COLLOIDS, SOLS, FOAMS, EMULSIONS, SOAPS, DETERGENTS and EMULSIFYING AGENTS,
Chemistry Notes: Colloids, sols, foams, emulsions, emulsifying
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
Doc Brown's chemistry exam study
revision notes: There are various sections to work through and all are
suitable for UK GCSE and IGCSE level and US grade 9 and grade 10
Water chemistry notes index
Colloids - Sols, foams, emulsions, paints and pigments
- A colloid
consists of one substance (or mixture of substances) very finely
in 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 phase.
- 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
- That is the fine dispersion of one substance in
another without a new solution forming:
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 solution.
- 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 or
- 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 purpose.
- 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.
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
- 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
- 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 negative.
- The situation is complicated further because the
charged colloid particles attract a sheath of oppositely charged ions
- This is called the electrical double layer
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 solution.
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
allows the colloid particles to
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 power:
- positive cations: Al3+ > Mg2+
- negative anions: [Fe(CN)6]3-
- and this explains why aluminium sulphate Al2(SO4)3
is used to precipitate (coagulate) colloidal clay in water treatment for
domestic water supplies of potable water.
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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 'runny' paint).
- 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
- 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
emulsifying agent 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 effect).
- 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!
emulsifying agent 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.
Water cycle, treatment, pollution
2. Colloids - sols, foam and
emulsions (this page)
Hard and soft water - causes and treatment
Gas and salt solubility in water and solubility
Calculation of water of crystallisation