COLLOID CHEMISTRYdept.ru.ac.bd/achem/images/Khairul-all-docs/2nd_year/22.pdf · ELECTRICAL...
Transcript of COLLOID CHEMISTRYdept.ru.ac.bd/achem/images/Khairul-all-docs/2nd_year/22.pdf · ELECTRICAL...
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.
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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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