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JEEMAIN.GURUCOLLOIDAL STATE & SURFACE CHEMISTRY
COLLOIDAL STATE :
When water soluble substances like sodium chloride, copper sulphate, sugar etc., are put into water they
dissolve and a homogeneous solution is obtained. The particles of the solute are not visible and their size is
molecular size. Such mixtures are called molecular solutions or true solutions.
Now suppose we take muddy river water or an insoluble substance like lead sulphate or calcium sulphate is put
into water. The particles of the solutes are visible even with the naked eye because their size is large. On
keeping for some time, particles settle down. These mixtures are called suspension.
In between these two extremes, there are particles which are bigger than molecules but are too small to be seen
even by a microscope. The colloidal state can thus be regarded as the intermediate state between molecules
and particles of a coarse suspension.
Thomas graham (1861) classified substances into two categories on the basis of their rates of diffusion-
Crystalloids : They diffuse rapidly in solution and can rapidly pass through animal or vegetable
membranes, e.g. urea, sugar, salts and other crystalline substances.
Colloids : They diffuse very slowly in solution and can not pass through animal or vegetable membranes,
e.g. starch, gelatin, silicic acid, proteins etc. Since this class of substances generally exist in amorphous
or gelatinous condition and hence the name colloid meaning "glue form".
N o t e : Actually every substance irrespective of its nature can be crystalloid or colloid under suitable conditions. For
example:
(i) NaCl though a crystalloid in water behaves like a colloid in benzene.
(ii) Soap is a colloid in water, while it behaves like a crystalloid in benzene.
Therefore colloidal state now a days may be defined as following "A substance is said to be in the colloidal state,
when it is dispersed in another medium in the form of ver y small par ticles having diameter between
10–4 to 10–7 cm (100 m to 1 m).
M o l ec u l a r Coa rs e
C olloid a l
s iz e in tr u e s u s pens io n
pa r tic le s iz e
s olu tion pa r tic le s iz e
1 0 –7 to 10 – 8 cm 1 0 –5 to 10 – 7 cm 1 0 –3 to 10 – 5 cm
COLLOIDAL SOLUTIONS :
They considered as a heterogeneous system consisting of the following three essential components:
(i) A dispersed phase: It is also known as discontinuous or inner phase. It consists of discrete particles
significantly larger than ordinary molecules and in this small particles of solute is diffused in solvent.
(ii) A dispersion medium or continuous phase or the outer phase : It is the medium in which
dispersed phase is present. This consists of continuously interlinked molecules.
(iii) A stabilising agent: This is a substance which tends to keep the colloidal particles apart. Some colloids
are self stabilizers.
Dispersed phase + Dispersion medium = Dispersion system (Colloidal solution)
Each of the two phases constituting a colloidal system may be a gas, a liquid or a solid. For example, in milk, the
fat globules are dispersed in water. Hence fat globules form a dispersed phase and water is the dispersion medium.
Sol : If dispersion of a solid in a liquid, solid or gaseous medium, the resulting solution is called sol.
Colloidal Solution : If dispersion of a solid (dispersed phase) in a liquid (dispersion medium), the
resulting solution is called colloidal solution.
Solid aerosol: The dispersion of a solid (dispersed phase) in a gas( dispersion medium).
Liquid aerosol : If the dispersed phase is a liquid and the dispersion medium is a gas, the resulting sol
is called a liquid aerosol.
Emulsion : When a liquid is dispersed in another liquid the resulting system is called an emulsion.
,JEEMAIN.GURU
Gel : If colloidal system becomes fairly rigid, it is termed as a gel.
CL ASSIFICATION OF COLLOIDS :
There are a number of basis for the classification of colloids.
(i) Depending upon the nature of the dispersed phase and that of dispersion medium the colloidal solutions are
divided into the following eight categories:
D is per s ed D is per s io n
S .No . Na m e E xa m pl e
Pha s e M ed iu m
Coloured glass,
1 Solid Solid Solid sol
gems, alloys
Paints, inks,
2 Solid Liquid Sol white of eggs
mud
3 Solid Gas Aerosol Smoke, dust
Curds,pudding,
4 Liquid Solid Gel
cheese, jellies
Milk, Cream,
5 Liquid Liquid Emulsion Butter',
Oil in water
Clouds, Mist,fog
6 Liquid Gas Liquid Aerosol
(water in air)
Cake, bread, lava,
7 Gas Solid Solid foam
pumice stone
Soap lather,froth
8 Gas Liquid Foam on beer, whipped
cream
Since the two gases are completely miscible with each other, they always form a true solution.
(ii) Depending upon the appearance of colloids :
On this basis colloids are divided into the following two main categories.
(a) Sol : When a colloidal solution appears as fluids, it is termed as sol. Sols are named after dispersion
medium. For example, when dispersion medium is water, they are called hydrosols when the dispersion
medium is alcohol they are called alcosols and so on.
(b) Gels : When a colloid has a solid-like appearance, it is termed as gel. The rigidity of gel varies from
substance to substance.
(iii) Depending upon the interaction of the two phases.
According to Perrin and Freundlich, colloids may be classified into lyophobic and lyophilic.
(a) Lyphobic or solvent-hating : When the dispersed phase has less affinity for the dispersion medium,
the colloids are termed as lyophobic. But when the dispersion medium is water, they are given the name
hydrophobic. Substances like metals, etc. which have particles of size bigger than the colloidal particles
or NaCl which has particles of size smaller than the colloidal size, fall in this category. Such substances
are brought into colloidal state with difficulty.
(b) Lyophilic or solvent loving : When dispersed phase has a greater affinity for the dispersion medium,
the colloids are termed as lyophilic and when the dispersion medium is water, they are given the name
hydrophilic. They are also called natural colloids, substances like proteins, starch and rubber etc. are
grouped under this category.
,JEEMAIN.GURU
(iv) Dependi ng upon t he electrical charge on t he disper sed phase :
On this basis the colloids may be divided into :
(a) Positive Colloids :
The dispersed phase carries the positive charge. The particles of Fe(OH) 3 sol in water are positively
charged. Examples of this type are also methylene blue and TiO2 sols.
(b) Negative Colloids :
This dispersed phase carries the negative charge. For example the particles of As 2S3 sol in water are
negatively charged. The other examples are copper or gold sol and certain dye-stuffs like eosin, congo
red etc.
(v) Depending on the str ucture of colloid par ticle s :
According to Lumiere and others, colloids can also be classified into molecular and micellar colloids. The particles
of molecular colloids are single macromolecules and their structure is similar to that of small molecules. The particles
of micellar colloids are aggregates of many molecules or groups of atoms which are held together by cohesive or van
der Waal's forces. The examples of molecular colloids are albumin, silicons, rubber etc. while that of micellar colloids
or sulphur, gold, soap detergents etc.
(a) Multimolecular colloids :
The multimolecular colloidal particles consists of aggregate of atoms of small molecules with diameter
less than 10–9 m or 1 nm. For example, a sol. of gold contains particles of various sizes having several
atoms. A sol.of sulphur consists of particles containing a thousand or so S2 molecules. These particles
are hold together by vandel Waal's forces. These are usually lyophobic sols.
(b) Macromolecular col loids :
The macromolecular colloidal particles themselves are large molecules. They have very high molecular
weights varrying from thousand to millions. These substances are generally polymers. Naturally occurring
macromolecules are such as starch, cellulose and proteins. Artificial macromolecules are such as
polyethylene, nylon, polysyrene, dacron, synthetic rubber, plastics, etc. The size of these molecules are
comparable to those of colloidal particles and therefore, their dispersion known as macromolecular
colloids. Their dispersion also resemble true solutions in some respect.
(c) The associated colloids or miscelles :
These colloids behave as normal electrolytes at low concentrations but colloids at higher concentrations.
This is because at higher concentrations, they form aggregated (associated) particles called miscelles.
Soap and synthetic detergents are examples of associated colloids. They furnish ions which may have
colloidal dimensions.
RCOONa
RCOO– + Na+
Sod. Stearate soap (R = C17H35)
The long-chain RCOO – ions associates or aggregate at higher concentrations and form miscelles and
behave as colloids. They may contains 100 or more molecules.
Sodium stearate C17H35COONa is an example of an associated colloid. In what it gives Na+ and sterate,
C17H35COO– ions. These ions associate to form miscelles of colloidal size.
Note : Sometimes the names Emulsoids and Suspensoids are also used for hydrophilic and hydrophobic
colloids respectively.
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