4.2.4.1Sizes of particles and their properties
Chemistry only — not assessed in Combined Science: Trilogy.
Nanoscience refers to structures that are 1–100 nm in size, of the order of a few hundred atoms.
Nanoparticles are smaller than fine particles (PM2.5), which have diameters between 100 and 2500 nm (1 × 10⁻⁷ m and 2.5 × 10⁻⁶ m). Coarse particles (PM10) have diameters between 1 × 10⁻⁵ m and 2.5 × 10⁻⁶ m. Coarse particles are often referred to as dust.
You should be able to compare 'nano' dimensions to typical dimensions of atoms and molecules, and this is where standard form, ratios and order of magnitude calculations are being examined.
An atom has a radius of about 0.1 nm, so a diameter of about 0.2 nm. A nanoparticle 100 nm across is therefore roughly 500 atoms wide — which is what the specification means by "of the order of a few hundred atoms".
Worked example
A nanoparticle has a diameter of 20 nm. An atom has a diameter of about 0.2 nm. How many times larger is the nanoparticle, and what order of magnitude is that?
- Write both in the same unit: 20 nm and 0.2 nm.
- Divide: 20 ÷ 0.2 = 100.
- So the nanoparticle is 100 times wider than an atom.
- 100 is 1 × 10², so this is two orders of magnitude.
Answer: 100 times larger in diameter — two orders of magnitude. (In volume it would be 100³ = 10⁶ times larger.)
As the side of a cube decreases by a factor of 10, the surface area to volume ratio increases by a factor of 10.
This is the key quantitative idea of the sub-topic, and it is worth proving to yourself rather than memorising, because you may be asked to calculate the surface areas and volumes of cubes.
Worked example
Calculate the surface area to volume ratio of a cube of side 10 nm, and of a cube of side 1 nm.
- A cube has 6 faces, so surface area = 6 × side².
- Volume = side³.
- For the 10 nm cube: surface area = 6 × 10² = 600 nm²; volume = 10³ = 1000 nm³; ratio = 600 ÷ 1000 = 0.6.
- For the 1 nm cube: surface area = 6 × 1² = 6 nm²; volume = 1³ = 1 nm³; ratio = 6 ÷ 1 = 6.
- Compare the two ratios: 6 ÷ 0.6 = 10.
Answer: 0.6 for the 10 nm cube and 6 for the 1 nm cube — a tenfold decrease in side length gives a tenfold increase in the surface area to volume ratio, as the specification states.
Nanoparticles may have properties different from those for the same materials in bulk because of their high surface area to volume ratio. It may also mean that smaller quantities are needed to be effective than for materials with normal particle sizes.
That second sentence is the practical consequence, and it appears again and again in the uses below: if almost every atom in a nanoparticle is at the surface, then almost every atom is available to react, absorb light or kill bacteria — so far less material does the same job.
Common mistake
Surface area to volume ratio increases as particles get smaller, not larger. Check by putting numbers in: a 1 nm cube has a ratio of 6, a 10 nm cube 0.6. If your answer says a bigger particle has the higher ratio, the division has been done the wrong way round.
- Nanoscience
- the study of structures 1–100 nm in size, of the order of a few hundred atoms
- Nanoparticle
- a particle 1–100 nm in size, smaller than fine particles, with a very high surface area to volume ratio
- Fine particles
- PM2.5: particles with diameters between 100 and 2500 nm
- Coarse particles
- PM10: particles with diameters between 2.5 × 10⁻⁶ m and 1 × 10⁻⁵ m, often referred to as dust
4.2.4.2Uses of nanoparticles
Chemistry only — not assessed in Combined Science: Trilogy.
Nanoparticles have many applications in medicine, in electronics, in cosmetics and sun creams, as deodorants, and as catalysts. New applications for nanoparticulate materials are an important area of research.
You should consider the advantages and disadvantages of the applications of these nanoparticulate materials, but you do not need to know specific examples or properties other than those specified.
You should be able to, given appropriate information, evaluate the use of nanoparticles for a specified purpose, and explain that there are possible risks associated with the use of nanoparticles.
An evaluation is not a list. Use the information you are given, weigh at least one advantage against at least one disadvantage, and finish with a judgement that answers the actual question asked — is this a good use of nanoparticles here, and why.
The general risk argument, which applies to nearly any use, is worth having ready. Nanoparticles are small enough to be breathed in and to pass into cells; their effects on health and on the environment have not been studied for long, because the materials are new; and the same high surface area that makes them so effective also makes them highly reactive. Because they are so new, there has not yet been time for long-term testing.
Worked example
A sun cream containing titanium dioxide nanoparticles blocks more ultraviolet light than the same mass of ordinary titanium dioxide powder, and is transparent rather than white. Evaluate its use.
- State the advantage from the information given: it blocks more ultraviolet light for the same mass, so it gives better protection against skin cancer, and being transparent it is more likely to be worn.
- Explain why, using the chemistry: the nanoparticles have a much higher surface area to volume ratio, so smaller quantities are needed to be effective.
- State the disadvantage: the particles are small enough that they may be absorbed through the skin, and the long-term effects are not known because the material is new.
- Reach a judgement, and say what would change it.
Answer: The protection against ultraviolet light is a clear, immediate benefit and needs less material, while the risk is uncertain rather than demonstrated — so on the information given the use is justified, but long-term testing of absorption through the skin would be needed to settle it.
Before you practise
- This sub-topic is chemistry only — it is not assessed in Combined Science: Trilogy.
- Nanoscience covers structures 1–100 nm in size, of the order of a few hundred atoms.
- Fine particles (PM2.5) are 100 to 2500 nm; coarse particles (PM10), often called dust, are larger still.
- Surface area of a cube = 6 × side²; volume = side³.
- As the side of a cube decreases by a factor of 10, the surface area to volume ratio increases by a factor of 10.
- Nanoparticles behave differently from the bulk material because of their high surface area to volume ratio, and smaller quantities are needed to be effective.
- Uses: medicine, electronics, cosmetics and sun creams, deodorants, catalysts.
- Evaluate by weighing advantages against risks and reaching a judgement — and remember that the risks are uncertain mainly because the materials are new.