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CHAPTER 6 COLLOIDAL SOLUTIONS Thomas Graham classified substances into crystalloids and colloids. Substances like salt, sugar and urea which in the dissolved state readily pass through a parchment paper or animal membrane are called crystalloids. On the other hand, substances like glue, gelatin, albumin, starch etc. which in the dissolved state do not pass through the parchment paper or animal membrane are called colloids (Greek word meaning glue like).

Thomas Graham (1805-1869) This division of substances was soon proved to be incorrect because a crystalloid could behave as a colloid under different conditions. For example, common salt, a typical crystalloid in an aqueous solution, behaves as a colloid in benzene medium.Colloids, therefore, does not represent a separate class of substances. Now it is replaced by the term colloidal state in which every substance can be obtained by a suitable method. The nature of the substance whether colloid or crystalloid depends upon the size of the solute particles. When the size of solute particles is between 1 to 100nm, it behaves as a colloid. On the other hand, if the size of solute particles is greater than 100 nm it exists as suspension and if particle size is less than 1 nm it exists as true solution and behaves like a crystalloid. Thus colloid is not a substance but a particular state of the substance which depends upon size of its particles. The colloidal state is the intermediate state between true solution and suspension. A colloidal system is a heterogeneous two phase system. One phase consists of dispersed particles of colloidal size and is called the dispersed phase. The other phase is the medium in which the colloidal particles are dispersed. It is called the dispersion medium. Classification of Colloidal state Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gases, eight types of colloidal systems are possible. A gas mixed with another gas forms a homogeneous mixture and hence is not a colloidal system.

XI ISC Notes

S.Narayana Iyer,M.Sc,M.Phil

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Types of Colloidal Systems Solid Solid Solid sol Some coloured glasses and gem stones Solid Liquid Sol Paints, starch in water Solid Gas Aerosol Smoke, dust Liquid Solid Gel Cheese,butter,jellies

Liquid Liquid Emulsion Milk, hair cream Liquid Gas Aerosol Fog,mist,cloud, insecticide sprays Gas Solid Solid sol Pumice stone, foam rubber Gas Liquid Foam Froth, whipped cream, soap lather

Colloidal systems may also be classified as lyophobic (solvent hating) and lyophilic (solvent loving) sols. Lyophobic sols are those which cannot be prepared easily by mixing the substance in a dispersion medium. They are prepared by special methods.eg. Gold sol. Lyophilic sols are those which can be prepared easily by mixing the substance in a dispersion medium. Eg. Starch sol Distinction between Lyophobic and Lyophilic Sols LYOPHOBIC SOL LYOPHILIC SOL 1. They are prepared easily by direct mixing

1. They are prepared by special methods.

2. They are reversible 2. They are irreversible. 3. They are very stable 3. They are unstable and requires

stabilizers 4. Addition of small amount of electrolyte has no effect

4. Addition of small amount of electrolyte can cause precipitation

5. Viscosity is higher than that of the dispersion medium

5. Viscosity is nearly the same as that of the dispersion medium

6. Surface tension is lower than that of the dispersion medium

6. Surface tension is the same as that of the dispersion medium.

Preparation of Colloidal Solutions

Colloidal systems of lyophilic colloids can be prepared by simple treatment with the dispersion medium. Starch, gum arabic, gelatin etc. form colloidal dispersion by merely treating with water.

Lyophobic sols are prepared by two methods called

1. Condensations methods. and 2. Dispersion methods.

1. Condensation methods

In these methods, small ions or molecules are condensed together to form particles of colloidal size. This may be achieved by chemical methods.

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(a) Oxidation: A colloidal solution of sulphur can be obtained by passing H2S

through an aqueous solution of sulphur dioxide.

SO2 + 2H2S → 2H2O + 3S

(b) Reduction: Colloidal solution of gold can be prepared by reducing gold chloride solution with stannous chloride solution.. 2AuCl3 + 3SnCl2 → 3SnCl4 + 2Au

(Sol) (c) Double decomposition: Arsenious sulphide sol can be prepared by passing H2S through a dilute solution of arsenious oxide.

As2O3 + 3H2S → As2S3 + 3H2O

(d) Exchange of solvent: Sulphur is soluble in alcohol, but less soluble in water. When alcoholic solution of sulphur is poured into water a colloidal solution of sulphur is obtained.

2. Dispersion Methods

In these methods large particles of the substance are broken down into particles of colloidal dimension in the presence of dispersion medium. Since the sols formed are highly unstable they are stabilized by adding some suitable stabilizer.

(a) Bredigs’ arc method: Preparation of gold sol: An electric arc is struck between two gold wires dipped in water. The water is cooled by using ice. Due to the intense heat of the arc the gold becomes vapour and it condenses under water to get particles of colloidal size.

Bredigs’ arc method (b) Peptisation: Peptisation is the process of conversion of a precipitate into colloidal solution by the addition of an electrolyte. Ferric hydroxide sol can be

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prepared by this method. Ferric hydroxide precipitate is suspended in water. It is shaken with a little ferric chloride solution. Reddish brown ferric hydroxide sol is obtained. This is due to the adsorption of ferric ions (Fe3+) over ferric hydroxide particles which cause them to disperse into solution because of electrostatic repulsion. The electrolyte used for this purpose is called peptising agent.

Purification of Colloidal Solutions

1. Dialysis: Animal membranes or parchment paper have very small pores. These allow small molecules and ions to pass through them but prevent the

bigger colloidal particles. The separation of a sol from the impure small molecules and ions by using animal membrane or parchment paper is called dialysis. The impure sol is placed in a bag of animal membrane and suspended in a large vessel containing pure water. The water is continuously renewed. The ions and small molecules will diffuse out of the bag. The diffusion of the electrolyte through the membrane can be increased by the application of electric field. The process is then called electrodialysis.

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2. Ultrafiltration: The separation of small molecules and ions from colloidal solution can also be carried out by ultrafiltration. Colloidal particles will pass through ordinary filter papers. But the pores of the filter paper can be made smaller by dipping them in gelatin and then hardening them by dipping in formaldehyde. Such filter papers are called ultrafilters. The process of separating colloidal particles from small molecules or ions by using ultrafilters is called ultrafiltration. The ultrafilter is supported on a wire mesh and the impure sol is poured over it. The electrolyte particles pass through it while the larger colloidal particles are retained. It can be speeded up by applying pressure on the impure sol. Properties of colloids

(1) Optical property a) Faraday-Tyndall effect: When a strong beam of light is passed through a sol contained in a glass vessel and placed in a dark room the path of the light becomes visible. This is due to the scattering of light by the colloidal particles. This phenomenon is called Faraday-Tyndall effect.

b) Ultramicroscope: Zsigmondy used Tyndall effect to set up an apparatus known as ultramicroscope. An intense beam of light is focussed on the colloidal solution contained in a glass vessel. The focus of the light is then observed with a microscope at right angles to the beam. Individual colloidal particles appear as bright spots against a dark background (dispersion medium). Under the ultramicroscope the actual colloidal particles are not visible, but the light scattered by them is visible Thus, ultramicroscope does not provide any information about the size and shape of colloidal particles.

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(2) Kinetic property: Brownian movement or Brownian motion. On observing a sol with an ultramicroscope it is found that the colloidal particles are always moving in a zig-zag manner. This irregular movement of colloidal particles is called Brownian movement. It was first observed by a Botanist Robert Brown using pollen grains in water. Hence the name Bownian movement. Kinetic theory offers an explanation for Brownian movement. It is due to the unequal bombardment of medium molecules on the colloidal particles.

Robert Brown (1773–1858)

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(3) Electrical property: a) Electrophoresis: The colloidal particles are either positively charged or negatively charged. So if an electric field is applied the colloidal particles move to the electrode bearing opposite charges and get deposited there. The colloidal particles of metallic oxides, hydroxides etc are positively charged while metallic sulphides, starch etc are negatively charged. The movement of colloidal particles under the influence of an electric field is called electrophoresis.

The colloidal solution is placed in a U-tube fitted with platinum electrodes. On passing an electric current, the charged colloidal particles move towards the oppositely charged electrode. Thus, if arsenious sulphide is taken in the U-tube its particles move towards the positive electrode. 4. Coagulation or precipitation: When an electrolyte is added to a sol the colloidal particles are precipitated. It is called coagulation or precipitation. The stability of a sol is due to charges on the colloidal particles. Due to similar charges the colloidal particles are unable to combine together; if the charge on the colloidal particles is neutralized they gather together and are precipitated. When an electrolyte is added the charged ions of the electrolyte neutralize the charge on the colloidal particles. So they are precipitated. Hardy and Schultz rule: The coagulating capacity of an electrolyte depends on the valency of its ions.” The higher the valency greater is its coagulating power”. It is called Hardy and Schultz rule.

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In the coagulation of a negative sol, the flocculating power is in the order: Al3+>Ba2+>Na+ Similarly, in the coagulation of a positive sol, the flocculating power is in the order: [Fe (CN)6]

4– > PO4 3– > SO4

2– > Cl–

The minimum concentration of an electrolyte in millimoles per litre required to cause precipitation of a sol in two hours is called coagulating value. The smaller the quantity needed, the higher will be the coagulating power of an ion. Coagulation of lyophilic sols There are two factors which are responsible for the stability of lyophilic sols. These factors are the charge and solvation of the colloidal particles. When these two factors are removed, a lyophilic sol can be coagulated. This is done (i) by adding an electrolyte and (ii) by adding a suitable solvent. When solvents such as alcohol and acetone are added to hydrophilic sols, the dehydration of dispersed phase occurs. Under this condition, a small quantity of electrolyte can bring about coagulation. Protection of colloids The lyophobic sols are unstable but the lyophilic sols are stable. The lyophilic sols can be protected from coagulation by the addition of lyophilic sol. It is called protection. It is believed that the lyophilic colloidal particles are adsorbed on the surface of lyophobic colloidal particles. This envelope protects them from the action of electrolyte. .Gold Number The protective power of a lyophilic sol is measured in terms of gold number. It was introduced by Zsigmondy. “It is the number of milligrams of protective colloid just sufficient to prevent the precipitation of 10 mL of a red gold sol by the addition of 1 mL of 10% sodium chloride solution. The smaller the value of gold number greater is its protective power. Applications of colloids 1. Food stuffs. Milk, ice cream etc are colloidal in nature. Milk is an emulsion of fat in water stabilized by milk protein. Ice cream is a dispersion of ice in cream. 2. Medicines. Colloidal medicines are more effective and are easily absorbed by the body system. Eg. Argyrol a silver sol is used as an eye lotion. Cold liver oil and skin ointments are emulsions. 3. Curdling of milk. The sugar present in milk produces lactic acid on fermentation. Ions produced by the acid destroy the charge on the colloidal particles present in milk which then coagulate and separate as curd. The same effect is produced if any other acid is added to the milk. 4. Purification of water. Alum is added to impure water. Alum provides Al3+ ions which neutralize the charge on the colloidal impurities. The colloidal impurities settle down and pure water can be decanted off.

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5. Sewage disposal. Sewage water contains dirt which are colloidal in nature and carry negative charge. These particles may be removed by using the phenomenon of electrophoresis. 6. Smoke precipitation. Smoke is a colloidal solution of carbon particles dispersed in air. Since carbon particles carry an electric charge. The charge on the carbon particles is removed by the use of electrical precipitator. It was devised by Cottrell. So it is called Cottrell’s precipitator.

7. Artificial rain. Artificial rain can be caused by throwing electrified sand from an aeroplane. Moist air contains colloidal particles of water which carry charge. Electrified sand helps to neutralize the charge on particles of water which coagulate and the result in rainfall. 8. Blood. Blood is a colloidal solution of albuminoid substance and can be coagulated by to clot by Al3+ or Fe3+ ions. Therefore bleeding can be stopped by rubbing alum against the cut which coagulates the blood and seals the blood vessels. 9. Blue colour of the sky. Colloidal dust particles along with water suspended in air scatter blue light which reaches our eyes and the sky appears blue to us. 10. Tail of the comets. When a comet flies with very high speed it leaves behind a tail of tiny solid particles suspended in air. These particles scatter light forming Tyndall’s cone which looks like the tail of the comet. 11. Formation of delta. River water is a colloidal solution of clay particles which carry negative charge. Sea water contains a number of electrolytes. When river water strikes the sea water Na+, Mg2+ ions etc present in the electrolytes of sea water coagulates the colloidal solution of clay which get deposited with the formation of delta at the mouth of the river.

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12. Rubber industry. Latex is a colloidal solution of negatively charged rubber particles dispersed in water and protected by gelatin. Rubber is obtained by coagulation of latex. 13. Photographic plates and films. Photographic plates or films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films. 14. Cleansing action of soaps. Soap is sodium or potassium salt of a higher fatty acid and may be represented as RCOO–Na+ (e.g., sodium stearate CH3 (CH2)16COO–Na+, which is a major component of many bar soaps). When dissolved in water, it dissociates into RCOO– and Na+ ions. The RCOO– ions, however, consist of two parts — a long hydrocarbon chain R (also called non-polar ‘tail’) which is hydrophobic (water repelling), and a polar group COO– (also called polar-ionic ‘head’), which is hydrophilic (water loving).

The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles. ((Micelles): There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids)

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Since the polar groups can interact with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus soap helps in emulsification and washing away of oils and fats. The negatively charged sheath around the globules prevents them from coming together and forming aggregates.