Thursday, August 22, 2013

Application of Colloids

Some important applications of colloidal are as follows:-
1.      Rubber Plating
                  The negatively charged rubber particles from rubber sol are deposited on wares handles of different tools, rubber gloves, etc by electroplating.
2.      Sewage Disposal
                  Sewage water contains particles of dirt, rubbish etc which are of colloidal size carry and charge and therefore do not settle down easily. The particles can be removed by electrophoresis. Dirty water is passed through a tunnel fitted with metallic electrode which is maintained at high potential difference. The particles migrate to the oppositely charged electrode, lose their charge and are coagulated. The deposited matter is used as a manure and water left behind is used for irrigation.
3.      Cottrel Smoke Precipitator
                  Smoke is made free of colloidal particles by passing it through cottrel’s precipitator installed in the chimney of an industrial plant. It consists of two metal discs charged to a high potential. The charge on colloidal dust and carbon particles of smoke is neutralised and they are precipitated down while free gases come out through chimney.

4.      Formation of Delta
                  River water is a sort of colloidal solution of clay in water. Sea water on other hand, contains large number of dissolved salts and is a sort of electrolyte. When river water comes in contact with sea water, coagulation of colloidal particles occurs. These coagulated clay particles settle down at a point of contact and gradually river bed starts rising. This lead river water to adopt different course and ultimately results in the formation of delta.

5.      Leather Industry
                  In leather industry, the tanning of leather is based on the mutual coagulation of oppositely charged colloids. The animal skin is positively charged colloidal system containing albumin and geletin in the colloidal state. When treated with negatively charged colloidal system of tanin, present in barks of trees, wood leaves etc, a mutual coagulation of two oppositely charged sols take place. As a result, the surface of leather becomes harder.
6.      Dyeing Industry
                  Dyeing is essentially an adsorption of dye by the colloidal particles of mordant used. The clothes like those of cotton, wool, silk etc are colloidal in nature and Mordants used are also colloidal in nature. The mordants like Al(OH)3, SnCl2, tanin etc are readily adsorbed by fibres. This is another typical case of colloidal state of matter.
7.      Artificial Rain
                  Artificial rain is caused by spraying oppositely charged colloidal dust or sand particles over the clouds by aeroplane. The colloidal water particles of cloud gets neutralised and coagulation causes artificial rain.
8.      Purification of Water
                  The colloidal particles present in water can be precipitated by addition of small amount of alum (K2SO4 . Al2 (SO4)3 . 24 H2O). The Al3+ ions furnished by alum helps coagulation of impurities which are present in colloidal form.
9.      Distinction of Natural from Artificial Honey
                  Natural honey is also colloidal in nature. The distinction between two is made by  Ley’s Test. It consists of treating few drops of honey with ammonical AgNO3 solution. In case of pure honey, metallic silver produced assumes reddish yellow colour due to the traces of Albumin or ethereal oils, which acts as protective colloids. The dark yellow or greenish precipitate is formed with artificial honey.

Micelles

Certain substances like soap and detergents behave as electrolytes at low concentration (the molecules of soap or detergent are smaller than colloidal particles). However, at high concentrations they constitute colloidal sol, which is known as Associated Colloids. The colloidal behaviour of such substances is due to formation of aggregates or clusters in solutions. Such clusters are of colloidal dimensions and are known as Micelles. Thus, micelles may be defined as the cluster or aggregated particles formed by associated colloids in solutions. The formation of micelles takes place above certain concentration called Critical Micellisation Concentration (CMC). Every micelle system has specific value of CMC.
Mechanism of Micelle Formation
Micelles are generally formed by the specific type of molecules which have lyophilic as well as lyophobic ends. Such molecules are known as Surface Active Molecules or Surfactant Molecules. Sodium oleate, C17 H33 COO Na+  is an example of such molecules.

The long hydrocarbon part of oleate radical (C17H-33) is lyophobic end while COO is lyophilic end. When the concentration of the solution is below CMC (3 x 10–3 Mol L–1) sodium oleate behaves as an electrolyte and ionises to Na+ and C17 H33 COO ions. As the concentration exceeds CMC, the lyophobic part starts moving away from solvent (it is solvent repelling) and are made to approach each other. However the polar COO part tends to interact with water (solvent). This ultimately lead to formation of cluster having dimensions of the colloidal particles. In each such cluster a large number of oleate groups (usually 100 or more) are clumped together in a spherical manner so that their hydrocarbon part interact with one another to reduce contact with solvent but COO part remains projected in water.

Wednesday, August 21, 2013

Emulsions

“An Emulsion is a colloidal solution in which both dispersion medium as well as dispersed phase are liquids. (The liquids should be Immiscible)”.
Preparation
Emulsions prepared by mere shaking of two liquids or by passing the mixture through a colloidal mill, called the Homogenizer, are unstable and tend to separate into two layers on standing. Thus, in order to get a stable emulsion, small quantities of certain other substances are added during preparation. The substances thus added to stabilize the emulsions are called Emulsifiers or Emulsifying Agents. The commonly used emulsifying agents are soaps, proteins, long chain sulphonic acids, basic sulphates of Fe, Cu, Ni etc.

Role of Emulsifier
                  The role of emulsifier can be explained by taking the example of soap as an Emulsifier. Soaps are sodium or potassium salts of higher fatty acids. A molecule of soap consists of two parts, the long hydrocarbon part (R–) which is oil soluble and the polar group COO- Na+ which is water soluble. Thus when oil and water mixture in presence of soap is shaken vigrously, the oil drop is surrounded by soap solution, the R – part of soap remains in oil and COO- Na+ part remains in water. Thus soap molecules are concentrated over the surface of oil drop. As a result interfacial tension between water and oil decreases and hence they do not coagulate or separate.

Types of Emulsion
Emulsion is colloidal solution of two immiscible liquids. Generally, one of the two liquids is water and the other which is immiscible with water, is designated as oil. Either of two liquids can constitute the dispersed phase. Thus there are two types of Emulsions.
i).      Oil-in-water type (o/w type)
ii).     Water-in-oil type (w/o type)
         i).      Oil-In-Water Emulsions
In this type of emulsions, oil acts as disperse phase and water acts as dispersion medium. Some examples of this type of emulsion are milk, vanishing cream etc.
         ii).     Water-In-Oil Emulsions
In this type of emulsions, water acts as dispersed phase and oil acts as dispersion medium. For example, butter, cod liver oil, cold cream etc.


Identification of Emulsions
The following tests may be employed to distinguish between two types of emulsions;
(i).     Dye Test 
To the emulsion some oil soluble dye is added. If the background becomes coloured, the emulsion is water-in-oil type and if the droplets becomes coloured the emulsion is oil-in-water type.
(ii).    Dilution Test
If the emulsion can be diluted with water, this indicates that water is dispersion medium and the emulsion is of oil-in-water type. In case the water added forms a separate layer than emulsion is water-in-oil type.

Applications of Emulsions
Some important applications of emulsions are as follows:-
(1).    Cleansing Action of Soap
Washing action of soap is due to emulsification of grease, dirt and taking it away in water.
(2).    Digestion of Fats
Digestion of fats in the intestines is aided by emulsification. A little of fat forms a sodium soap with alkaline solution in the intestines and this soap emulsifies the rest of fat. It is easier for digestive enzymes to carry out their functions on emulsified fat.
(3).    In Medicines
A wide variety of pharmaceutical preparations are emulsions. For example: cod liver oil, disinfectant phenyl results in emulsion when poured in water.
(4).    In Metallurgical Operations
Emulsions play an important role in industry. The metal ores are concentrated with froth-floatation process which involves the treatment of the pulversied ore in emulsion of pine oil.

Gold Number

The lyophilic colloids differ widely in their power of protection. The protective action of different colloids is measured in terms of the Gold number introduced by Zsigmondy. The gold number is defined as:
“The number of milligrams of a lyophilic colloid that will just prevent the precipitation of 10ml of standard gold sol (containing 0.5 to 0.6gm of gold per litre) when 1 ml of 10% sodium chloride solution is added”.
         The precipitation of gold sol is indicated by a colour change from red to blue. (When the particle size increases colour changes). The smaller the gold number of lyophilic colloid, the greater is its protecting power. Geletin has small gold number and is an effective protective colloid.

The gold number of few protective colloids is as follows:

Geletine = 0.005 – 0.01
Haemoglobin = 0.03 – 0.07
Egg Albumin = 0.08 – 0.10
Gum Arabic = 0.10 – 0.15
Potato Starch = 25
Casein = 0.01 – 0.02

In the preparation of ice cream, geletin is used as protecting agent to the colloidal particles of ice. If the ice particles coagulate, the smooth texture of ice cream is lost.

Protective Colloids

Lyophobic sols like those of metals (Au, Ag etc) are unstable and are easily coagulated by addition of electrolyte. However, it is observed that when certain lyophilic colloids such as gum, geletin, Agar – Agar etc. are added to a lyophobic sol, the stability of the lyophobic colloids is markedly increased i.e. the addition of small amount of electrolytes does not cause coagulation of lyophobic colloids. This action of lyophilic colloids to prevent the coagulation of lyophobic colloid by addition of electrolyte is called Protection of Colloidal Sol and the lyophilic colloid is called Protective Colloid.
Explanation
The Particles of protective colloid is believed to be adsorbed by lyophobic colloidal particles and thus forms a covering over the surface of lyophobic colloidal particles. The lyophobic colloid thus behaves as lyophilic colloid and is precipitated less easily by electrolytes.
It may be noted that protective colloid not only increases stability of lyophobic colloids but it also makes them reversible.

Tuesday, August 20, 2013

Hardy-Schultz Rule

     The coagulation of a colloidal solution by an electrolyte does not take place until the added electrolyte has certain minimum concentration in the solution. The minimum amount of an electrolyte (millimoles) that must be added to one litre of a colloidal solution so as to bring about complete coagulation or flocculation is called the Coagulation or Flocculation Value of the Electrolyte. Thus smaller is the flocculation value of electrolyte, greater is its coagulation or precipitating power.
Different electrolytes have different coagulation values. The coagulation behaviour of various electrolytes was studied by Hardy and Schultz. They gave a generalisation known as Hardy – Schultz Law, which states,        
“Greater the valency of oppositely charged ions of the electrolyte being added, the faster is coagulation”.    
So, for coagulation of sols carrying negative charge Al3+ ion is more effective than Ba2+ions or Na+ ions. Similarly for coagulation of positively charged sols ion is more effective than  or  ions.
Thus in case of positively charged sol the coagulating power of anions is in the order of
  and in case of negatively charged sols, the coagulating power of cations is in the order of  Al3+ > Ba2+ > Na

Coagulation or Flocculation of Colloidal Solutions

The colloidal sols are stable by the presence of electric charges on colloidal particles. Because of electric repulsion the particles do not come close to one another and hence do not aggregate. The removal of charge by any means will lead to the aggregation of particles and hence precipitates immediately.
The process by means of which the particles of the dispersed phase in a sol are precipitated is known as Coagulation or Flocculation.
The coagulation can be effected by different methods as follows:-
         1.      Heating or Cooling
                        In some cases, heating the sol results in coagulation. E.g. When egg is boiled, the albumin gets coagulated. Similarly, in some cases, cooling the sol results in the coagulation. E.g. Coagulation of milk i.e. on cooling milk, fat starts flowing on the surface.
2.      Mutual Action of Sols
                        When two sols carrying opposite charges are mixed together, their charges get neutralized and they get coagulated. Thus when negatively charged arsenic sulphide sol is added to the positively charged ferric hydroxide sol, in suitable proportions, coagulation of both sols take place simultaneously.
3.      Electrophoresis
When an electric current is passed through colloidal dispersion, particles migrate towards oppositely charged electrode and their charges get neutralized due to opposite charge on electrode. They thus get deposited on the electrode or are coagulated. Example: Rubber cloth can be prepared by electrophoresis of rubber latex.
4.      By Prolonged Dialysis
The stability of a colloidal sol is due to presence of small electrolyte. On prolonged dialysis the electrolyte is completely removed. As a result the colloidal sol becomes unstable and gets coagulated.
5.      Addition of Electrolyte
Presence of small concentration of appropriate electrolyte is necessary to stabilize the colloidal solutions. However, when large amount of electrolyte is added the colloidal particles get neutralized by the opposite charged ion of the electrolyte. Due to the neutralization of colloidal particles they aggregate and are coagulated.
For example: When aluminium sulphate or barium chloride is added to arsenic sulphide sol, it takes up barium ion or aluminium ion while the positively charged particles of Ferric hydroxide takes up chloride ion or sulphate ion, as the case may be. This causes neutralization of the charge on colloidal particles and lowers the stability of the sols, leading to coagulation.

Electrical Properties of Colloidal Solution (Electrophoresis and Electro-Osmosis)

(I).    Electrophoresis
                When a potential difference (electric field) is applied across two platinum electrodes immersed in a colloidal solution, the particles of dispersed phase move towards either the positive or negative electrode. This observation was first discovered by Rauss in 1807 and was investigated later by Linder and Picton.
The movement of colloidal particles under the action of electric field is known as Electrophoresis.
If the colloidal particles move towards the positive electrode (Anode) they carry negative charge. On the other hand if the sol particles migrate towards negative electrode (Cathode), they are positively charged. From the direction of movement of colloidal particles it is possible to find out the charge on colloidals.

Demonstration of Electrophoresis
                 The demonstration of electrophoresis is as follows:-
Take a colloidal sol say AS2S3 sol in a U – tube. Place an electrolyte, having density less than that of solution (say distilled water). The electrolyte provides distinct boundary between electrolyte and colloidal sol.
Place two platinum electrodes in two arms of U – tube such that they dip in the colloidal sol. When a high potential difference of about 100 volts is applied across the two platinum electrodes, it is observed that the level or Boundary of colloidal solution falls on the negative electrode side and rises up on positive electrode side. On reaching the positive electrode, the colloidal particles get discharged. As a result of neutralisation of charge, the colloidal particles aggregate and settle down at the bottom.

Electro-Osmosis
A colloidal solution as a whole is electrically neutral in nature i.e., dispersion medium carries an equal and opposite charge to that of the particles of dispersed phase. When the movement of dispersed phase of colloidal solution is prevented by suitable means, the dispersion medium can be made to move under the influence of an applied electric field or potential. This phenomenon is referred to as Electro-Osmosis. Thus electro-osmosis may be defined as the movement of the dispersion medium under the influence of an applied electric field when the particles of dispersed phase are prevented from moving.

Demonstration of Electro-Osmosis
The phenomenon of electro-osmosis can be demonstrated experimentally as follows:-
The demonstration of electro-osmosis is carried out in a specially designed apparatus. The apparatus consists of a bigger tube having two side tubes T and T/ attached to its ends. The bigger tube is divided into three compartments A, B and C by means of two semi-permeable membranes. A tube carrying a stop-cock is attached to the central compartment A. Two platinum electrodes are inserted in the outer compartments B and C.
A colloidal dispersion is placed in the central compartment A and the outer compartments B and C are filled with water. The water in compartments B and C also extends to the side tube T and T/. The function of membrane is to prevent the movement of colloidal particles. Therefore, when a potential difference is applied across the electrodes held close to the membranes in the compartment B and C, dispersion medium begins to move.

If the particles carry positive charge, the dispersion medium would start moving towards the anode and the level of water in the side tube T would be seen to rise, indicating the presence of negative charge on the dispersion medium. If the particles carry negative charge, the dispersion medium would be seen to move towards cathode and water in the side tube T would start rising.
Electro osmosis is utilizing for dewatering moist clay and drying of dye pastes.

Mechanical Property (Brownian Movement)

Robert Brown, a Botanist discovered in 1827 that pollen grains placed in water do not remain at rest but move about continuously and randomly. Later on, this phenomenon was observed in case of colloidal particles when they were seen under the ultramicroscope. The particles were seen to be in constant motion in zig – zag path in all possible directions.
This zig – zag motion of colloidal particles is called Brownian Movement.

Cause of Brownian Movement
Brownian Movement arises because of impact of the molecules of the dispersion medium with the colloidal particles. It has been postulated that the impacts of the particles of dispersion medium on colloidal particles are unequal which ultimately causes its zig – zag motion. However, as the size of particle increases, the probability of unequal impacts decreases and the Brownian movement becomes slow. Ultimately, when the dispersed particles become bigger enough to acquire the dimensions of suspension, no Brownian movement is observed.
Significance of Brownian Movement
(i).     Brownian Movement provides direct demonstration of ceaseless motion of molecules as postulated by Kinetic Theory.
(ii).    It neutralizes the force of gravity acting on colloidal particles and hence helps in providing stability to colloidal sols.

Optical Properties of Colloidal Solution (Tyndall Effect)

Tyndall Effect
Tyndall in 1869, observed that if a strong beam of light is passed through a colloidal solution placed in a dark place and viewed at right angles to the direction of light beam, the path of light shows up as a hazy beam or cone. This is due to the fact that sol particles absorb light energy and then emit it in all directions in space. This emission of light known as Scattering of Light,illuminates the path of beam in colloidal dispersion.
“The phenomenon of scattering of light by the sol particles is called Tyndall Effect. The illuminated beam or cone formed by the scattering of light by the sol particles is often referred as Tyndall Beam or Tyndall Cone. It is not shown by pure solvents.

Explanation
The Tyndall Effect is due to scattering of light by colloidal particles. The colloidal particles first absorb light and then a part of absorbed light is scattered from the surface of the colloidal particles. Maximum scattering intensity being in a plane at right angles to plane of incident light, the path becomes visible when seen from that direction. The particles of pure solvents or true solution are too small to scatter light.

General Properties of Colloidal Solutions

         a).     Filterability
                        Colloidal particles can pass through ordinary filter paper (diameter of pores in ordinary filter paper is about 10-6m).
         b).     Molecular weight
                        Colloidal particles are formed by aggregation of a large number of small molecules, e.g starch (C6H10O5)n. Hence they possess very high molecular weights.
         c).     Osmotic Pressure
                        As colloidal particles is an aggregation of large number of particles, the number of particles in sol is very small. Therefore, osmotic pressure of sol is very low. The measurement of osmotic pressure has been used for determination of molecular weights of colloids.
         d).     Colour
                        The colour of sol depends on the wavelength of light. Scattered by colloidal particles which in turn depends on the size of the particles on which the wavelength of scattered light depends. Gold sol can be red, blue or purple depending upon size of particle.

Monday, August 19, 2013

Origin of Charge on Colloidal Particles

The colloidal particles are known to carry positive or negative charge and all the particles of same sol carry same kind of charge. Various views have been put forward to explain the charge of colloidal particles.
(i). Frictional Electrification
The origin of charge on the colloidal particles may be due to frictional electrification. It is believed that frictional electrification due to rubbing the dispersed phase particles with those of dispersion medium results in some type of charge on colloidal particles of the sol. This view is not satisfactory.
(ii). 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. This view is not valid in all cases.
(iii). Dissociation Of Surface Molecules
The dissociation of surface molecules leads to electric charge on colloidal particles of the sol. For example, Consider an aqueous solution of soap which undergoes dissociation into ions.
Herein, the cation (Na+) passes into the solvent while the anions (C15H31 COO-) have a tendency to form negatively charged aggregates due to weak attractive forces present in the long hydrocarbon chains. These aggregates are of colloidal dimensions and are negatively charged. This is not valid for colloidal solutions of non-electrolytes such as clay, smoke etc.
(iv). 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 instance, protein molecules give rise to formation of colloidal solutions. Thus the particles of protein sol either have positive charge or negative charge depending upon the PH of 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 acidic medium and a negatively charged particle in basic or alkaline medium. This can be illustrated as follows:

At an intermediate PH point referred to as Iso-electric point, the protein will exist as uncharged molecules.
(v). Selective Adsorption of ions from Solutions
Colloidal particles adsorb preferentially positive or negative ions present in the dispersion medium. When two or more ions are present in the dispersion medium, the selective adsorption of ions common to the colloidal particles takes place resulting in the formation of positively charged or negatively charged particles in a colloidal solution. In simple words, the electric charge on the colloidal particles essentially originates by the selective adsorption of ions common to the colloidal particles from the dispersion medium.
For illustration, consider some typical examples of selective adsorption of ions.
(i). Positively charged Ferric hydroxide sol.
Fe(OH)3 sol is prepared by shaking Ferric hydroxide precipitate with dilute solution of Ferric Chloride.

(ii). Positively charged silver chloride sol.
It is obtained by shaking silver chloride precipitate with dilute solution of silver nitrate.

(iii). Negatively charged Arsenious Sulphide Sol.
It is prepared by passing H2S gas slowly through solution of AS2O3.

(iv). Negatively charged Silver Chloride Sol.
It is prepared by shaking silver chloride precipitate with dilute solution of hydrochloric acid.


Purification of Colloidal Solutions

The colloidal solutions prepared by various methods contains appreciable amount of electrolyte and other soluble Impurities. These Impurities may destablise the colloidal sol. Hence, they are to be removed. A very important method of removal of soluble impurities from sol is Dialysis.

Dialysis
Animal membranes (bladder) or those made of parchment paper and cellophane sheet, have very fine pores. These pores permit ions (or small particles) to pass through, but not the large colloidal particles. Thus particles of true solution can pass through parchment membrane but colloidal particles cannot pass through these membranes. A bag made of such membrane is filled with the colloidal solution and is then suspended in fresh water (distilled water). After some time whole of crystalloid in solution passing out leaving the colloidal behind. The distilled water is renewed frequently to avoid accumulation of the crystalloid as otherwise they may start diffusing back into the bag. “The process of separating the colloidal particles from those of crystalloids by diffusion of mixture through a parchment or animal membrane is known as dialysis”.
The above process can be quickened if an electric field is applied around the membrane, the process is then known as Electrodialysis.

Ultrafiltration
In this method, colloidal solutions are purified by carrying out filtration through special type of graded filters called Ultra – filters. These filter papers allow electrolytes to pass through them but do not allow colloidal particles. These filter papers are made from ordinary filter papers impregnating them with colloidal particles. In order to quicken the filtration increased pressure or suction is required.
Ultra-Centrifugation
In this method, the colloidal solution is place 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 water to form the colloidal solution again.

Monday, August 12, 2013

Preparation of Colloids

Lyophilic sols may be prepared by simply warming the solid with liquid dispersion medium. E.g. Starch with water. On the other hand, lyophobic sols have to be prepared by special methods. These methods fall into two categories;
  1. Condensation or Aggregation Methods.
  2. Dispersion Methods.
  1. Condensation or Aggregation Methods
  2. These methods consists of chemical reactions or change of solvent whereby the atoms or molecules of the dispersed phase appearing first, aggregate to form colloidal particles. The conditions (temp., conc. etc) used are such as permit the formation of sol particles but prevent the particles becoming too large and forming precipitate.
    The important chemical methods for preparing lyophobic sols are as follows;
    1. Double Decomposition:
    2. An Arsenic Sulphide (AS2S3) sol is prepared by passing a slow stream of hydrogen sulphide gas through cold solution of arsenious oxide (AS2O3). This is continued till a yellow colour of sol attains maximum intensity.
      AS2O3 + 3 H2S-------->AS2S3(Yellow Sol) + 3H2O
      Sols of silver halide are obtained by mixing dilute solutions of silver salts and alkali halides in equivalent amounts. Silica gel sol is prepared by this method between dilute solutions of sodium silicate and HCl.
    3. Oxidation:
    4. A Colloidal sol of sulphur can be obtained by passing hydrogen Sulphide into solution of sulphur dioxide in water or through a solution of an oxidising agent (Bromine water, nitric acid).
      SO2 + 2H2S-------->3S + 2H2O
      H2S + (O)-------->S + H2O
    5. Reduction:
    6. A colloidal solution of a metal like silver, gold and platinum can be prepared by reducing their salt solutions with suitable reducing agents, such as stannous chloride, formaldehyde, hydrazine, tannic acid etc.
      2AuCl3 + 3SnCl2-------->2Au(Gold sol.)+ 3 SnCl4
      or AuCl3 + Tannic acid-------->Au (Sol.)
    7. Hydrolysis:
    8. The method is used to prepare hydroxides and oxides of weakly electropositive metals like Fe, Al or Sn. A red sol of ferric hydroxide, is obtained by adding few drops of 30% ferric chloride solution to a large volume of almost boiling water and stirred with a glass rod. FeCl3 + 3H2O-------->Fe(OH)3(Red Sol.)+ 3HCl.

    The important physical methods for preparing lyophobic sols are:
    1. By Exchange of Solvent:
    2. When a true solution is mixed with an excess of the other solvent in which the solute is insoluble but solvent is soluble, a colloidal solution is obtained. For Example, when a solution of sulphur in alcohol (ethanol) is added to an excess of water, a colloidal solution of sulphur is obtained due to decrease in solubility.
    3. By Excessive Cooling:
    4. The colloidal solution of ice in an organic solvent such as CHCl3 or ether can be obtained by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution separately combines to form particles of colloidal size.
  3. Dispersion Methods
  4. In this method large particles of the substances are broken, into particles of colloidal dimensions in presence of dispersion medium. Since the sols formed are highly unstable. They are stabilized by adding some suitable stabilizer. Some of the methods employed for carrying out dispersion are as follows:
    1. Mechanical Dispersion:

    2. In this method, the coarse particles along with dispersion medium is brought into colloidal state by grinding it in colloidal mill, ball mill or ultrasonic disintegrator. The solid particles along the dispersion medium are fed into the colloidal mill. The mill consists of two steel plates nearly touching each other and rotating in opposite directions with high speed (7000 rev/min). The solid particles are torn off to colloidal size and are then dispersed in liquid to give the sol colloidal graphite (lubricant) and printing inks are made by this method. Recently, a mercury sol has been prepared by disintegrating a layer of mercury into sol particles in water, by means of ultrasonic vibrator.
    3. Bredig’s Arc Method:
    4. This process involves dispersion as well as aggregation. Colloidal solutions of metals such as gold, silver, platinum etc. can be prepared by this method. In this method electric arc is struck between electrodes of metal immersed in the dispersion medium. The intense heat produced vapourises some of metal, which then condenses to form particles of colloidal size.
    5. Peptisation:
    6. Peptisation may be defined as the process of converting a precipitate into colloidal form by shaking it with dispersion medium in the presence of small amount of electrolyte. The electrolyte used for this purpose is called Peptizing Agent. This method is applied, generally, to convert fresh precipitate into colloidal solution because such precipitates are simply clusters of particles of colloidal size held by weak forces.

    Cause Of Peptisation
    During peptisation, the precipitate adsorbs one of the ion of the electrolyte on its surface. The adsorbed ion is generally common with those of the precipitate. This causes the development of positive or negative charge on precipitates which ultimately breaks up into smaller particles having the dimensions of colloids. For example: When freshly precipitated Fe(OH)3 is shaken with aqueous solution of FeCl3 (Peptising agent) it adsorbs Fe3+ ions and thereby breaks up into small sized particles of type Fe(OH)3 / Fe3+. Similarly a precipitate of AgCl on shaking with dilute solution of AgNO3, adsorbs Ag+ ion and gets peptised to colloidal particles of type AgCl / Ag+. In some cases, peptisation can also be achieved by organic solvents. For example: Cellulose nitrate is peptised by ethanol. The colloidal solution so obtained is known as Colloidion.