2. COLLOIDS, SOLS, FOAMS, EMULSIONS,
SOAPS, DETERGENTS and EMULSIFYING AGENTS, examples explained
Colloids (e.g. sol, foam, emulsion) are described
and explained with examples. There is also a section on
'paints and pigments' e.g. explaining water-based emulsion paints or traditional oil-based paints
(Suitable for AQA, Edexcel and OCR GCSE
ALL my GCSE Chemistry Notes
Find your GCSE
science course for more help links to revision notes
mobile phone or ipad etc. in 'landscape' mode
This is a BIG
website, you need to take time to explore it [SEARCH
Colloids - Sols, foams, emulsions, paints and pigments
- A colloid
consists of one substance (or mixture of substances) very finely dispersed
another substance (or a mixture of substances) without a new true
solution forming. So a
colloid is a mixture of a dispersed phase
and a continuous phase (disperse
or continuous medium) BUT the dispersed phase is NOT dissolved in the continuous
- A colloid is NOT a solution, although the
colloid particles are not usually seen under a microscope, they are much
bigger than molecules, and much bigger than the molecules of the
continuous phase (disperse medium e.g. water).
- In a solution the solvent or solute particles are
usually of comparable size and completely mixed at the 'individual
particle level' i.e. completely homogeneous in the same phase.
- A colloid can be thought of as
intermediate between a true solution and a mixture of e.g. a liquid and
an insoluble solid. No filtration separation is possible with solutions
but filtration is easy and effective with an insoluble solid.
Similarly, most colloid particles
are too small to be filtered, but separation from truly dissolved
substances is possible with a membrane.
- The colloidal particles of the disperse
phase are equivalent to the solute of a solution and the continuous
phase is equivalent to the solvent. The mixture is sometimes referred
to as the 'colloidal solution'. These descriptors can be somewhat
'blurred' by the intermediate particulate nature of colloidal systems!
- The particles in a colloid are so small
that they remain 'suspended' (the mixture is called a 'suspension') in the disperse medium (e.g. colloidal clay
particles in water) with little tendency to settle out. However the
colloidal particles are big enough for their surface area properties
to be significant (see
- Examples of
- That is the fine dispersion of one substance in another without a new
sol is a solid dispersed in a
liquid e.g. tiny particles of clay in water (the dispersion medium). The
particles are so small and well separated and weakly bonding to the
liquid that they do not readily coagulate and separate out. You are
dealing with microscopic particles held in suspension in the fluid.
foam is a gas dispersed in
a liquid e.g. a well shaken soap solution or shaving cream foam.
emulsion is a liquid
dispersed or suspended in another liquid in the form of fine drops ...
- and is a mixture of two immiscible
liquids like oil and water, one liquid is NOT dissolved in the other
and both phases are true liquids, though the mixture is NOT a true
- Oil and water are two immiscible liquids
and would normally separate out into two layers, that is after shaking
to disperse the two sets of minute droplets in each other,
flocculation-coagulation takes place and when the drops become larger
and eventually the two layers reform.
- Emulsions are thicker than either
liquid e.g. the emulsion 'French dressing', is thicker than olive oil
- With time, the two layers settle out, so
the less dense oil floats on top of the aqueous/water layer.
- One way to inhibit the two layers
settling out, or at least to slow down coagulation of the fine droplets
of the liquids, is to use an emulsifier - a chemical agent that facilitates
emulsification and promote emulsion stability.
- An emulsifying agent stabilises an
emulsion and prevents the two immiscible liquid layers from
separating out, or at least, considerably slows the process down like in
salad dressing which does usually need shaking before use.
- Two of the most commonly used
emulsifiers are lecithin (E322) and the mono- and
di-glycerides of fatty acids (E471), and are classified as food
additives in the E number system.
- Egg yolk also acts as an emulsifying agent
(because it contains lecithin).
- Examples of emulsions.
- (i) milk (aqueous solution + insoluble,
but dispersed fats), this is an 'oil-in-water' emulsion
- (ii) French dressing in salads (based on vinegar +
olive oil), but these do reform the oil and aqueous layers quite
easily which is why they are shaken before use)
- (iii) Mayonnaise-salad cream is a mixture of
oil, water, emulsifier and other ingredients.
contain emulsifiers to stop the salty water from separating out and mayonnaise also contains an emulsifier to stop the oil and aqueous
based components separating out. Margarine is an 'water-in-oil'
- (v) Cosmetic foundation creams and
brushless shaving creams are oil-in-water emulsions. Cold creams and
cleansing creams are water-in-oil emulsions.
- One of the problems with useful
emulsions is that the two main components, the two immiscible liquids,
tend to separate out rendering the emulsion useless for its designed
- The way round this is to use an
emulsifying agent (emulsifier) which inhibits the separation of the
emulsion back into two layers.
- Emulsions are very important in food
preparations, pharmaceutical products, cosmetic preparations,
insecticide sprays, oil-based paints an water-based emulsion paints.
- All of these emulsion products need to
be stabilised by an emulsifying agent which slows down the coagulation
of the dispersed tiny drops to reform two separate layers (or phases).
- Emulsifiers are usually, what are called
'surface-active agents' or surfactants and it is these compounds
that slow down the coagulation process by reducing the tendency of the
dispersed liquid droplets to come together.
Emulsifier molecules have
a 'water loving'/'oil hating' (hydrophilic) part and a 'water
hating'/'oil loving' part (hydrophobic). So one end of an emulsifying
molecule is attracted to water and the other end attracted to oil or
fat. Therefore they can interact
with the different components and keep the different types of molecules
dispersed in each other.
Colloidal particles may be
- (Note: So
far the discussion has been confined to hydrophobic ('water hating')
colloids which do NOT interact strongly with the continuous phase.
- In contrast 'gels' for example, are
hydrophilic ('water liking') colloids, in which the colloid
particles are very solvated* and
stabilised by the continuous phase).
- Solvated means the particle is
weakly attracted to layers of surrounding 'solvent' molecules of the
dispersal medium e.g. water.
Colloidal particles of a sol absorb ions,
- but not in
electrically balanced proportions.
- Depending on which ion(s) are preferentially
absorbed from the water,
the net charge on the colloid particle can be positive or
- The situation is complicated further because the
charged colloid particles attract a sheath of oppositely charged ions
- This is called the electrical double layer effect.
This means neighbouring colloid particles have the same 'outer charge'
and so are repelled, rather than attracted together.
- The sol
itself is overall electrically neutral like any other
Colloids are destroyed when the
particles of the disperse phase join together and separate out from the
- This process is called coagulation.
- For sols,
any disturbance of the double layer can cause coagulation to happen.
- It can be caused by boiling the sol, the increased random
thermal collisions disturb the electrical balance and
colloid particles to collect together.
Sols are also very sensitive to the
presence of ions, so any electrolyte ions present can affect the
electrical double layer (the theory is complex but just think of the
ions charge as affecting the stability of the double layer). The more
highly charged the ion, the greater the electrical field
force effect, so the greater its coagulating
power. The ions reduce the repulsion between the colloid
particles and allow coagulation to occur.
- Examples of coagulating
- positive cations: Al3+ > Mg2+ > Na+
- negative anions: [Fe(CN)6]3- > SO42-
- and this explains why aluminium
sulphate Al2(SO4)3 is used to
precipitate (coagulate) colloidal clay in water treatment for domestic
TOP OF PAGE
Paints and Pigments
- Paints derive their colours from
specific pigments or mixtures of pigments to produce a whole range of
colours to suit are aesthetic taste.
- To enable the paint to be applied easily
to a surface, with the minimum of pigment to give the right intensity of
colour, paints prepared as a colloid.
- In the case of paints, these colloids
consist of tiny particles of pigment dispersed in some kind of
continuous liquid (technically this mixture is called a sol, a
- Other colloidal paint mixtures consist
of a gel, where the liquid molecules are held together by
dissolved polymer molecules, but the pigment particles are still
dispersed in the same way as any other colloid.
- The particles are so tiny they do not
readily coagulate and form a solid deposit in the liquid.
- Paints are a mixture that usually
consists of a solvent (the dispersal medium), a binding medium
(often dissolved in the solvent) and particles of pigment (the
dispersed material in the emulsion) and with modern water based emulsion
paints an emulsifying agent (maybe the binding medium itself) to
stabilise the mixture and give the can of paint a good shelf-life.
- The solvent contains the dispersed
pigment, binding medium, emulsifying agent etc. and is quite runny or an
easily spreadable gel, so that you can spread the paint easily and
evenly with a paint brush.
- The pigment consists of very
finely dispersed particles in the mixture and obviously gives the paint
its characteristic colour.
- After the solvent has evaporated
as the paint dries, the binding agent hardens and holds all
the pigment particles together to form the hard layer of completely
not shown separately
- Modern emulsion paints (above) consist
of a water-based emulsion since the solvent is usually water. The
binding agent is often a dissolved polymer like polyvinyl acetate (PVA).
After applying the paint, the water evaporates leaving the thin surface
layer of binding agent and pigment which hardens further as the paint
fully sets (the PVA molecules bind together to give the hardening
- A thin layer of water-based emulsion
paint dries quite quickly and is very convenient for painting inside
(with no solvent fumes) or outside too for that matter - and I do
appreciate non-drip gel emulsion paints!
not shown separately
- 'Older' traditional paints and 'artists
oil paint' are oil-based colloidal emulsions. In this case the binding
agent is an oil that when exposed to air hardens and cross-links to bind
the pigment particles together. Oil paint mixtures dry and set in two
stages. First the solvent evaporate to leave the oil, binding agent and
pigment particles. Then oxygen in the air, oxidises the oil which causes
the oil molecules to cross-link via covalent bonds to form a hard solid
3D matrix which holds the layer of pigment together. Lecithin, in egg
yolk, has been used in the past (and still is?) to act as both an
emulsifying agent and linseed oil as the binding agent.
- Although oil paints are glossy, hard
wearing with good waterproofing properties, they do take longer to dry.
- They are more appropriate for outdoor
painting (wood or metal), especially as they give off harmful fumes as
the solvent is evaporating.
Extra Aqueous Chemistry
Index: 1. Water
cycle, treatment, pollution
Colloids - sols, foam and emulsions (this page)
and soft water - causes and treatment
Gas and salt solubility
in water and solubility curves
Calculation of water of crystallisation
|Website content © Dr
Phil Brown 2000+. All copyrights reserved on revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
permitted. Exam revision summaries & references to science course specifications