₃ is taken in a ‘U’ shaped glass tube. The sol particles of the sol are negatively charged. Now the dispersion medium with little quantity of electrolyte is introduced over the colloidal solution. There should be a sharp boundary between the sol and the dispersion medium.

Electric potential is applied across the two platinum electrodes dipped in a sol in two limbs, it is observed that the level of sol drops at the negative electrode and rises at the positive electrode side. This shows that sol particles have migrated to the positive electrode, indicating that the particles are negatively charged.

If colloidal particles are allowed to reach the electrode, their charges are neutralised and coagulation takes place.

Applications of Electrophoresis:

• Electrophoresis is used to detect the nature of charge on colloidal particles.
• It is used in the removal of carbon particles from chimney gases.
• It is used in electro-deposition of rubber on metal, wood or cloth surfaces from latex.
• It is used to bring about coagulation of sol.

Electro-osmosis:

The migration of the dispersion medium of a colloidal solution under the influence of the electric field when the movements of colloidal particles are prevented is called as electro-osmosis.

Cause of Electro-osmosis:

Since the sol as a whole is electrically neutral, dispersion medium has an opposite electric charge as compared with that of the sol particles. If the dispersed phase has a positive charge we say that the dispersion medium has a negative charge.

Illustration:

A sol of As₂S₃ is filled in a glass tube. The sol particles of the sol are negatively charged. Hence the dispersion medium (water) is positively charged. The colloidal solution and pure dispersion medium in a glass tube are separated by a semipermeable membrane.

When an electric potential is applied across the platinum electrodes dipping in each arm, sol particles cannot pass through the semipermeable membrane but dispersion medium (water) move to the negative electrode through the semipermeable membrane.  The level of sol drops at the +ve electrode and rises at -ve electrode. This movement of dispersion medium towards -ve electrode shows that the charge on the dispersion medium is positive.

Applications of Electro-osmosis:

• Electro-osmosis is used in dewatering of moist clay
• It is used in the drying of dye-pastes
• It is used in the removal of water from peat.

Applications of Electrical Properties of Colloids

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.

Science > Chemistry > Colloids > Charge on Colloidal Particles

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In this article, we shall study the general, mechanical and optical properties of colloids.

General Properties of Colloids:

Heterogenous Character:

The ultramicroscopic examination indicates that colloidal dispersion is a heterogeneous system consisting of a continuous dispersion medium and discontinuous disperse phase.

Visibility:

Colloidal particles cannot be seen through naked eyes or ordinary microscope due to their very small size. The shortest wavelength in visible spectra is about 4000 Å. Hence we cannot see any object less than 200μm and colloidal particles have sizes less than 200μm.

Recently new techniques like Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), and Scanning Transmission Electron Microscope (STEM) are used to determine the size and shape of colloidal particles.

Filterability:

The colloidal particles readily pass through ordinary filter paper. The range of particle size of colloidal substance is in between 5 × 10^-9 m to 2 × 10^-7 m. The pore size of ordinary filter paper is bigger i.e. of order 10^-7 m. So Colloidal particles can pass through it and thus filter paper can be used to separate colloidal particles from coarse suspension.

Sols and true solutions pass through filter paper. The colloids cannot pass (diffuse) through parchment membrane but crystalloids can pass through parchment membrane. The process of separating colloids from other dissolved substance using parchment membrane as the filter is called dialysis. This process is used for purification of colloids.

When the impure sol is placed in specially created ultrafilter, with small pores, the sol particles being bigger than the pores remain behind while dispersion medium and dissolved electrolyte pass through. This process is known as ultra-purification.

Surface Tension and Viscosity:

Lyophilic sols have a higher viscosity and lower surface tension than dispersion medium and lyophobic sols have a nearly same viscosity and surface tension as the dispersion medium.

Molecular Mass:

Colloidal particles are of two types i) Multimolecular and ii) Macromolecular

• Multimolecular colloidal particles are aggregates of a number of small molecules or atoms. e.g. Sulphur sol, gold sol.
• Macromolecular particles are very big molecules or polymers. e.g. Starch, proteins.

As the colloidal particles are aggregates of a number of molecules or a large molecule themselves their molecular mass is very high.

Colligative Properties:

Colloidal particles are bigger aggregates. The colligative properties depend on the number of particles. Due to less number of particles compared to true solution colligative properties are lower. Hence the values of colligative properties like osmotic pressure., depression in freezing point and elevation in boiling points are of small order compared to the values shown by true solutions at the same concentration.

Colour:

Many sols are coloured. Sol particles are able to scatter light rays. Colour of the sol depends upon the wavelength of scattered light by the sol particles and which again depends on the size of the sol particles. The colour of colloidal solution also changes with the way the observer receives the light.

Let us consider silver sol (colloidal solution of the same substance) having different types of particles. It is found that the sols show different colours. 

Notes:

• The blue sky is due to the blue light scattered by small dust particles in the atmosphere. The atmosphere is a colloidal system consisting of dust particles suspended in air.
• The red sky is due to red light scattered by larger dust particles in the atmosphere.
• Depending on the size of the dust and water particles, different colours are seen in the cloud.
• Fine gold sol is red but as particle size increases it becomes blue or purple.

Mechanical Properties of Colloids:

Brownian Movement:

The English Botanist Robert Brown, in 1927 observed that colloidal particles exhibit continuous random motion in all directions in a straight line.  He found such movement when pollen grains were suspended in water. The phenomenon of continuous zig-zag movement of colloidal particles in straight line paths in a random direction is known as a Brownian movement.

Explanation:

Colloidal particles are surrounded by a large number of dispersion medium molecules which constantly bombard the colloidal particles. On unequal bombardment, the colloidal particles get pushed in certain directions. Since colloidal particles possess like charge, they repel each other.

Factors Affecting Brownian Movement:

• Brownian movement depends on the viscosity of the dispersion medium. Brownian movement is more in less viscous solution.
• Brownian movement depends on the size of the particle. If the particles are of smaller size. The Brownian movement is more rapid.

Applications Brownian Movement:

• Due to the Brownian movement colloidal particles hardly settle down and prevent aggregation of colloidal particles. Thus colloidal solution becomes stable.
• Avogadro’s number can be calculated by Brownian movement.  

Optical Properties of Colloids:

Tyndall effect:

When an intense beam of light is passed through the colloidal solution (taken in a glass vessel) placed in a dark the path of light through the colloidal solution is clearly visible due to the scattering of light by sol particles.  This effect is known as Tyndall effect.

This fact was first noted in 1857 by Faraday and then studied in  details by Tyndall in 1868. True solutions do not exhibit Tyndall effect.

The emitted light emerges in the form of a bright cone called Tyndall cone. Through ultramicroscope, each colloidal particle appears a bright point against the dark background, due to the scattering. Thus the colloidal particles become self-luminous. As a result, the path of the beam of light through colloidal solution becomes clearly visible. The nature of scattering depends on the size of the sol particle and the refractive indices of sol particle.

Explanation:

Colloidal particles are not large enough like suspension particles to reflect the light nor they are small enough, like true solution particles to allow the light to pass through them. Due to the intermediate size of colloidal particles, they scatter part of the absorbed light, from their surfaces in all directions. Thus the cause of Tyndall effect is a scattering of light by colloidal particles.

Conditions to be Satisfied for Viewing Tyndall Effect:

• The diameter of the dispersed particle is not much smaller than the wavelength of light used.
• There should be a large difference between the magnitudes of refractive indices of the dispersed phase and the dispersion medium.

Applications of Tyndall Effect:

• Tyndall effect is useful to distinguish colloidal solution from the true solution
• To test the purity of gases in the manufacture of H₂SO₄ by the contact process.
• Count the number of colloidal particles in colloidal sols using ultra-microscope.

Science > Chemistry > Colloids > Properties of Colloids

The post Properties of Colloids appeared first on The Fact Factor.

In this article, we shall study different methods of preparation of colloids.

Preparation of Lyophilic Sols:

For preparing lyophilic sol, the dispersed phase is directly added to dispersion medium in cold or by warming.

Colloidal solutions of starch, glue, gelatin, etc. in water can be prepared by this method. Solutions of colloidal electrolytes such as soaps and dyestuffs can also be prepared by this method.

Preparation of Lyophobic Sols:

For preparing lyophobic sol, the substance in bulk is broken down into particles of colloidal dimensions (Dispersion) or aggregating smaller particles into particles of colloidal dimensions (condensation). To improve the stability of sol certain substances are added to the sol, the substances added are called stabilizers.

Dispersion methods:

In the dispersion method particle of larger size are broken down to the colloidal size in the dispersion medium. Starting with the material in massive form, a colloidal solution is prepared by using suitable devices to disintegrate it into particles of colloidal size. Normally this is carried out by physical methods.

Mechanical dispersion method :

The substance which is to be dispersed is finely ground. It is then mixed with the dispersion medium, protective materials or stabilizer is also added when a coarse suspension is obtained. This suspension is then passed through a colloid mill.  A colloid mill consists of two heavy metal discs placed one above the other separated by a very small gap from each other.  They are rotated in the opposite directions at a very high speed of about 7000 r.p.m. The sol results due to the large shearing effect. Protective material used prevents particles from coagulation.

Using this method sols of indigo, sulphur, toothpaste, printer ink, paints, ointments etc. are prepared.

Electrical Dispersion or Bredig’s Arc Method:

This method used to prepared metal sols like platinum, silver, gold, copper in water. A dispersion medium (conductivity water) and a trace of sodium hydroxide (the stabilising agent) is taken in porcelain or glass (non conducting) vessel. The vessel containing dispersion medium is surrounded by a freezing mixture. Metal to be dispersed is dipped in the vessel in the form of electrodes. Electrodes are connected to the high voltage source. The ends of electrodes in the dispersion medium are very near to each other. A very high voltage is applied and then an electrical arc is struck between the tips of electrodes. This creates large heat due to which metal rods melt, evaporate and suddenly cooled due to freezing mixture gives rise to the colloidal solution of the metal.

Functions of the freezing mixtures are

• Freezing mixture helps in condensation of metal vapours forming the colloids
• It prevents vapourisation of water.
• It prevents coagulation of colloids, by keeping sol cold.

Peptization or Chemical Dispersion:

Redispersion of freshly prepared precipitate into the sol by adding an electrolyte containing common ion is called as peptization. An electrolyte used for peptization is called as the peptizing agent. Peptization is a reverse process of coagulation. The peptization action is due to the preferential adsorption of one of the ions of the electrolyte on the precipitate.

Example – 1: 

Freshly prepared Fe(OH)₃ precipitate when treated with dilute solution of FeCl₃, reddish brown ferric hydroxide sol is formed (Fe³⁺ being common ion). In this case, FeCl₃ is the peptizing agent.

Fe (OH)₃ +   FeCl₃ →  [Fe(OH)₃] Fe³⁺

Example – 2: 

Fresh Silver chloride precipitate when treated with a small amount of dilute HCl, a silver chloride sol is formed.

Example – 3:

Cadmium sulphate can be peptized with the help of hydrogen sulphate.

Ultrasonic Dispersion:

Ultrasound is a very effective processing method in the generation and application of colloidal size particles. High-intensity ultrasonic waves are used for this purpose. During sonicating of liquids the ultrasonic waves that propagate through dispersion medium result in alternating high-pressure (compression) and low-pressure (rarefaction) cycles. This mechanical stress causes Ultrasonic cavitation in liquids. It creates high-speed liquid jets of up to 1000km/h. Such jets press liquid at high pressure between the particles and separate them from each other. Smaller particles are accelerated with the liquid jets and collide at high speeds.

Various substances like oils, mercury, sulphur, sulphides and oxides of metal can be dispersed in a colloidal state by this method.

Condensation methods:

These methods involve chemical reactions. In these methods factors like temperature, pressure, concentrations.  etc. are properly maintained.  The unwanted ions present in the sol are removed by dialysis , as these ions may eventually coagulate the sol.

Chemical Methods:

Oxidation Method:

Preparation of Colloidal Sulphur: 

When H₂S in water (aqueous solution) is exposed to air, it slowly gets oxidised to sulphur. The sulphur so formed remains in water in the colloidal state and the solution so formed remains in water in the colloidal state and the solution has a slightly milkish appearance.

H₂S   +     O2  →   H₂O  +    2S (colloidal)

A sol of sulphur can also be prepared when H₂S gas is bubbled through an aqueous solution of SO₂.

H₂S  +     SO₂ →  2 H₂O   +  3S (colloidal)

Reduction Method:

Preparation of Gold Sol: 

A number of metals like silver, gold, platinum, mercury lead can be obtained in the colloidal state by the reduction of their salt solutions (dilute) using suitable reducing agents like hydrogen sulphide, formaldehyde, stannous chloride, tannic acid etc.

Gold sol can be obtained when AuCl₃(dil) solution is treated with stannous chloride.

2 AuCl₃ +  3 SnCl₂ → 3 SnCl₄ +  2 Au (colloidal)

Similarly, silver, platinum mercury sols are prepared.

AgNO₃  +  Tannic acid  →       Ag sol

AuCl₃  +  Tannic acid  →       Au sol

Hydrolysis Method:

Preparation of Ferric Hydroxide Sol:

A colloidal solution of ferric hydroxide is obtained by boiling a dilute solution of ferric chloride.

FeCl₃ + 3H₂O →  Fe(OH)₃   + 3 HCl

Preparation of Silicic Acid Sol:

By hydrolysis of a dilute solution of sodium silicate with a hydrochloric acid, the colloidal solution of silicic acid is obtained.

Double Decomposition Method:

Preparation of Arsenious Sulphide Sol: 

Arsenious sulphide, As₂S₃ is a lyophobic colloid. It is obtained by the hydrolysis of arsenious oxide (AS₂0₃) with boiling distilled water, followed by passing H₂S gas through solution obtained. In the colloidal solution of arsenious sulphide, each particle is surrounded by HS- ions, produced by the dissociation of H₂S. This sulphide ion layer is further surrounded by the counter ion layer of H⁺ ions.

As₂O₃  + 3 H₂O  →     2As(OH)₃  (boiling)

2 As(OH)₃ + 3H₂S   →   As₂S₃  + 6H₂O

(light yellow sol)

By Exchange Solvent Method:

There are a number of substances whose colloidal solutions can be prepared by taking a solution of the substance in one solvent and pouring it into another solvent in which the substance is relatively less soluble.

Preparation of Sulphur or Phosphorous Sol:

If a solution of sulphur or phosphorus prepared in alcohol is poured into water, a colloidal solution of sulphur or phosphorus is obtained due to the low solubility of sulphur or phosphorus in water.

By change of physical state: 

A colloidal solution of certain elements such as mercury and sulphur are obtained by passing their vapours through cold water containing a stabilizer ( an ammonium salt or a citrate).

Excessive Cooling Method:

A colloidal solution of ice in an organic solvent like ether or chloroform can be prepared by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution, separately combine to form particles of colloidal size.

Purification of Colloidal Solution

Dialysis:

The process of separating the particles of colloid from those of crystalloid, by means of diffusion through a suitable membrane (animal membrane or parchment paper) is called dialysis. The apparatus used for the performing dialysis is called dialyser.

Principle:

The colloidal particles can not pass through a parchment or cellophane membrane while the ions of the electrolyte (crystalloids) can pass through it.

Process:

A bag made up of suitable semipermeable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flown. The molecules and ions of crystalloids diffuse through the membrane into the water and are washed away. Thus the sol in the bag is purified.

Dialysis can be used for removing HCl from the ferric hydroxide sol.

Electrodialysis:

The ordinary process of dialysis is slow. (ii) To increase the process of purification, the dialysis is carried out by applying an electric field. This process is called electrodialysis. The ions present in the colloidal solution migrate towards oppositely charged electrodes.

The important application of electrodialysis process in the artificial kidney machine used for the purification of the blood of the patients whose kidneys have failed to work.

Ultrafiltration:

The pores of ordinary filter paper are large, hence colloidal particles pass through them easily. If the pores of the ordinary filter paper are made smaller by soaking the filter paper in a solution of gelatin of colloidion (it is a mixture of 4% nitro-cellulose in alcohol and ether) and subsequently hardened by soaking in formaldehyde

The treated filter paper may retain colloidal particles and allow the true solution particles to escape. Such filter paper is known as ultrafilter and the process of separating colloids by using ultrafilters is known as ultrafiltration.

The colloidal particles left on ultrafilter paper are then washed with a fresh dispersion medium to get a pure colloidal solution.

Ultracentrifugation:

In this method, the colloidal solution is placed in a high-speed centrifugal machine. On centrifuging, the colloidal particles settle down. The impurities remain in the centrifugate and are removed. The settled colloidal particles are mixed with the dispersion medium to form the colloidal solution again.

Science > Chemistry > Colloids > Preparation of Colloids

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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.

Science > Chemistry > Colloids > Associated Colloids (Micelles)

The post Associated Colloids (Micelles) appeared first on The Fact Factor.

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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Depending upon the size of particles in solution, solutions are classified as true solutions, colloidal solutions, and suspensions. In this article, we shall study colloids i.e. colloidal solutions.

Historical Background:

The foundation of colloidal chemistry was laid by English chemist Thomas Graham in 1861. Other prominent chemists contributed to this field are Tyndall, Hardy, Zsigmondy, N. R. Dhar, S.S. Bhatnagar. Thomas Graham classified the soluble substances into two categories. namely, a) crystalloids and b) colloids

Crystalloids:

Many crystalline substances like common salt, sugar, urea when dissolved in water can pass through parchment membrane are termed as crystalloids and their solution with water is called as a true solution. In true solution particle size of solute is very small. The diameter of the particles is about 1 × 10^-9 m. They cannot be seen by any instrument.

Colloids:

The substances like gum, glue which when dissolved in water do not pass through a parchment membrane.  A solution of colloidal substance with water is known as a colloidal solution. In colloidal solution particle size varies from 5 × 10^-9 m to 2 × 10^-7 m. in diameter. Colloidal particles can be seen clearly under a high-resolution microscope or ultramicroscope. Any substance can be converted into a colloidal state by suitable means.

Types of Solutions on the Basis of Size of Particles:

True Solution:

Crystalline substances like sugar, sodium chloride are called crystalloids and are soluble in water. The aqueous solution of crystalloids is called a true solution.

Characteristics of True Solutions:

• It is a homogeneous solution as the particles are not seen even under ultramicroscope.
• The solution is clear and transparent.
• The solution is a single phase, consisting of solute (sugar, sodium chloride) and solvent (water)
• Size of solute particles is 1 × 10^-9 m. Therefore they can pass through both the parchment membrane as well as through filter paper.
• The particles can be neutral molecules as in the case of sugar or contain cations (Na⁺) and anions (Cl^–) as in case of NaCl.
• The true solution does not exhibit the Tyndall effect. When a powerful beam of light is passed through a true solution kept in a dark, the path of the beam of light through the true solution is invisible.
• A true solution does not exhibit Brownian movement. The zig-zag motion of particles like pollen grains in a random direction in straight lines is called Brownian movement.
• Particles of solute never settle down under gravity.
• They are not visible by any optical means.
• They can diffuse rapidly.

Colloidal Solution or Colloidal Dispersion:

The word colloids is derived from the Greek word Kolla (glue) and oid (like). Thus colloid means glue-like. When a thin paste of amorphous substances like glue, gum or starch etc. is added to boiling water, taken in a beaker, with continuous stirring a solution obtained is called as a colloidal solution or colloidal dispersion.

Characteristics of Colloidal Solutions:

• It is a heterogeneous solution consisting of two immiscible phases. They are visible under ultramicroscope.
• The solution is turbid.
• There are two phases present in the solution. One phase is present in the form of small particles, dispersed in the medium is called as the dispersed phase or discontinuous phase. The other phase in which particles are dispersed is called as dispersion medium or continuous phase.
• The particle size of the dispersed phase (diameter) is in the range of 5 × 10^-9 m to 2 × 10^-7 m. Hence they can pass through filter paper but not through parchment membrane.
• All colloidal particles in solution carry the same charge either positive or negative. Due to same nature of charge on all colloidal particles in the solution, the particles repel each other and thus impart stability to the solution.
• Colloidal solutions exhibit the Tyndall effect
• Colloidal solutions exhibit Brownian movement
• Particles do not settle under gravity.
• They diffuse slowly.

Suspension:

When sand is stirred into water, the solution obtained is called a suspension.

Characteristics of Suspension:

• It is a heterogeneous solution consisting of two immiscible phases. They are visible to the naked eye.
• The solution is turbid.
• There are two phases present in the solution. One phase is present in the form of small particles. Dispersed in the medium is called as the dispersed phase or discontinuous phase. The other phase in which particles are dispersed is called as dispersion medium or continuous phase.
• The dispersed particles are aggregates of millions of molecules.  The diameter of the particles are aggregates of millions of molecules.  The diameter of particles is greater than 10 -7 m. Hence they can not pass through filter paper and not through a parchment membrane.
• Particles do not carry any charge.
• Suspensions do not exhibit Tyndall effect
• Suspensions do not exhibit Brownian movement
• Particles settle under gravity.
• They do not diffuse.

Note:

The colloidal state is intermediate between crystalloids and suspensions. The colloidal solution is a solution of colloidal substance.  The colloidal solution is consists of two phases.

The Terminology of Colloidal Solutions:

Colloidal State:

The colloidal state is a heterogeneous dispersion of two immiscible phases which possess certain distinguishing characteristics.

Colloidal Solution:

The colloidal solution is a heterogeneous system consisting of two phases.

Dispersed Phase:

The colloidal substance which is dispersed in a solvent is called as dispersed phase or inner phase or internal phase or discontinuous phase.

e.g. In smoke, carbon particles are dispersed phase.

Dispersion Medium :

The medium in which the colloidal substance is dispersed is known as dispersion medium or outer phase or external phase or continuous phase.

e.g. In starch solution, water is a dispersion medium. another e.g. for a copper sol, copper particles constitute dispersed phase and water the dispersion medium.

Applications of Colloids:

Foods:

• Majority of our food material are colloidal in nature.
• Emulsion -Milk, Cod Liver oil, Buttermilk
• Gels – Jelly, Curd, Butter, Cheese,
• Juicy fruits like mango, apple,
• Badami halva.
• Solid foam – Biscuits, Cake, Bread, Dryfruits
• Foam – Whipped cream

Medicines:

• A number of medicines are colloidal. Colloidal medicines are easily assimilated by body tissues.
• Argyrol and protargol are protected colloidal solutions of silver is used against granulation
• Colloidal gold, calcium injections are used to raise the vitality of the human physiological system against diseases like T.B. and rickets.
• Colloidal sulphur is used as germs killer in plants
• Colloidal antimony is used in curing kala-azar
• Milk of magnesia is used in the treatment of stomach acidity.

Industrial Goods:

• Many industrial goods in our daily life are colloids. e.g. soaps, toothpaste, gum, shoe polish, enamels, resins, leather, paints, varnishes, cosmetics, etc.

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