COLLOID CHEMISTRY

MD. KHAIRUL ISLAM

HISTORICAL BACKGROUND

Thomas Graham (1861) observed that crystalline substances such

as sugar, urea, and sodium chloride passed through the membrane,

while others like glue, gelatin and gum arabic did not. The former he

called crystalloids and the latter colloids (Greek, kolla = glue ; eidos =

like). Graham thought that the difference in the behavior of

‘crystalloids’ and ‘colloids’ was due to the particle size. Later it was

realised that any substance, regardless of its nature, could be

converted into a colloid by subdividing it into particles of colloidal

size.

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1°A True solution 10ºA Colloidal solution 2000ºA Suspension

DEFINITION

o A colloid is a type of chemical mixture in which

one substance is dispersed evenly throughout

another – homogenous mixture.

o The substance distributed as the colloidal

particles is called the Dispersed phase. The

second continuous phase in which the colloidal

particles are dispersed is called the Dispersion

medium.

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COLLOIDAL SYSTEM CONSISTS OF TWO

SEPARATE PHASES

1. a dispersed phase or also known as internal

phase (for solute)

2. a continuous phase or dispersion medium (for

solvent)

A colloidal system may be solid, liquid, or

gaseous.

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CLASSIFICATION OF COLLOIDS

Medium / Phases Dispersed Phase

Gas Liquid Solid

Continuous

Medium

Gas

NONE

(All gases are

mutually

miscible)

Liquid Aerosol

Examples: fog,

mist, hair

sprays

Solid Aerosol

Examples:

smoke, cloud,

air particulates

Liquid

Foam

Example:

whipped cream

Emulsion

Examples: milk,

mayonnaise,

hand cream

Sol

Examples:

pigmented ink,

blood

Solid

Solid Foam

Examples:

aerogel,

styrofoam,

pumice

Gel

Examples: agar,

gelatin, jelly,

silicagel, opal

Solid Sol

Example:

cranberry glass

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TYPES OF SOL

Sols are colloidal systems in which a solid is dispersed in a liquid.

These can be subdivided into two classes :

(a) Lyophilic sols (solvent-loving)

(b) Lyophobic sols (solvent-hating)

Lyophilic sols are those in which the dispersed phase exhibits a definite

affinity for the medium or the solvent. The examples of lyophilic sols are

dispersions of starch, gum, and protein in water.

Lyophobic sols are those in which the dispersed phase has no attraction for

the medium or the solvent.The examples of lyophobic sols are dispersion of

gold, iron (III) hydroxide and sulphur in water.

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CHARACTERISTICS OF LYOPHILIC AND LYOPHOBIC SOLS

Some features of lyophilic and lyophobic sols are listed below-

Ease of preparation

Charge on particles

Solvation

Viscosity

Precipitation

Reversibility

Tyndall effect

Migration in electronic field

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COMPARISON OF LYOPHILIC AND LYOPHOBIC SOLS

Lyophilic Sols Lyophobic Sols

Prepared by direct mixing with dispersion medium Not prepared by direct mixing with the medium

Little or no charge on particles. Particles carry positive or negative charge

Particles generally solvated. No solvation of particles

Viscosity higher than dispersion medium; set to a

gel

Viscosity almost the same as of medium; do not

set to a gel

Precipitated by high concentration of electrolytes Precipitated by low concentration of

electrolytes

Reversible. Irreversible.

Do not exhibit Tyndall effect. Exhibit Tyndall effect.

Particles migrate to anode or cathode, or not at all. Particles migrate to either anode or cathode.

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PREPARATION OF SOLS

Lyophilic sols may be prepared by simply warming the solid with

the 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 :

(a) Dispersion Methods in which larger macro-sized particles are

broken down to colloidal size.

(b) Aggregation Methods in which colloidal size particles are built

up by aggregating single ions or molecules.

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DISPERSION METHODS

Mechanical dispersion using Colloid mill: (Ex. Colloidal

graphaite, printing inks)

Bredig’s Arc Method: (used for preparing hydrosols of metals e.g., silver, gold and platinum)

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DISPERSION METHODS (CONTD)

By Peptization: (ferric hydroxide )

[The dispersal of a precipitated material into colloidal solution by the action of

an electrolyte in solution, is termed peptization. The electrolyte used is called a

peptizing agent. Peptization is the reverse of coagulation of a sol]

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AGGREGATION METHODS

Double Decomposition

As2O3 + 3H2S ⎯⎯ As2S3 (sol) + 3H2O

Reduction

AgNO3 + tannic acid ⎯⎯ Ag sol AuCl3 + tannic acid ⎯⎯ Au sol Oxidation

2H2S + SO2 ⎯⎯ 2H2O + S

Hydrolysis

FeCl3 + 3H2O ⎯⎯ Fe(OH)3 (red sol)+ 3HCl

Change of Solvent

When a solution of sulphur or resin in ethanol is added to an excess of water,

the sulphur or resin sol is formed owing to decrease in solubility. The substance

is present in molecular state in ethanol but on transference to water, the

molecules precipitate out to form colloidal particles.

4/15/2016 Md. Khairul Islam, Lecturer, ACCE, R.U.

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PURIFICATION OF SOLS

Dialysis

Electrodialysis

Ultrafiltration

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PROPERTIES OF SOLS Optical properties

Sols exhibit Tyndall effect

Ultramicroscope shows up the presence of individual particles

Sol particles can be seen with an Electron microscope

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PROPERTIES OF SOLS (CONTD)

Kinetic properties

Brownian Movement

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PROPERTIES OF SOLS (CONTD) Electrical properties

The sol particles carry an electric charge

Electrophoresis

Electro-osmosis

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ELECTRICAL PROPERTIES OF SOLS (CONTD)

Coagulation or Precipitation: The flocculation and settling down of the

discharged sol particles is called coagulation or precipitation of the sol.

The coagulation or precipitation or destabilization of a given sol can be brought

about in four ways

By addition of electrolytes

By electrophoresis

By mixing two oppositely charged sols

By boiling

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ELECTRICAL PROPERTIES OF SOLS (CONTD)

Protective action of sols: The property of lyophilic sols to prevent the

precipitation of a lyophobic sol is called protection.The lyophilic sol used to

protect a lyophobic sol from precipitation is referred to as a Protective

colloid. If a little gelatin (hydrophilic colloid) is added to a gold sol

(hydrophobic sol), the latter is protected. The ‘protected gold sol’ is no longer

precipitated on the addition of sodium chloride. Because the lyophilic colloid

forms a coating around the lyophobic sol particles.

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 hydrophilic colloid that will just prevent the

precipitation of 10 ml of a gold sol on the addition of 1 ml of 10 per cent

sodium chloride solution.

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ORIGIN OF CHARGE ON SOL PARTICLES

Adsorption of ions from the aqueous medium

Ionisation of surface groups

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STABILITY OF SOLS

Presence of like charge on sol particles

Presence of Solvent layer around sol particle

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ASSOCIATED COLLOIDS

Substances whose molecules aggregate

spontaneously in a given solvent to form

particles of colloidal dimensions are called

Associated or Association Colloids.

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EMULSIONS

An emulsion may be defined as a dispersion of

finely divided liquid droplets in another liquid.

There are two types of emulsions.

(a) Oil-in-Water type (Milk)

(b) Water-in-Oil type (Ex- Stiff greases)

Preparations

Properties

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GELS

A gel is a jelly-like colloidal system in which a liquid is dispersed

in a solid medium.

Elastic gels (Gelatin, starch and soaps )

Non elastic gels (silica gel)

Properties of Gels

Hydration

Swelling

Syneresis

Thixotropy

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APPLICATIONS OF COLLOIDS

Foods

Medicines

Non-drip or thixotropic paints

Electrical precipitation of smoke

Clarification of Municipal water

Formation of Delta

Artificial Kidney machine

Adsorption indicators

Blue colour of the sky

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