n. vamsi krishna. colloidal system are the ones in which one of three states (solid, liquid and gas)...

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N. Vamsi Krishna

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Page 1: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

N. Vamsi Krishna

Page 2: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

• Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another.

• Colloids were named first in the early 19th century by the Father of Physical Chemistry, Thomas Graham (1805-1869). In 1920's and 1930's, the importance of colloids to industrial processes and biochemistry changed everything making it a hot field.

• A colloidal system consists of an internal phase (dispersion phase), which is the material of colloidal dimensions, and an external phase (dispersion medium) .Similar to the terms solute and solvent used for simple solutions.

• As the particles of a colloid system become smaller and smaller, we go over imperceptibly from a two-phase colloid to a single-phase solution, and there is no definite boundary i.e. true solution.

Introduction:

Page 3: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Properties Suspension Colloid Solution

1. Particle size >100nm 1-100nm <1nm

2. Separation1)ordinary filtration2) ultra filtration

possiblepossible

not possiblepossible

not possiblenot possible

3. Settling Settles under gravity

Settles onCentrifugation

Does notsettle

4. Appearance opaque Generally clear clear

5. Diffusion Not possible Diffuses slowly Diffuses rapidly

6. Brownian motion shows shows Not observable

7. Tyndall effect shows shows Not observable

Table 1: Characteristics of Suspension, Colloid and Solution

Page 4: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

• Total interfacial area (NAp) per 1cm3 of colloid is given by, NAp = N4πR2 = 4π R2Ф/(4/3π R3 ) = 3Ф/RWhere R = radius, Ф = volume fraction Number of particles per 1cm3 of colloid N = Ф/Vp = Ф/ (4/3π R3 )

• The large area emphasizes surface effects relative to volume effects, giving colloids different properties than those of bulk matter.

• Because of their various properties, colloidal systems have many applications in various industries like food industry (diary products, chocolate etc), pharmaceutical and cosmetic industry (gels and emulsions), Photographic industry (films, carbon paper, ink etc), Electrical and electronic industry (liquid crystals, isolating materials etc), paint industry and agrochemical industry.

Page 5: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Classification of colloids:

Colloids can be classified mainly by 3 ways; • Classification Based on the State of the Dispersed Phase and Dispersion Medium • Classification of Colloids Based on Type of Particles of the Dispersed Phase

• Classification Based on the Nature of Interaction Between Dispersed Phase and Dispersion Medium

Page 6: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Dispersion Medium

 Dispersed phase

Type of colloid Example

Gas  Liquid Aerosol Fog, clouds

Gas Solid Aerosol Smoke

 Liquid Gas Foam  Whipped cream, soda water

 Liquid Liquid Emulsion Milk, hair cream

 Liquid Solid Sol  Paints, cell fluids

Solid Gas Foam  Pumice, plastic foams

Solid Liquid Gel  Jelly, cheese

Solid Solid Solid Sol Ruby glass

Table 2: Types of colloids

Classification Based on the State of the Dispersed Phase and Dispersion Medium

Page 7: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Classification of Colloids Based on Type of Particles of the Dispersed Phase

a) Multimolecular colloids usually have lyophobic character. Ex: gold and sulphur sols

b) Macromolecular colloids resemble true solutions Ex: proteins, cellulose, starch and polymers such as polyethylene, nylon and

polystyrene

c) Associated colloids type of micelle it forms depends on the nature of solvent (hydrophilic or hydrophobic)

Kraft Temperature (Tk ) Critical micelle concentration (CMC) Ex: soaps and synthetic detergents

MicelleSurfactant

Page 8: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Property Lyophilic sols (suspensoid) Lyophobic sols (Emulsoid)

Surface tension Lower than that of the medium Same as that of the medium

Viscosity Much higher than that of the medium

Same as that of the medium

Reversibility Reversible Irreversible

Stability More stable Less stable

Visibility Particles can’t be detected even under ultra

microscope

Particles can be detected under ultra microscope.

Migration Particles may migrate in either direction or do not migrate in an electric field because do not carry any charge.

Particles migrate either towards cathode or anode in an electric

field because they carry charge.

Action of electrolyte Addition of smaller quantity of electrolyte has little effect

Coagulation takes place

Hydration Extensive hydration takes place

No hydration

Examples Gum, gelatin, starch, proteins, rubber etc.

Metals like Ag and Au, hydroxides like Al(OH3), Fe(OH)3 metal

sulphides like AS2S3 etc.

Table 3: Distingustion between lyophilic and lyophobic colloids

Classification Based on the Nature of Interaction Between Dispersed Phase and Dispersion Medium

Page 9: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Preparation of colloids:

1) lyophilic sols easy to prepare

2) lyophobic sols unstable and irreversible, difficult to prepare a) Condensation or aggregation method b) Dispersion method

Page 10: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

a) Condensation or aggregation method

Chemical methods:

• By Reduction: Metal sols are generally prepared by this method. Ex: HAuCl4¯ (aq) Au (sol)

AgO (aq) Ag (sol)

• By Oxidation: Sulphur sols are easily obtained by the oxidation. Ex: 2H2S+SO2 2H2O + 3S (sol)

• By double decomposition:

Ex: As2O3 (aq) + 3H2S As2S3 (sol)+ 3H20

• By hydrolysis: To obtain sols of oxides or hydroxides of weakly electropositive metals. Ex: FeCl3 + 3H2O Fe(OH)3 (sol) + 3HCl

HCHO

H2, 50-60 0C

Page 11: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Physical methods:

• By excessive cooling: used to prepare a colloidal solution of ice.

• By exchange of solvent:

Ex: S (alc) + H2O S (sol)

• By change of physical state: Sols of substances like mercury and sulphur are prepared

Page 12: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Figure1: Pictorial view of colloidal mill

b) Dispersion method

• Mechanical disintegration: Colloidal solutions of black ink, paints, varnishes, dyes etc. and food products like concentrated milk, food additives etc.

Page 13: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

• Peptization: The process of dispersing a precipitate in to a colloidal solution by adding small quantity of electrolyte is called peptization.

Ex: Fe(OH)3 ↓+ FeCl3 [ Fe(OH)3Fe3+] (sol) + 3Cl¯

Page 14: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Figure 2: Bredig’s arc method

• Bredig’s arc method:

used to prepare sols of platinum, silver, copper or gold. Traces of KOH are required to stabilize the colloidal solution.

Page 15: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Generally used methods to prepare other colloidal systems are;

− Aerosol is formed by passing gas jet to a liquid spray.

− Emulsions are usually prepared by vigorously shaking the two constituents together, often with the addition of an emulsifying agent . The phase in which emulsifier is more soluble forms the outer layer. lyophobic colloids .

− Gels are often formed by cooling lyophilic sols that contain large linear molecules and have a much greater viscosity than the solvent. Elastic and Rigid gels.

− Foams are formed when gas and liquid are mixed together in a container and shaken along with a foaming agent.

Page 16: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Purification of colloidal solutions:

• Colloidal solutions prepared by the above methods contain some soluble impurities and excess of electrolytes; these have to be removed to obtain pure sols.

•Generally a) Dialysis, b) ultra filtration and c) ultra centrifugation are the techniques used.

• Ultra filtration: colloidal sols are filtered through ultra-filtrers. pore size of filter paper is decreased such that it will restrict the passage of colloidal particles.

• Ultra-centrifugation: Centrifugation is carried out at very high speeds such that the colloidal particles settle down at the bottom of the tube and the impurities remain in the solution .

• It is important to note that above methods to purify colloidal solution do not produce100% solution.

Page 17: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Figure 3: Electro-dialysis

• used for purification of blood in case of kidney failure.

Page 18: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Figure 4: Size dependent change of colour in Au sol

Properties of colloids:

a) Optical properties

Page 19: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Figure 6: Schematic drawing of ultra microscope

Opalescence in colloids is due to scattering of light by particles. This effect was studied by Tyndall and is generally known as tyndall effect. Tyndall observed that scattered beam to be polarized and intensity of the same to be dependent on position of observer, nature of system and wavelength of light used.

Page 20: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

b) Colligative Properties

• The magnitudes of these properties for colloidal solutions are much smaller than those obtained for true solutions. They exhibit measurable osmotic pressures. This property is used for the determination of the average molecular masses of the colloids.

c) Kinetic properties

liquid undergoes continuous chaotic and random motions. This motion of the particles is called Brownian motion. This Brownian motion is found to decrease by increase in particle size or by increase in viscosity of medium.

r = (3bυ/ 4π nd) ⅓ where, b = number of grams of substance per dm3 n = number of particles observed in view υ = volume dm3 d = density of dry substance

Page 21: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

c) Electrical properties

Lyophobic sols carry charge, because of which they get repelled on approaching another particle avoiding their coagulation.

i) Due to presence of electrolytesEx: positive charge on ferric hydroxide sol prepared by hydrolysis of ferric chloride is due to adsorption of Fe3+ ions on surface.

ii) dissociation of molecular electrolytes adsorbed on the surface of particles. Ex: H2S molecules get adsorbed on sulphides during precipitation. H2S undergoes dissociation and the hydrogen ions are lost. The particles become negatively charged due to (S2-) which are left on the colloidal particles.

iii) by dissociation of surface molecules Ex: soaps, proteins

Page 22: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Figure 7: Electrical double layer

The influence of net charge decreases with distance and so the number of oppositely charged ions, equaling the number of ions of both charges prevailing electro neutrality.

Page 23: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

The difference in potential between the surface of shear plane and electro neutral region of solution is called Zeta potential (ζ), which is given by Helmholtz- Smoluchowski equation. ζ = 4πηu/εr

where η = viscosity of dispersion medium εr = relative permittivity of dispersion medium u = mobility of colloidal particle

Page 24: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

• In case of lyophobic colloids;(i) Coagulation is brought about by ions having opposite charge to that of the sol.(ii) The efficiency of an ion to cause coagulation depends upon the square of its valency.Thus efficacy of cations Al3+>Mg2+> Na+ and PO4

3->SO42->Cl-.

(iii)The minimum concentration of an electrolyte required to cause coagulation of a sol is called its flocculation value. The other methods to coagulate sol are by mutual mixing of oppositely charged sols, persistent dialysis and by mechanical means.

• In case of lyophilic sols, i) water of solvation (cage of water surrounding) prevents particles from coagulation. ii) Their coagulation is due to removal of salvation water a s electrolytes binds them. Thus efficacy of cations Al3+>Mg2+> Ca2+> Ba2+>Na+>K+ and efficacy of anions PO4

3-

>SO42->Cl->NO3

-> ClO3->I- for coagulation of lyophilic sols and this is known as lyotropic

series or Hofmeister series.iii) Zsigmondy quantified stabilizing action of protective colloids by gold number, which is defined as largest number of milligrams of a protective colloid when added to 10 ml of standard gold sol, just fails to prevent the colour change from red to blue upon addition of 1 ml of 10% sodium chloride solution

Page 25: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Properties of colloids:

• Purification of water by alum (coagulation)

• In rubber platting

• In tanning

• Artificial rains

• Formation of deltas (coagulation)

• Blood clot formation

• Colloidal medicine: Argyrol and protargyrol are colloidal solution of silver and are used as eye lotions .Colloidal sulphur is used as disinfectant and colloidal gold, calcium and iron are used as tonics. A wide variety of medicines are emulsions.

• Coating of Photographic plates

• Sewage disposal

• Metallurgy

Page 26: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named

Figure 8: Schematic diagram of Cottrell smoke precipitator

• Smoke precipitation (Coagulation)

Page 27: N. Vamsi Krishna. Colloidal system are the ones in which one of three states (solid, liquid and gas) is finely dispersed in another. Colloids were named