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School Chemistry Notes: Colloids, sols, foams, emulsions, emulsifying agents

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

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 science-chemistry students

Water chemistry notes index

2. Colloids - Sols, foams, emulsions, paints and pigments

  • Colloidal clay:
    • A colloid consists of one substance (or mixture of substances) very finely dispersed 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 electrical properties below).
  • Examples of colloids
    • That is the fine dispersion of one substance in another without a new solution forming:
    • A 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.
    • A foam is a gas dispersed in a liquid e.g. a well shaken soap solution or shaving cream foam.
    • An 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 vinegar
      • 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.
      • (iv) margarines 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' emulsion.
      • (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.
    • 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.
      • Diagram A represents a true solution where the black dots represent the dissolved individual molecules - they do NOT clump together.
      • oil-in-water emulsion, no emulsifier
      • Diagram B represents an emulsion of oil droplets dispersed in water (oil in water emulsion).
        • Each oil droplet will have millions of oil molecules in it.
        • The oil is the disperse phase and the water is the continuous phase.
        • This is NOT a true solution. Milk is an oil-in-water emulsion.
        • Semi-skimmed or full fat milk is like this, droplets of fat (~1-3% oil) are dispersed in water.
        • Single cream (~18% oil), double cream (~50% oil) are oil in water emulsions.
        • Whipped cream and ice cream are oil in water emulsions.
          • Air is whipped or whisked into cream to give it a soft frothy texture to use as a topping.
          • Whipping air into ice cream gives it a softer texture so you can scoop out portions easily.
        • Mayonnaise is an emulsion of sunflower oil or olive oil with vinegar, and these mixtures are used in salad dressings and sauces. A salad dressing coats the salad materials better than either the olive oil or vinegar.
        • Some non-food examples of oil in water emulsions include moisturising creams and other cosmetic lotions.
      • water-in-oil emulsion, no emulsifier
      • Diagram C represents an emulsion of water droplets dispersed in an oil (water in oil emulsion).
        • Each water droplet will have millions of water molecules in it.
        • The water is the disperse phase and the oil is the continuous phase.
        • This is NOT a true solution. Butter and margarine are water-in-oil emulsions.
        • In margarine or butter there will be far more of the oil/fat than water, but the diagram is just meant to give an idea of how an emulsion is stabilised. The diagram below is better representation of margarine with its emulsifying agent. The hydrocarbon tails sticking out from the minute water globules, make the water compatible with the hydrogenated vegetable oils.

      • water-in-oil emulsion with emulsifier

      • Diagram E shows the stabilisation of a water-in-oil emulsion by an emulsifying agent molecule.

      • oil-in-water emulsion with emulsifier
      • Diagram D above represents the effect of mixing an oil and water with an emulsifying agent (edible substances like lecithin are used in processed food, or a soap/detergent in a washing action!).
        • You can see one end of an emulsifying molecule is attracted to water (hydrophilic end) and the other end attracted to oil or fat (hydrophobic end). Therefore they can interact with the different components and keep the different types of molecules dispersed in each other.
        • Diagram D also illustrates the mechanism by which soaps wash oily/greasy clothes or surfaces.
        • The washing process is described and explained below diagrams E1, E2 and S3.
      • You will get repulsion between negative hydrophilic ends of the soap/detergent molecule, but its more to with making the emulsified particles of dirt/oil/grease etc. being more compatible with the water by weakly bonding with the water, hence the contaminants can be washed away.
      • Diagrams E1 and E2 show the basic structure of a soap 'molecule' or other 'surface-active agents', known as surfactants. Soaps and detergents enable surfaces to be 'wetted' by lowering the surface tension, essential to getting a cleaning action to remove grease or oil stains from clothes or plates etc. This effect keeps the particles or dirt, grease, oil etc. in a dispersed state so it is washed away.
      • Diagrams E1, E2 and S3: Emulsifying molecules like soap/detergents have a negative ionic hydrophilic 'head' ('water liking'/'oil hating' end of molecule) and a hydrophobic 'tail' ('water hating'/'oil liking' end of molecule').
        • eg the stearate ion from the soap sodium stearate shown above.
        • When you shake soap with an oily/greasy material (washing clothes or scrubbing a surface), the oil/grease breaks up into tiny droplets or globules. Why? ...
        • The hydrocarbon hydrophobic tail of the soap dissolves in the oil or grease globule and the negative head is on the surface of the globules/droplets.
        • The hydrophobic tail can only interact with oil/grease i.e. is attracted to oil and grease.
        • The ionic negatively charged hydrophilic head can only interact with water i.e. is attracted to water and weakly bonds with water molecules.
        • Two hydrophilic heads cannot interact with each other and tend to repel each other especially if the hydrophilic head carries a negative charge (ionic), therefore you get repulsion between the oil/fat globules - though this argument is only part of the 'mechanism story' - read on!
        • In effect, the globules of oil/fat get a surface coating of the emulsifier inhibiting them coming together.
        • Although I've seen arguments in textbooks describing the repulsion of the negative ends of soap/detergent molecules as the 'cleaning mechanism'...
        • ... its more to do with the soap/detergent making the emulsified particles of dirt/oil/grease etc. more compatible with the water via the 'attached' hydrophobic tail bonding to the dirt/oil/grease particles ...
        • ... and these emulsified particles weakly bonding with the water via the hydrophilic head of the soap/detergent molecule, hence the contaminants can be washed away.
        • A general name for these emulsifying molecules is surfactants and includes soaps, detergents and naturally occurring molecules like lecithin found in egg yolk. Lecithin is a complex mixture of molecules with a hydrophilic head and hydrophobic tail and in the margarine industry 'non-ionic' mono/diglyceride esters are used to stabilise 'spreads'. Please note that surfactant emulsifiers do not have to be ionic, natural emulsifiers like lecithin in eggs
        • So, the oil/fat/grease particles cannot re-clump together to form a separate layer on the clothes or surface being cleaned, and so the emulsion is stabilised (see diagram below)
        • In the context of washing, the dirt/oil/grease particles remain dispersed in the soapy washing water and hence washed away.
        • In other contexts eg food, you use a soap like molecule, but harmless and edible!, to do exactly the same effect, that is, emulsifying the mixture to make a stable emulsion which doesn't separate back into two layers.
          • In the food industry emulsifiers are very important for stopping recipe components separating out from emulsions and give processed foods greater stability and longer shelf-life and helps to produce less fatty food and still retain acceptable texture for the consumer. There can be some diet restrictions for some people eg if you are allergic to eggs then any processed food using egg yolk as an emulsifying agent is a no go area! As with any processed food, if you have a sensitive constitution, you must carefully check the ingredients.
          • Incidentally, the emulsifier molecule does not have to be an ionic compound like soap.
          • It can be a non-ionic neutral molecule like lecithin BUT the molecule must have a hydrophilic head that bonds with water and a hydrophobic tail that bonds with oil/grease.
            • The bonds formed are intermolecular bonds (from intermolecular forces of attraction) and NOT chemical bonds like ionic or covalent bonds.
        • Detergents are also emulsifiers, and not just used for washing in the home, they are used to help disperse oil spilt from tankers into rivers, seas and oceans. Much of the oil spill can be contained by booms and pumped off the surface of the water - but not all unfortunately. Dispersed oil droplets break down (biodegrade) more quickly than large patches of oil, but the process is very slow. Rescued seabirds coated in oil can be washed with detergent to clean them BUT their own natural protective oils are also washed away so their lives are still in danger and the birds need care and rehabilitation.
        • See also ...
    • Colloidal particles may be electrically charged.
      • (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 negative. 
      • The situation is complicated further because the charged colloid particles attract a sheath of oppositely charged ions around them.
      • 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 solution.
    • Colloids are destroyed when the particles of the disperse phase join together and separate out from the continuous phase.
      • 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 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 power:
      • positive cations: Al3+ > Mg2+ > Na+
      • negative anions: [Fe(CN)6]3- > SO42- > Cl- 
      • 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 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 dried paint.
  • 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.


Extra Aqueous Chemistry Index:

1. Water cycle, treatment, pollution

2. Colloids - sols, foam and emulsions (this page)

3. Hard and soft water - causes and treatment

4. Gas and salt solubility in water and solubility curves 

5. Calculation of water of crystallisation

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