1. COLLOIDS
Mohammad Safiqul Islma
NSTU

2. • Classification Based on the State of the Dispersed
Phase and Dispersion Medium
• The below table lists various types of colloids with
various examples.
• Types of colloids
• Dispersion Medium Dispersed Phase Type of
colloid Example Gas Liquid Aerosol Fog,
mist Gas Solid Aerosol Smoke Liquid Gas Foam
Whipped
cream Liquid Liquid Emulsion Mayonnaise, 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 (glass with
dispersed metal)

3. Classification Based on the State of the
Dispersed Phase and Dispersion Medium
Dispersion Medium Dispersed Phase Type of colloid Example
Gas Liquid Aerosol Fog, mist
Gas Solid Aerosol Smoke
Liquid Gas Foam Whipped cream
Mayonnaise, hair
Liquid Liquid Emulsion
cream
Liquid Solid Sol Paints, cell fluids
Solid Gas Foam Pumice, plastic foams
Solid Liquid Gel Jelly, cheese
Ruby glass (glass with
Solid Solid Solid Sol
dispersed metal)

4. • Fog and smoke are aerosol which are liquid
droplets or solid particles dispersed throughout a
gas. When liquid droplets are dispersed
throughout another liquid phase this results in
emulsion, as in the case of butterf at dispersed
throughout homogenized milk. A sol consists of
solid particles dispersed in a liquid. Foam consists
of gas being dispersed in a liquid phase as in the
case of whipped cream.
• Out of the various types of colloids, sols, gels and
emulsions are very common. In the later sections
'sols' and 'emulsions' are discussed in some detail.

5. Classification Based on the Nature of
Interaction Between Dispersed Phase and
Dispersion Medium
• Colloidal systems, depending on the nature of
attraction between the dispersed phase and the
dispersion medium are classified into lyophobic
(solvent hating) and lyophilic (solvent loving). If
water is the dispersion phase is water, then the
colloids are either hydrophilic or hydrophobic.
• 1) Lyophilic colloids
• In this type of colloids sols, the dispersed phase has
great attraction for the dispersion medium. In such
colloids, the dispersed phase does not precipitate
easily and the sols are quite stable. If the dispersion
medium is separated from the dispersed phase, the
sol can be reconstituted by simply remixing with the
dispersion medium. Hence, these sols are called
reversible sols.

6. • Examples of lyophilic sols include sols of gum,
gelatine, starch, proteins and certain polymers in
organic solvents
• 2) Lyophobic colloids
• In this type of colloidal sols, the dispersed phase has
little affinity for the dispersion medium. These colloids
are easily precipitated on the addition of small
amounts of electrolytes, by heating or by shaking and
therefore are not stable. Once precipitated, it is not
easy to reconstitute the sol by simple mixing with the
dispersion medium. Hence, these sols are called
irreversible sols. Examples of lyophobic sols include
sols of metals and their insoluble compounds like
sulphides and oxides. Lyophobic sols need stabilizing
agents to keep the dispersed phase from precipitating
out.

7. • Hydrophobic sols are often formed when
rapid crystallization takes place. With rapid
crystallization, many centres of
crystallization called nuclei are formed at
once. Ions are attracted to these nuclei
and very small crystals are formed. These
small crystals are prevented from settling
out by the random thermal motion of the
water molecules.

8. Classification of Colloids Based on Type of
Particles of the Dispersed Phase
• 1) Multimolecular colloids
• 2) Macromolecular colloids 3) Associated colloids.
• Multimolecular colloids
In this type of colloids the colloidal particles are
aggregates of atoms or small molecules with
molecular size less than one nanometer (1 nm).
For e.g., gold sol consists of particles of various
sizes which are clusters of several gold atoms.
Similarly, sulphur sol consists of colloidal particles
which are aggregates of S8 molecules. The
molecules in the aggregates are held together by
Van der Waal forces.

9. • Associated colloids (Micelles)
Certain substances behave as strong electrolytes at low
concentration but at higher concentrations these substances
exhibit colloidal characteristics due to the formation of
aggregated particles. These aggregated particles are called
micelles. Micelles are called associated colloids. The
formation of micelles takes place only above a particular
temperature called Kraft Temperature (Tk) and above
particular concentration called the Critical micelle
concentration (CMC). On dilution, these colloids revert back to
individual ions. Surface active molecules such as soaps and
synthetic detergents form associated colloids in water. For
soaps, the CMC is about 10-4 to 10-3 mol L-1. Micelles have
both a lyophilic and lyophobic parts. Micelles may consists of
more than 100 molecules.

10. • Micelles are formed by specific molecules which
have lyophilic as well as lyophobic ends.
Ordinary soap which contains sodium stearate
(C17H35COONa) forms micelle in water. The
stearate ion has a long hydrocarbon end that is
hydrophobic (because it is nonpolar) and a polar
carboxyl group (COO-) that is hydrophilic.

13. Aggregation of RCOO- ions to form
a micelle

14. • When the concentration of sodium stearate is
below its CMC, then it behaves as a normal
electrolyte and ionizes to give Na+ and
C17H35COO- ions. As the concentration exceeds
the CMC, the hydrophobic end starts receding
away from the solvent and approach each other.
However, the polar COO- part interacts with water.
This leads to the formation of a cluster having the
dimensions of a colloid particles. In each cluster a
large number of stearate groups clump together in
a spherical manner such that the hydrocarbon
parts interact with one another and the COO-
groups remains projected in water.

15. • CHARACTERISTICS OF LYOPHlLIC AND LYOPHOBIC
SOLS
• Some features of lyophilir and IYopho~ic sols are listed
• (J) Ease of preparation
• Lyophilic sols can be obtained straightaway by mixing the
material (starch, protein) with asuitahle solvent. The giant
molecules of the material are of colloidal size and these at
once pass into the colloidal form on account of interaction with
the sol vent.
• Lyophobic sols are not obtained by simply mixing the solid
material with the solvent.
• (2) Charge on particles:
• Particles of a hydrophilic sol may have Iitlle or no charge at
all.
• Particles of a hydrophohic sol carry positive or negative
charge which gives them stability.

16. • (3) Solvation
• Hydrophilic sol particles are, generally
solvated, TIlat is,they arc surrouned by an
adsorbed layer
• of the dispersion medium which does not
permit them to come together and coagulate.
Hydration
of gelatin is an example,
• There is no solvallon of the hydrophobic sol
particles for want of interaction wilh tbe

17. • (4) Viscosity
• Lyophilic sols are viscous as the particle size
increases due to solvation. and the proportion
of free medium decreases.
• Viscosity of hydrophobic sol is almost the
same as of the dispersion medium itself.
• (5) Precipitation
• Lyophilic sols are precipitated (or coagulated)
only by high concentration of the electrolytes
when the sol particles are desolvated,

18. • Lyophobic sols are precipitated even by Iow
concentration of electolytes, the protective layer being
absent.
• (6) Reversibility:
• The dispersed phase of lyophilic sols when separated
by coagulation or by evaporation of the medium, can
be reconverted into the colloidal form just on mixing
with the dipersion medium, Therefore this type of sols
are designated as Reversible sols.
• On the other hand, the lyophobic sols once
precipit:ued cannot be re-formed merely by mixing
with dispersion medium. These are terefore, called
Irreversible sols.

19. (7) Tyndall effect
• On account of relatively small panicle size, lyophilic
sols do not scatter light and show no Tyndall effect.
• Lyophobic sol particles are large enough to exhibit
Tyndall effect.
• (X) Migration in Electric field
• Lyophilic sol particles(proteins) migrate to anode or
cathode, or not at all, when placed in electric field.
• Lyophobic sol particles move either to anode or
cathode, according as they carry negative or positi ve
charge.
• The differences

20. 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:
• 1. Dispersion Methods in which larger macro-
sized particles are broken to colloidal size,
• 2. Aggregation Methods in which colloidal
size particles arc built up by aggregating
single ions or molecules.

21. AGGREGATION METHODS
• These methods consist of chemical reactions or
change of solvent whereby the atoms or
molecules of the dispersed phase appearing first,
coalesce or aggregate to form colloidal particles.
The conditions (temperature, concentration, cte.)
used are such as permit the formation of sol
particles but prevent the particles becoming too
large and forming precipitate. The unwanted ions
(spectator ions) present in the sol are removed by
dialysis as these ions may eventually coagulate
the sol.
• The more important methods for preparing
hydrophobic sols are listed below:

23. • Excess hydrogen sulphide (electrolyte) is
removed by passing in a stream of hydrogen.
• (2) Reduction:
• Silver sols and gold sols can be obtained by
treating dilute solutions of silver nitrate or
gold
chloride wilh organic reducing agent like
tannic acid or ethanal (HCHO)
• AgNO + tannic acid Ag sol
• AuCI) + tannic acid - Au sol

24. • (3) Oxidation
• A sol of sulphur is produced by passing
hydrogen sulphide into a solution of
sulphur dioxide.

26. • (4) Hydrolysis
• Sols of the hydroxides of iron, chromium and
aluminium are readily prepared by the
hydrolysis
of salts of the respective metals. In order to
obtain a red sol of ferric hydroxide, a few mls
of 30%
• ferricchloride solution is added to a large
volume of almost boiling water and stirred
with a glass

28. • (5) Change of Solvent
• When a solution of SlIlphur 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 slate in ethanol but
on transference to water, the molecules
precipitate out to form colloidal particles.

29. Dispersion methods
• (I) Mechanical dispersion using Colloid mill
The solid along with the liquid dispersion
medium is fed into a Colloid mill. The mill
consists of
two steel plates nearly touching each other
and rotating in opposite directions with high
speed. The solid particles are ground down to
colloidal size and are dispersed in the liquid
to give the sol.
• 'Colloidal graphite' (a lubricant) and printing

31. • Recently, mercury sol has been prepared by disintegrating a
layer of mercury into sol particles in water by means of
ultrasonic vibration.
• (2) Bredig's Arc Method
• It is used for preparing hydrosols of metals e.g .• silver, gold
and platinum. An arc is struck betwccn the two metal
electrodes held close together beneath de-ionized water. The
water is kept cold by immersing the container in ice/water
bath and a trace of alkali (KOH) is added. The intense heat of
the spark across the electrodes vaporises some of the metal
and tile vapour condenses under
• water. Thus the atoms of tile metal present in the vapor
aggregate to form colloidal particles in water. Since the metal
has been ultimately converted into sol particles (via metal
vapour), this
• method has been treated as of dispersion.

33. • (3) By Peptization
• Some freshly precipitated ionic solids are
dispersed into colloidal solution in waler by
the addition of small quantities of
electrolytes, particularly those containing a
common ion. The precipitate adsorbs the
common ions and electrically charged
particles then split from the precipitate as
colloidal particles.

35. • 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.
• Examples of preparation of sols by
peptization
• (I) Silver Chloride, Ag-Cl, can be converted
into a sol by adding hydrochloric acid (Cl
being common ion).
• (2) Ferric hydroxide, Fe(OH) , yields a sol by
adding ferric chloride (Fe' being common

36. PURIFICATION OF SOLS
In the methods of preparation stated above, the resulting
sol frequently contains besides colloidal particles
appreciable amounts of electrolytes. To obtain the pure
sol,these electrolytes have to be removed. This
purilication of sols can he accomplished by three
methods:
• (a) Dialysis
• (b) Electrodialysis
• (e) Ultrafiltration

37. Dialysis
• Removal of soluble impurities from sols by the
use of semipermeable membrane is known as
dialysis.
Solutes present in a true solution can pass
through a semipermeable membrane such as
parchment paper or cellophane. However, sol
particles cannot pass through such membranes.
When a bag made up of such a membrane is
filled with the colloidal sol and then placed in
fresh water, the soluble particles such as
electrolytes pass through the membrane and go
into the water leaving behind the colloidal sol.

38. • By using a continuous flow of fresh water, the concentration of the
electrolyte outside the membrane tends to be zero. Thus diffusion of
the ions into pure water remains brisk all the time. In this way,
practically I the electrolyte present in the sol can be removed easily.
• The process of removing ions (or molecules) from a sol
by diffusion through a permeable membrane is called
Dialysis. The apparatus used for dialysis is called a
Dialyser.
• The most important application of dialysis is in the purification of
blood with the aid of an artificial kidney machine. The dialysis
membrane permits excess ions and waste products like urea
molecules to pass through and does not allow the colloidal particles
of haemoglobin to pass through.

41. Electrodialysis
• In this process. dialysis is carried under the influence of
electric field. Potential is applied between the metal
screens supporting the membranes. This speeds up the
migration of ions to the opposite electrode. Hence
dialysis is greatly accelerated. Evidently electrodialysis is
not meant for non·electrolyte impurities like sligar and
urea.

42. Ultra-Filtration
• Sols pass through an ordinary filter paper. Its pores are
too large to retain the colloidal particles. However, if the
filter paper is impregnated with collodion or a
regenerated cellulose such as cellophane or visking, the
pore size is much reduced. Such a modified filter paper
is called an ultrafilter. The separation of the sol particles
from the liquid medium and electrolytes by filtration
through an ultrafilter is called ultrafiltration. Ultrafiltration
is a slow process. Gas pressure (or suction) is to be
applied to speed it up. The colloidal particles are left on
the ultrafilter in the form of slime. The slime may be
stirred into fresh medium to get back the pure sol. By
using graded ultrafilters, the technique of ultrafiltration
can be employed to separate sol particles of different
sizes.

44. PROPERTIES OF SOLS
• COLOUR
• The colour of a hydrophobic sol depends on the
wavelength of the light scallered by the dispersed
particles. The wavelength of the scattered light again
depends on the size and the nature of the
particles. This is in the case of silver sols.

45. The colour changes produced by varying particles size have
been observed in many other Cases
OPTICAL PROPERTIES OF SOI.S
I) Sols exhibit Tyndall effect:
When a strong beam of light is passed through a sol and is
viewed at right angle_" the path of light shows up as a hazy
beam of cone. This is due to the fact that sol particles absorb
light energy and then emit it in all directions in space.
The phenomenon of the scattering of light by the sol particles is
called Tyndall effect.
The illuminated beam or cone formed by the scattering of light by
the sol particles is often referred as Tyndall beam or Tyndall
cone.

46. • True solutions do not show the tyndall effectt. Since ions
or solute molecules arc too small to scatter light, the
beam of light passing thuough a true solution is not
visible when viewed from the side. Thus Tyndall effect
can be used to distinguish a colloidal solution from a true
solution.