10
COLLOIDS
10.1 INTRODUCTION - SOME QUICK REVISION
You should recall that there are two common types of mixtures - homogeneous mixtures and
heterogeneous mixtures.
Homogeneous mixtures are more commonly called solutions, and are composed of two or
more components - solutes and solvents. The particles which make up the mixture are molecular in
size, and the solute is uniformly distributed throughout the solvent. It will not settle out with time, and
the solute and solvent can only be separated by some physical means such as evaporation.
Heterogeneous mixtures are commonly called suspensions, with the dispersed particles being
much larger in size (e.g. fine sand in water). The particles are initially suspended fairly evenly
throughout the suspending medium, but after a small period of time they settle out onto the bottom of
the container. The particles may be separated by simple techniques such as filtration.
10.2 C OLLOIDS - A THIRD TYPE OF M IXTURE
If we consider homogeneous mixtures and heterogeneous mixtures to be the two opposite extremes,
then what about those mixtures which contain particles which are made up of many molecules, but
still not big enough to settle out? These intermediate types of mixtures are called colloids or
sometimes colloidal dispersions. They are simply defined as mixtures whose dispersed particles are
larger than solutions, but not large enough to settle out. In terms of size this generally means they
range between 10-6 and 10-4 meters in diameter, although this only a rough approximation - smaller
and larger examples exist.
In a colloid the particles are known as the dispersed phase (which is the equivalent of the
solute in a solution), whilst the medium in which the particles are suspended is called the dispersing
medium (the equivalent of the solvent in a solution).
Important examples of colloids include blood, colloidal clay, ink, homogenised milk, paint and
many others.
Some solutions with very large solvent particles exhibit colloidal behaviour. This is because
even though their solute particles consist of one molecule, they are so large that they behave like
colloidal particles. Examples include large dye molecules and those of some polymers.
10.3 HOW TO R ECOGNISE COLLOIDS
The properties of colloids are very different from both solutions and suspensions. This allows to be
easily identified. All colloidal solutions:
 scatter light (known as the Tyndall Effect)
 have particles which remain in constant motion (called Brownian Movement)
 may be destroyed by very fine filtration, or techniques which force the colloidal particles to clump
together.
The Tyndall Effect
If we fill a beaker with a true solution (such as salt in water), and we fill another beaker with a
transparent colloid (such as colloidal clay), then the two solutions will appear visually homogeneous
to the naked eye. The colloidal solution, will however appear somewhat cloudy in strong light. This is
due to the colloidal particles being large enough in size to scatter the light shining into the beaker into
all directions. The same effect occurs when the projector beam is visible at the cinema or when car
headlights shine through a foggy or dusty atmosphere. Here the colloid is dust or smoke (dispersed
phase) in air (the dispersing medium). Molecular particles are too small to do this, so it is an easy
method for identifying a colloidal fluid.
10. Colloids
PRACTICAL WORK
1. Obtain 20% NaCl solution, a 20% potassium nitrate solution and a 6% starch solution.
2. Place 400mL of each solution into separate beakers.
3. In a semi-darkened room shine a strong light source with a focussed beam through each
solution.
4. Note your observations and identify which solution if any is colloidal.
Brownian Movement
When colloidal sized particles in solution are examined
under a microscope, each particle can be seen to move
in an erratic zigzag fashion and never comes to rest.
What causes this? You may recall from Chapter ? that
the particles in any liquid or gas are in constant random
motion, moving about in an erratic
fashion. As the dispersing medium is generally a liquid
or a gas its particles move in this fashion, and constantly
collide with the colloidal sized particles suspended in it.
As millions of these collisions occur every second, the
colloidal particles are kept constantly moving in a
random pattern. The smaller the size of the colloidal
FIGURE 10.1 - Brownian Movement
particles, the greater the movement. This is because
smaller particles have less mass and therefore are moved a greater distance by collision with the
molecules of the dispersing medium.
Several factors increase Brownian Movement. Increasing the temperature increases the effect
(as the molecules in the dispersing medium have more kinetic energy, and are travelling at a greater
velocity), and increasing viscosity decreases the effect.
10.4 PHYSICAL PROCESSES INVOLVING C OLLOIDS
Preparing and Stabilising colloids
As we know that colloidal particles are intermediate in size between molecular particles and those
found in suspensions it would seem obvious that there are two ways of preparing a colloid - either
breakdown larger particles to the appropriate size (called the dispersion method) or build up smaller
ones (called the condensation method). In practice both methods are used.
An example of the dispersion method is the production of Indian ink. Here carbon powder, gum
and water are ground in a special mill which reduces the particles to colloidal size. Another process
called peptization involves shaking a small amount of a precipitate with a few drops of en electrolyte
such as HCl. An example of this is the formation of aluminium hydroxide colloid used in some antacid
drinks. Other methods which use electrical and chemical methods for decreasing particle size are also
used.
The condensation method is less commonly used than the dispersion method. Chemical and
physical condensation methods are both used, with the most common being the mixing of two
immiscible solvents. An example of this is the formation of a colloidal sulfur solution. Sulfur is
soluble in ethanol (alcohol), but insoluble in water. If sulfur is dissolved in ethanol, then poured into
water, the sulfur particles precipitate out at colloidal size in the water ethanol mixture.
Destroying (destabilising) Colloids
There are several methods for destroying colloidal solutions, including chemical, mechanical and
electrical techniques.
Chemistry 2
10.2
10. Colloids
Most techniques for destroying colloids take
advantage of the fact that colloidal particles often
have a layer of electrical charge around them.
Initially colloidal particles are neutral, but their
size allows them to readily adsorb ions onto their
surface. Another layer of ions of opposite charge
pair up with these to make the colloidal particles
electrically neutral, but their outer surface is coated
with ions of similar charge. The layer of similar
charge means that the outside of the colloidal
FIGURE 10.2 - Repulsive forces between colloid
particle may be positive or negative - depending
particles
on the type of ion adsorbed onto the surface. This
ensures that the colloidal particles do not join together as the similar charges on their surface mean
they repel one and other (called electrostatic repulsion). Methods for destroying colloids overcome
the electrostatic repulsion, and make the particles join up to form new particles too big to be
suspended. This allows separation of the dispersed phase from the dispersing medium by filtration or
precipitation. Common methods for performing this include:
 electrostatic precipitation
 coagulation
 electrophoresis
 dialysis.
Electrostatic precipitation is an industrial technique. The most common use of this is to filter
colloidal smoke particles out of air, allowing the reduction of solid fallout from smoke stacks. It is
very important in reducing air pollution. The technique uses layers of fine wires which are stretched
out across the outlet of the smoke stack. Both positive and negatively charged wires are used. As the
colloidal smoke particles are charged, they are attracted to the fine wires where they are held on by
electrostatic attraction. In time large non-colloidal particles are formed by all of the smaller ones
sticking together. These may then be removed from the wire by a simple shaking procedure. This is
shown in figure 10.3.
Coagulation is very similar to electrostatic precipitation at the molecular level, but occurs in
solution. When the particles of a colloidal solution become large enough to settle out, the solution is
said to be coagulated. This process can be achieved through several means including the addition of a
high concentration of electrolyte or heating.
If a solution containing very large concentrations of ions (a strong electrolyte) is added to a
colloidal solution, the ions remove the charges on the surface of the colloidal particles and allow them
to coalesce - resulting in precipitation. This is what happens at the mouth of river flowing into the
ocean. The fresh river water is often a colloidal suspension of clay and soil. The high concentration of
ions in the sea water however, destroys this colloid and causes the precipitation of much soil and clay
in the river mouth forming a muddy area called a delta.
Another example of coagulation is the laboratory technique known as ‘digestion’. Here very
fine colloidal precipitates are forced to form suspensions by gently boiling them for extended periods.
This allows them to be filtered by normal filter papers, and their mass determined. The heat from the
boiling process gives the particles greater kinetic energy, causing them to crash into one and other
often, overcoming the electrostatic repulsions caused by their charged outer surface.
Electrophoresis is another method for destroying colloids which uses electrical separation
techniques. It is like a solution version of the electrostatic precipitator. Here positive and negative
electrodes are inserted into the dispersing medium, and the charged colloidal particles are attracted to
the electrode of opposite charge. At the electrode they coalesce and form a precipitate. This is a very
important technique in biological sciences.
Chemistry 2
10.3
10. Colloids
high DC voltage
soot free gas escape
charged electrodes
soot laden smoke
inlet
earth point
removal of soot particles
Figure 10.3 - A typical electrostatic precipitator
Dialysis is a mechanical form of colloid destruction which uses a very fine filter system to separate
out particles of molecular size (as found in the dispersing medium) from those of colloidal size (as
found in the dispersed phase). Normal filter papers allow colloidal particles to pass through, as their
pores are too large, but special filters (called dialysis membranes) can be used to separate them out.
Examples of materials which will act as dialysis membranes include cellophane, gelatin, parchment
and specially treated filter papers. Dialysis membranes form the basis of the ‘kidney machine’ used to
remove poisonous substances from the blood of people who have kidney disease (see below).
10.5 THE IMPORTANCE OF C OLLOIDS
Many important colloids use water as the dispersing medium. These colloids can be classified as either
hydrophobic (water hating) or hydrophilic (water loving). Hydrophilic colloids have dispersed phase
particles whose surface is covered in polar water soluble groups. Examples of this are blood and
starch. Hydrophobic colloids contain particles with little affinity for water, meaning that they must be
stabilised in some way to keep the colloidal particles from coalescing. An example of this is
mayonnaise which is a dispersion of oil in water. The oil droplets are maintained at colloidal size by
adding egg white which acts like a detergent to suspend them in solution. The egg white is referred to
as an emulsifying agent as it allows the emulsion to be stabilised and stops the oil droplets from
reforming a separate oil layer.
Different Types of Colloids
Colloids are classified according to the nature of their dispersed phase and dispersing medium. Table
10.1 lists the common types of colloids, the nature of their dispersed phase and dispersing medium,
and some important examples.
Chemistry 2
10.4
10. Colloids
TABLE 10.1 Examples of some important colloids
Colloid Type
aerosol
Dispersed
Phase
liquid
Dispersing
Medium
gas
Examples
fog, aerosol sprays
aerosol
foam
emulsion
sol
solid
gas
liquid
solid
gas
liquid
liquid
liquid
smoke
whipped cream
milk, mayonnaise
paint, blood, starch
solid foam
solid emulsion
solid sol
gas
liquid
solid
solid
solid
solid
marshmallow, foams
cheese, butter
ruby glass
Using Colloids and Their Properties
You may deduce from table 10.1 that many of the substances regularly used in our daily lives are
colloidal in nature. Many process and service industries make colloids, or use their properties to derive
material benefits.
One of the most important is the kidney machine which uses the colloidal properties of blood to
allow it to be purified. Blood is a colloid of mostly red and white blood cells (the dispersed phase) in a
0.9% salt solution (the dispersing medium) which is often called the plasma. In a healthy human body
most of the toxins which are produced are released into the blood plasma then filtered out by the
kidneys, and finally excreted in the urine. If the kidney fails however, they must be removed by other
means, or the person dies. The kidney machine takes the blood and circulates it through dialysis tubing
which is bathed in a 0.9% salt solution. This means that any toxic substances in the blood move out of
the blood and into the kidney machine by diffusion. The blood cells however, being colloidal cannot
pass through the dialysis membrane and are returned to the body along with the purified blood plasma.
WHAT YOU NEED TO BE ABLE TO DO
 compare and contrast the properties of homogeneous mixtures and heterogeneous mixtures with
colloids
 describe methods of identifying colloids
 outline methods for preparing and destroying colloids
 describe some commercially significant colloids and give details of their dispersed phase and
dispersing medium
Chemistry 2
10.5