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N. Vamsi Krishna



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

Colloids were named f irst in the ear ly 19th century by the Father of Physical Chemistry, Thomas Graham (1805-1869). In 1920's and 1930's, the importance of

colloids to industrial pr ocesses and biochemistry changed everything making it ahot f ield.

A colloidal system consists of an internal phase (dispersion phase), which is thematerial 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 f r om a two-phase colloid to a single-phase solution, and there is no def inite boundary i.e. true solution.

Intr oduction:



Pr operties Suspension Colloid Solution

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

2. Separation1)ordinary f iltration2) ultra f iltration

possible

possible

not possible

possible

not possible

not possible

3. Settling Settles under gravity

Settles onCentrif ugation

Does notsettle

4. Appearance opaque Generally clear clear

5.Diff usion N

o

t po

ssibl

e Diff

uses slo

wl

y Diff

uses rapidl

y6. Br ownian motion shows shows Not observable

7. Tyndall eff ect shows shows Not observable

Table 1: Characteristics of Suspension, Colloid and Solution



To

tal

inter f

acial

area (NAp) per 1cm3 of

collo

id is given by,NAp = N4R2 = 4 R2/(4/3 R3 ) = 3/RWhere R = radius, = volume f ractionNumber of particles per 1cm3 of colloid

N = /Vp = / (4/3 R3 )

The large area emphasizes sur f ace eff ects relative to volume eff ects, givingcolloids diff erent pr operties than those of bulk matter.

Because of their various pr operties, colloidal systems have many applicationsin various industries like food industry (diary pr oducts, chocolate etc),pharmaceutical and cosmetic industry (gels and emulsions), Photographicindustry (f ilms, carbon paper, ink etc), Electrical and electr onic industry (liquidcrystals, isolating materials etc), paint industry and agr ochemical industry.



Classif ication of colloids:

Colloids can be classif ied mainly by 3 ways;

Classif ication Based on the State of the Dispersed Phase and Dispersion Medium

Classif ication of Colloids Based on Type of Particles of the Dispersed Phase

Classif ication Based on the Nature of Interaction Between Dispersed Phase andDispersion Medium



Dispersion Medium Dispersed phase Type of colloid Example

Gas Liquid Aer osol Fog, clouds

Gas Solid Aer osol Smoke

Liquid Gas Foam Whipped cream,soda water

Liquid Liquid Emulsion Milk, hair cream

Liquid Sol

id 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

Classif ication Based on the State of the Dispersed Phase and Dispersion Medium



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) Macr omolecular colloids resemble true solutionsEx: pr oteins, cellulose, starch and polymers such as polyethylene, nylon andpolystyrene

c) Associated colloids type of micelle it forms depends on the nature of solvent(hydr ophilic or hydr ophobic)

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

Micelle

Sur f actant



Pr operty Lyophilic sols (suspensoid) Lyophobic sols (Emulsoid)

Sur f ace tension Lower than that

of the medium Same as that

of the medium

Viscosity Much higher than that of themedium

Same as that of the medium

Reversibility Reversible Irreversible

Stability More stable Less stable

Visibility Particles can¶t be detectedeven under ultra

micr oscope

Particles can be detected under ultra micr oscope.

Migration Particles may migrate in either direction or do not migratein an electric f ield becausedo not carry any charge.

Particles migrate either towardscathode or anode in an electric

f ield because they carrycharge.

Action of electr olyte Addition of smaller quantity of electr olyte has little eff ect

Coagulation takes place

Hydration Extensive hydration takesplace

No hydration

Examples Gum, gelatin, starch, pr oteins,

rubber etc.

Metals like Ag and Au, hydr oxides

like Al(OH3), Fe(OH)3 metal sulphides like AS2S3 etc.

Table 3: Distingustion between lyophilic and lyophobic colloids

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



Preparation of colloids:

1) lyophilic sols easy to prepare

2) lyophobic sols unstable and irreversible, diff icult to prepare

a) Condensation or aggregation method

b) Dispersion method



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 hydr olysis: To obtain sols of oxides or hydr oxides of weakly electr opositive metals.

Ex: FeCl3 + 3H2O Fe(OH)3 (sol) + 3HCl

HCHO

H2, 50-60 0C



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



Figure1: Pictorial view of colloidal mill

b) Dispersion method

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



Peptization:The pr ocess of dispersing a precipitate in to a colloidal solution by adding small quantityof

electr

olyte is ca

lled peptizati

on.

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



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

stabil

ize the collo

idal

sol

utio

n.



Generally used methods to prepare other colloidal systems are;

í Aer osol is formed by passing gas jet to a liquid spray.

í Emulsions are usually prepared by vigor ously shaking the two constituents together,of ten with the addition of an emulsif ying agent . The phase in which emulsif ier is moresoluble forms the outer layer. lyophobic colloids .

î Gels are of ten formed by cooling lyophilic sols that contain large linear molecules and

have a much greater viscosity than the solvent.Elastic and Rigid gels.

î Foams are formed when gas and liquid are mixed together in a container and shakenalong with a foaming agent.



Purif ication of colloidal solutions:

Colloidal solutions prepared by the above methods contain some soluble impurities andexcess of electr olytes; these have to be removed to obtain pure sols.

Generally a) Dialysis, b) ultra f iltration and c) ultra centrif ugation are the techniques used

Ultra f iltration: colloidal sols are f iltered thr ough ultra-f iltrers. pore size of f ilter paper is

decreased such that it will restrict the passage of colloidal particles.

Ultra-centrif ugation: Centrif ugation is carried out at very high speeds such that thecolloidal particles settle down at the bottom of the tube and the impurities remain in thesolution .

It is important to note that above methods to purif y colloidal solution do not pr oduce100% solution.



Figure 3: Electr o-dialysis

used for purif ication of blood in case of kidney f ailure.



Figure 4: Size dependent change of colour in Au sol

Pr operties of colloids:

a) Optical pr operties



Figure 6: Schematic drawing of ultra micr oscope

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



b) Colligative Pr operties

The magnitudes of these pr operties for colloidal solutions are much smaller than thoseobtained for true solutions. They exhibit measurable osmotic pressures. This pr operty isused for the determination of the average molecular masses of the colloids.

c) Kinetic pr operties

liquid undergoes continuous chaotic and random motions. This motion of the particlesis called Br ownian motion. This Br ownian motion is found to decrease by increase inparticle size or by increase in viscosity of medium.

r = (3b/ 4 nd)

where, b = number of grams of substance per dm3n = number of particles observed in view = volume dm3d = density of dry substance



c) Electrical pr operties

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

i) Due to presence of electr olytesEx: positive charge on f erric hydr oxide sol prepared by hydr olysis of f erric chloride is dueto adsorption of Fe3+ ions on sur f ace.

ii) dissociation of molecular electr olytes adsorbed on the sur f ace of particles.Ex: H2S molecules get adsorbed on sulphides during precipitation. H2S undergoesdissociation and the hydr ogen ions are lost. The particles become negatively charged dueto (S2-) which are lef t on the colloidal particles.

iii) by dissociation of sur f ace moleculesEx: soaps, pr oteins



Figure 7: Electrical double layer

The influence of net charge decreases with distance and so the number of oppositely

charged ions, equa

ling the number

of ions

of both charges prevai

ling e

lectr

oneutra

lity.



The diff erence in potential between the sur f ace of shear plane and electr o neutral regionof solution is called Zeta potential (), which is given by Helmholtz- Smoluchowskiequation.

= 4u/r

where = viscosity of dispersion mediumr = relative permittivity of dispersion mediumu = mobility of colloidal particle



In case of lyophobic colloids;

(i) Coagulation is br ought about by ions having opposite charge to that of the sol.(ii) The eff iciency of an ion to cause coagulation depends upon the square of its valency.Thus eff icacy of cations Al3+>Mg2+> Na+ and PO4

3->SO42->Cl-.

(iii)The minimum concentration of an electr olyte 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 dial

ysis and by mechanical

means.

In case of lyophilic sols,i) water of solvation (cage of water surr ounding) prevents particles f r om coagulation.ii) Their coagulation is due to removal of salvation water a s electr olytes binds them.Thus eff icacy of cations Al3+>Mg2+> Ca2+> Ba2+>Na+>K+ and eff icacy of anions PO4

3-

>SO4

2->Cl->NO3

-> ClO3

->I- for coagulation of lyophilic sols and this is known as lyotr opicseries or Hof meister series.iii) Zsigmondy quantif ied stabilizing action of pr otective colloids by gold number, which isdef ined as largest number of milligrams of a pr otective colloid when added to 10 ml of standard gold sol, just f ails to prevent the colour change f r om red to blue upon additionof 1 ml of 10% sodium chloride solution



Pr operties of colloids:

Purif ication of water by alum (coagulation)

In rubber platting

In tanning

Artif icial rains

Formation of deltas (coagulation)

Blood clot formation

Colloidal medicine: Argyr ol and pr otargyr ol are colloidal solution of silver and are usedas eye lotions .Colloidal sulphur is used as disinf ectant and colloidal gold, calcium and

ir on are used as tonics. A wide variety of medicines are emulsions.

Coating of Photographic plates

Sewage disposal

Metallurgy



Figure 8: Schematic diagram of Cottrell smoke precipitator

Smoke precipitation (Coagulation)

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