Colloids
The matter, with the particle size ranging from approximately 1 nm to 100 nm is said to be colloidal state. Colloids
are mixtures in which the dispersed particles do not settle out and the particles can pass through the hole of the filter
paper (not filterable), but not through a semipermeable membrane. All living tissue is colloidal (many individual
molecules and larger biological molecules are colloidal in size). Between a true solution and a suspension are to be
found the colloidal system A true solution such as one of sugar or a salt in water, the particles of the solute
distributed in the solvent consist of single molecule or ion. On the other hand, a suspension has large dispersed
particles that are large enough to be seen by the naked eye and the particles do not pass through any membrane. It is
then a heterogeneous mixture which settles out on standing and its components can be separated by filtration.
Application of colloids
In industry, colloid science is important in the manufacture of paints, ceramics, plastics, textiles, photographic paper
and films, glues, inks, cements, rubber, leather, dressings, jellies, adhesives and other products.
Colloid and its Basic Behaviors
Dispersed phase and dispersion medium
Dispersed phase:
phase forming the particle
Dispersed medium: medium in which the particles are distributed or dispersed
The important characteristic of the colloidal state is the presence of particles larger than molecules but not large
enough to be seen in the microscope.
Comparative properties of solutions, colloids and suspensions
Particle size
The type of dispersion
system
particles
< 1nm
True solutions
Ions and
molecules
Sol
1-100 nm
Colloidal
System
suspension
>100 nm
1
Colloidal
particles
Macromolecular
solution
Macromolecules
Associated
colloid
micelles
Coarse
particle
properties
example
Be homogeneous
Dispersed particles diffuse rapidly
Pass through both filter paper and a
semipermeable membrane
Be heterogeneous
Dispersed particles diffuse rapidly
Pass through both filter paper and a
semipermeable membrane
NaCl
NaOH
C6H12O6
Be homogeneous
Dispersed particles diffuse slowly
Pass through filter paper but not a
semipermeable membrane
Be homogeneous
Dispersed particles diffuse slowly
Pass through filter paper but not a
semipermeable membrane
Be heterogeneous
Dispersed particles diffuse slowly
Don’t pass holes of filter paper or a
semipermeable membrane
Iron( III)
hydroxide
sol
Silver
iodide sol
proteins
Sodium
stearate
Latex ,
river silt
Types of Colloidal Systems
Colloids are characterized according to the state (solid, liquid, or gas) of the dispersed phase and of the dispersion
medium. There are eight types of colloidal systems. A list of the eight types with examples is given in the following
table.
1
2
3
4
5
6
7
8
Dispersion medium
Gas
Gas
Liquid
Liquid
Liquid
Solid
Solid
Solid
Dispersed phase
Liquid
Solid
Gas
Liquid
Solid
Gas
liquid
solid
Name
Aerosol
Aerosol
foam
Emulsion
Sol
Foam
Gel
Solid sol
Example
Fog ,cloud, spray
Smoke , dust
Whipped cream
Milk, cosmetic lotion, Mayonnaise
Paints, detergents, gold sol
Pumice(light rock full of air spaces), Foam Rubber
Jellies, gelatin
Ruby glass
Lyophopic and lyphilic Colloids
Colloidal solution with a liquid dispersion medium i.e. sol may be divided roughly into two categories,
referred to as Lyophilic: Lyophilic colloids are liquid loving colloids (Lyo means solvent and philic means
loving).
Lyophopic: Lyophopic colloids are liquid hating colloids (Lyo means solvent and phobic means hating).
Examples
Lyophilic: Sols of organic substances like gelatin, gum, starch and proteins.
Lyophopic: Sols of inorganic substances like Iron (Fe(OH)3) and Platinum.
If water is dispersion medium, the terms hydrophobic and hydrophilic, respectively are used.
Lyophilic Colloids
Prepared easily by directly mixing with the liquid
dispersion medium.
They are quite stable and are not easily precipitated or
coagulated.
Lyophopic Colloids
Cannot be prepared directly. Prepared by special
methods only.
They are easily precipitated by addition of a small
amount of a suitable electrolyte.
Specific Surface Area
In a true solution the system consists of one phase only and there is no true surface of separation between the
molecular particles of the solute and solvent. Colloidal solutions, however, are two phase systems, and for each
particle there is a definite surface of separation between the colloid and the liquid medium. When two immiscible
substances are in contact, the contact surface is called interface.
The essential properties of colloidal solutions can be described to the fact the ratio of the surface area to the volume
of the particles is very large. The ratio of the surface area to the volume of the particles is called the specific surface
area and represents the degree of the dispersion.
S0 = S/V
Where S0 is the specific area; S is the total area of the particles; V is the volume of the particles
2
Surface Tension
Molecules within a liquid are pulled in all directions. There is no tendency for them to be pulled in one way.
However, the molecules at the surface are pulled downward and sideways but not upward. These attractions tend to
pull molecules in to the liquid and cause the surface to behave as if it were tightened like an elastic film.
A measure of the elastic like force existing in the surface of a liquid is surface tension, symboled σ or
γ.
The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive
forces between molecules
Definition: The surface tension of a liquid is the amount of energy required to stretch or increase the
surface by unit area
.
Adsorption
An important property of a surface is known as adsorption; this term is used to describe the existence of a higher
concentration of any particular substance at the surface of a liquid and solid than is present in the bulk.
Sol
Formation of the Colloidal Particles
Many substances, particularly those of high molecular weight forming lyophilic sols, pass into colloidal solution
when they are warmed with a suitable dispersion medium. Instances of the preparation of the colloidal solutions in
this manner are gelatin and starch in water and rubber in benzene. Lyophopic sols have generally to be prepared by
special methods which yield particles of the appropriate size.
The Properties of Sol
A. Optical Behaviors of Sol: Tyndall effect
If a light is allowed to pass through a true solution, some of the light will be absorbed, and some will be
transmitted. The particles in a solution are not large enough to scatter the light. However, if light passed
through a colloid, the light is scattered by the larger colloidal particles, we do not see the particles
themselves, and rather we see the light that is scattered by them.
B. Electric behaviors of Sol: Electrophoresis
One of the most important properties of dispersed colloidal particles is that they are usually electrically
charged. The charges in the colloidal particles result from the adsorption of ions that exist in the dispersion
medium. Colloids selectively adsorb ions on their surface. If a colloid adsorbs ions, it becomes a negatively
charged colloid. If a colloid adsorbs positive ions, it becomes a positively charged colloid.
If an electric potential is applied to a colloid, the charged colloidal particles move toward to oppositely
charged electrode this migration is known as electrophoresis.
Electrophoresis is the movement of charged particle relative to liquid it suspends in under the influence of
an applied electric field.
Stability of Sol particles
Why are colloidal dispersion is stable? The answer can be found for sols in both their electric charge and
their extensive salvation.
3
Macromolecule
The chief macromolecule, for example, starch, proteins etc, naturally occurring, and their colloid properties
are biological importance. One of the most striking differences between sols and macromolecular solutions
is the high viscosity of the latter.
Macromolecules
Sols
Homogeneous
Heterogeneous
High viscosity and osmotic pressure
Low viscosity and osmotic pressure
Stable systems
Unstable system (aggregating of particles and
sedimentation )
A strong attraction between the dispersed phase A lack of attraction between the dispersed phase and
And dispersed medium.
the dispersed medium
Without Tyndall effect
Tyndall effect
Small amount of electrolytes have no
The addition of small amount electrolytes to sols
coagulation effect
results in precipitation of the dispersed substance.
QUESTIONS
1.
2.
3.
4.
5.
6.
4
Classify the following colloids according to the type and give the name of the colloid
a. Gold in water
b. Soap foam
c. Clouds
d. Mayonnaise
e. Whipped cream
Define terms , dispersed phase, dispersed medium, sol, lyophilic, and lyophopic,
What are characteristic properties of colloidal solutions, how do they differ from true solutions?
What is meant by the surface tension and the specific surface area?
Describe the phenomenon of electrophoresis.
What are factors determine the stability of sol? How does it differ from a macromolecule?