Colloids

Colloids

Solutions are homogenous systems e.g. sugar solution. If sand is stirred in water, it slowly settles down and is called a suspension. Between the extremes of suspensions and solutions a large group of systems called colloidal dispersion or simply colloids exist. A colloidal is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium. The essential difference between a solution and a colloidal is one of particle size. In a solution, the particles or ions are small molecules. In a colloid, the dispersed phase may consist a particles of a simple macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions or molecules. Colloidal particles are larger than simple molecules but small enough to remain suspended. They have a range of a diameter between 1 and 1000 nm (10^-9 to 10^-6m).

Illustration 1. Comment on the statement that "colloid is not a substance but a state of substance".

Solution: Any substance (solid, liquid or gas) using special method can be brought into colloidal state. For example, NaCl in water forms true solution but in benzene forms colloidal solutions in alcohol but colloidal in water.

Classification of colloids

Colloids are classified on the basic of the following criteria:

(a) Physical state of dispersed phase and dispersion medium.

(b Nature of interaction between dispersed phase and dispersion medium.

(c) Type of particles of the dispersed phase.

1. Classification based on physical state of dispersed phase and dispersion medium

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 homogenous mixture and hence is not a colloidal system. The examples of the various of colloids along with their typical names are listed in table:

Types of colloidal systems

Out of various types of colloids given above, the most common are sols (solid in liquids), gels (liquids in solids) and emulsions (liquid in liquids). Further, it may be mentioned that depending upon the dispersion medium, the sols are given special names as follows:

Illustration : 2

Solution: (a) – (vii), (b) – (vi), (c) – (v), (d) - (iv), (e) – (iii), (f) – (ii), (g) – (i)

2. Based on the nature of interaction between dispersed phase and dispersion medium:

On this basis, colloidal sols are divided into two categories namely, lyophilic and lyophobic. If water is the dispersion medium, the terms are hydrophilic and hydrophobic.

(i) Lyophilic colloids: The word 'lyophilic' means solvent loving. Colloidal sols directly formed by substances like gum, gelatine, starch, rubber etc., on mixing with a suitable liquid (the dispersion medium) are called lyophilic sols. If the dispersion medium is separated from the dispersed phase (by evaporation), the sol can be reconstituted by simply remixing with the dispersion medium. That is why these sols are also called reversible sols.

(ii) Lyophobic colloids: The word 'lyophobic' means solvent hating substances. When substances are simply mixed with the dispersion medium they do not form the colloidal sol. These sols are readily precipitated (or coagulated) on the addition of small amounts of electrolytes, by heating or by shaking and hence are not stable. Further, once precipitated, they do not give back the colloidal sol by simple addition of the dispersion medium. Hence these sols need stabilizing agents for their preservation.

3. Classification based on types of particles of the dispersed phase: Multimolecular macromolecular and associated colloids

Depending upon the type of the particles of the disperised phase, colloids are classified as: multimolecular, macromolecular and associated colloids.

(i) Multimolecular colloids

When on dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having size (with diameters less than 1 nm) in the colloidal range, the species thus formed are called multimolecular colloids. For example sulphur sol consist of particles containing a thousand or more of S₈ sulphur molecules.

(ii) Macromolecular colloids: Macromolecules have large molecular masses. These on dissolution in a suitable solvent form a solution in which the size of the macromolecules may be in the colloidal range. Such systems are called macromlecular colloids. These colloids are quite stable and resemble true solutions in many respects.

Examples of naturally occuring macromolecules are starch, cellulose, proteins and enzymes. Examples of man – made macromolecules are polyethene, nylon, polystyrene, synthetic rubber etc.

(iii) Associated Colloids (Micelles): There are some substances which at low concentrations behaves as normal, strong electrolytes but at higher concentrations exhibit colloidal behaviour due to the formation of aggregated particles. The aggregated particles thus formed are called micelles. These are also known as associated colloids. The formation of micelles take place only above a particular temperature called kraft temperature (T_k) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is $\sim$10^-4 to 10^-3 molL^-1. These colloids have both lyophobic and lyophilic parts. Micelles may contains as many as 100 molecules or more.

Illustration 3. Why medicines are more effective in colloidal state?

Solution: A colloidal state has larger surface area of sol particles and this shows more effective adsorption. Thus medicines in colloidal state are more effectively absorbed and give better results.

Preparation of colloidal sols

For the preparation of lyophobic and lyophilic sols different methods are employed.

(A) Preparation of lyophobic sols

Lyophobic sols can be prepared by two methods

(i) Condensation methods

(ii) Dispersion methods

(i) Condensation method

In these methods particles of atomic or molecular size are induced to combine to form aggregates having colloidal dimensions. For this purpose chemical as well as physical methods can be applied.

(a) Chemical method

Colloidal solutions can be prepared by chemical reactions leading to formation of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate leading to formation of sols.

$A{s}_2{O}_3+3H_{2}S\stackrel{doubledecompostion}{\to }A{s}_2{S}_3\left(sol\right)+3H_{2}O$

$S{O}_2+2H_{2}O\stackrel{oxidation}{\to }3S\left(sol\right)+2H_{2}O$

$2AuC{l}_3+3HCHO+H_{2}O\stackrel{reduction}{\to }2Au\left(sol\right)+3HCOOH+6HCl$

$FeC{l}_3+3H_{2}O\stackrel{hydrolysis}{\to }Fe{\left(OH\right)}_3\left(Sol\right)+3HCl$

(b) Physical methods

(i) Exchange of solvent: When a true solution is mixed with an excess of the other solvent in which the solute is insoluble but solvent is miscible a colloids sol is obtained. For example, when a solution of sulphur in alcohol is poured in excess of water, a colloidal sol of sulphur is obtained.

(ii) Excessive cooling: The colloidal sol 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 combine to form particles of colloidal size.

2. Dispersion methods

In these methods large particles of the substance are broken into particles of colloidal dimensions in the presence of dispersion medium. These are stabilized by adding some suitable stabilizer. Some of the methods employed for carrying out dispersion are given below:

(a) Mechanical dispersion

In this method, the coarse suspension of the substance is brought into a colloidal state in the dispersion medium by grinding it in a colloid mill, ball mill or ultrasonic disintegrator the colloid mill consists of two metal discs. Close together, rotating at high speed (7000 revolution per minute) in opposite directions. The suspension particles are form to the colloidal size.

(b) Electrical disintegration or Bredig's arc methods

This process involves dispersion as well as condensation. Collodial sols of metals such as gold, silver, platinum etc can be prepared by this method. In this method electric arc is struck between electrodes of the metal immersed in the dispersion medium. The intense heat produced vaporizes the metal, which then condenses to form particles of colloidal size.

(c) Peptization

Peptization may be defined as the process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this purpose is called peptizing agent. This method is applied, generally, to convert a freshly prepared precipitate into a colloidal sol.

For example, when freshly precipitated Fe(OH)₃ is shaken with aqueous solution of FeCl₃(peptizing agent) it adsorbs Fe⁺³ ions and thereby breaks up into small – sized particles.

(B) Preparation of lyophilic sols

Lyophilic sols are quite stable and can be easily prepared by shaking the lyophilic material with dispersion medium. Some examples colloidal sols of gelatin, gum, starch, egg.

Purification of colloidal sols

The colloidal sols obtained by various methods are impure and contain impurities of electrolytes and other soluble substances. These impurities may destabilize the sol. Hence, they have to be removed. A very important method of removal of soluble impurities from sols by a semipermeable membrane is known as dialysis.

(i) Dialysis

Particles of true solutions can pass through parchment paper or cellophone membrane, sol particles can not pass through these membranes. A bag made up of such membrane is filled with the colloidal solution and is then suspended in fresh water. The electrolyte particles pass out leaving behind the colloidal sol.

Movement of ions across the membrane can be expendited by applying electric potential through two electrodes. This method is faster than simple dialysis and is known as electrodialysis.

(ii) Ultra –filtration

In this method, colloidal sols are purified by carrying out filtration through special type of graded filters called ultra – filter. These filter papers allow only the electrolytes to pass through. These filter papers are made of particular pore size by impregnating ordinary filter paper with colloidal particles. In order to accelerate the filtration through such filter papers, increased pressure or section is employed.

(iii) Ultra-centrifugation

In this method, the colloids sol is taken in a tube which is placed in an ultra – centrifuge. On rotation of the tube at high speeds, the colloidal particles settle down at the bottom of the tube and the impurities remain in the solution called the centrifugate. The settled colloidal particles are mixed with an appropriate dispersing medium to regenerate the sol.

Important properties of colloidal sols

(i) Colligative properties: Colloidal sols show the colligative properties, viz., relative lowering of vapour pressure, elevation in boiling point, depression in freezing point and osmotic pressure.

(ii) Optical properties-Tyndall effect: If a strong beam of light is passed through a colloidal sol placed in a dark place, the path of the beam gets illuminated. This phenomenon is called tyndall effect.

True solutions do not exhibit tyndall effect because the particles in them are too small in size and do not cause any scattering.

Illustration 4. Sky appear blue. Why?

Solution: Colloidal particles of dust, dirt in air scatter blue light to the maximum extent.

(iii) Mechanical properties

Brownian movement

The colloidal particles are seen to be in constant zig-zag motion. This zig – zag motion is called Brownian movement.

Brownian movement arises because of the impact of the molecules of the dispersion medium with colloidal particles. It has been postulated that the impact of the molecules of the dispersion medium with the colloidal particles. As the size of the particle increases, Brownian movement becomes slow. Ultimately, when the dispersed particles become big enough to acquire the dimensions of suspension, no Brownian movement is observed.

(a) Brownian movement provides a direct demonstration of ceaseless motion of molecules as postulated by kinetic theory.

(b) It counters the forces of gravity acting on colloidal particles and hence helps in providing stability to colloidal sols by not allowing then to settle down.

(iv) Electrical properties (Electrophoresis): The particles of the colloids are electrically charged and carry positive or negative charge. The dispersion medium has an equal and opposite charge making the system neutral as a whole due to similar nature of the charge carried by the particles. They repel each other and do not combine to form bigger particles. That is why a sol is stable and particles do not settle down. Arsenious sulphide, gold silver and platinum particles in their respective colloidal sols are negatively charged while particles of ferric hydroxide, aluminium hydroxide are positively charged. The existence of the electric charge is shown by the phenomenon of electrophoresis. It involves the movement of colloidal particles either towards the cathode or anode, under the influence of the electric field.

(iv) Coagulation of colloids: The presence of small amounts of appropriate electrolytes is necessary for the stability of the colloids. However, when an electrolyte is added in larger concentration, the particles of the sol take up the ions, which are oppositely charged and thus get neutralized. The neutral particles then start aggregating giving particles of larger size which are then precipitated. This process of aggregation of colloidal particles into an insoluble precipitate by the addition of some suitable electrolyte is known as coagulation.

At lower concentration of electrolytes, the aggregation of particles is called flocculation.

The minimum amount of an electrolyte (in milli moles) that must be added to one litre of a colloidal solution so as to cause its compete coagulation is called the precipitation or coagulation value of the electrolyte.

Different electrolytes have different coagulation value.

(i) The ions carrying charge opposite to that of sol particles are effective in causing the coagulation of the sol.

(ii) Coagulation power of an electrolyte is directly proportional to the fourth power of the valency of the ions causing coagulation.

Illustration 5. Which of the following can act as a protective colloid?

(A) gelatin (B) silica gel

(C) oil-in-water emulsion (D) all correct

Solution: (A)

Illustration 6. The coagulation of 100 ml of a colloidal sol of gold is completely prevented by addition of 0.25 g of starch to it before adding 1 ml of 10 % NaCl solution. Find out the gold number of starch.

Solution: Starch added to 100 ml of gold sol to completely prevent coagulation by 1 ml of 10% NaCl sol = 0.25 g = 250 mg.

Starch required to be added to 10 ml of gold sol to completely prevent coagulation by 1 ml of 10 % NaCl sol = 25 mg

$\therefore$ By definition, gold number of starch = 25

EMULSIONS

Emulsions are colloids in which both dispersed phase and dispersion medium are liquids. Emulsion can be broadly classified into two types.

(i) Oil in water emulsions: In this type of emulsions, oil acts as (organic solvent) dispersed phase and water acts as dispersion medium. Some examples of this type of emulsions are milk, vanishing cream, etc. In milk, liquid fat is dispersed in water.

(ii) Water in oil emulsions: In this type of emulsions, water acts as dispersed phase and oil (organic solvent) acts as dispersion medium. Cold cream, butter etc, are examples of oil emulsions.

Identification of Emulsion

The following tests may be employed to distinguish between the two types of emulsions:

(i) Dye test: Some oil soluble dye is added to the emulsion. If the background becomes coloured, the emulsion is water – in – oil type and if the coloured droplets are seen, the emulsion is oil – in – water type.

(ii) Dilution test: If the emulsion can be diluted with water, this indicates that water is the dispersion medium and the emulsion is of oil – in – water type. In case the added water forms a separate layer, the emulsion is water – in – oil type.

Preparation of Emulsion

The process of making an emulsion is known as emulsification. Emulsion may be obtained by vigorously mixing both the liquids. The dispersed drops at once come together and form separate layers. To stabilize an emulsion, the addition of a small quantity of a third substance known as emulsifying agent or emulsifier is essential. Soaps and detergents are most frequently used as emulsifiers.

The other common stabilizing agents are proteins, gum and agar-agar.

Illustration 7. Why gelatin is added to ice cream?

Solution: To stabilize to prevent coagulation of ice cream particles.

Demulsification

The separation of an emulsion into its constituent liquids is called demulsification. The various techniques applied for demulsification are freezing, boiling, centrifugation, electrostatic precipitation or chemical methods, which destroy the emulsifying agents.

APPLICATION OF COLLOIDS

Colloids including emulsions find a number of uses in our daily life and industry. Some of the uses are given below:

(i) Rubber plating: Latex is a colloidal solution of negatively charged rubber particles. Rubber plated articles are prepared by depositing negative charged particles over the article to be rubber plated by making that article an anode in a rubber plating bath.

(ii) Medicines: Medicines in colloidal form are easily absorbed by the body tissues and hence are more effective.

(iii) Sewage disposal: Colloidal particles of dirt, mud etc. carry electric charge. Hence when sewage water is passed through the plates kept at a high potential, the colloidal particles are coagulated due to electrophoresis and the suspended matter gets removed.

(iv) Purification of water: The precipitation of colloidal impurities present in water can be done by adding certain electrolytes like alum etc. The negatively charged colloidal particles of impurities get neutralized by the Al³⁺ ions and settle down and pure water can be decanted off.

(v) Formation of delta: River water contains charged colloidal particles of clay, sand and many other materials. Sea water is a very big store – house of a variety of electrolytes dissolved in it. As soon as river water comes in contact with sea water, the electrolytes present in sea water coagulate the suspended colloidal particles which ultimately settle down at the point of contact and thus the level of the river bed rises. As a result, water adopts a different course and delta is formed in due course of time.

(vi) Smoke screen: In warfare, smoke screens are used which are colloidal dispersion of certain substance in the air.

Illustration 8.

Solution. (a) – (iv), (b) – (i), (c) – (ii), (d) – (iii)