9.10.1 Particle Size and Properties

Syllabus

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 (PM_2.5), which have diameters between 100 and 2500 nm (1 x 10^-7 m and 2.5 x 10^-6 m).
Coarse particles (PM₁₀) have diameters between 1 x 10^-5 m and 2.5 x 10^-6 m. Coarse particles are often referred to as dust.
As the side of cube decreases by a factor of 10 the surface area to volume ratio increases by a factor of 10.
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.
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.
Students should
1. be able to compare ‘nano’ dimensions to typical dimensions of atoms and molecules.
2. consider advantages and disadvantages of the applications of these nanoparticulate materials, but do not need to know specific examples or properties other than those specified.
3. be able to evaluate the use of nanoparticles for a specified purpose, given appropriate information, and explain that there are possible risks associated with the use of nanoparticles.

What does this mean?

What is a nanometer?

We know what we mean by a centimeter (cm) - we mean 1/100 of a metre or 1 x 10^-2 m

We know what we mean by a millimeter (mm) - we mean 1/1000 of a metre or 1 x 10^-3 m

You may have heard of a micrometer - we mean 1/1000000 of a metre or 1 x 10^-6 m

A nanometer (nm) - means 1/1000000000 of a metre or 1 x 10^-9 m

So nanoparticles are between 1 and 100 nm in diameter.

Nanoparticles

Nanoparticles are a lot smaller than any particle you can see.

A coarse sand grain is around 1mm in diameter or 0.001 m

This is 1x10^-3m

A fine sand grain is around 0.1 mm in diameter or 0.0001 m

This 1x10^-4m

But when scientists talk about particles they're not thinking of sand.

They refer to coarse particles as dust and include particles between 1 x 10^-5 m and 2.5 x 10^-6 m, which is smaller than fine sand grains.

To make things easier they refer to this range of size as PM₁₀ - particles less than 10 micrometers.

They think of fine particles as being PM_2.5 - particles smaller than 2.5 micrometers.

It seems unlikely that the examiner will expect you to recall these numbers even though they are stated on the syllabus but you would be expected to know that Nanoparticles are smaller even than this.

What's good about Nanoparticles?

It's all about surface area.

Strangely, the more we break up a particle the more surface it has.

Breaking up the particle into smaller pieces doesn't increase its size (volume) though.

So, the smaller the particles the higher the surface are to volume ratio.

You are expected to know that decreasing the length of the sides by a factor of ten increases the surface area to volume ratio by a factor of 10.

And for solids reactions happen at the surface.

So the smaller the particle, the larger the surface area and the faster its reactions may happen.

This may mean that we can use less material to do the same job.

For instance, nano sized salt tastes stronger than normal salt due to its large surface.

So we can use less salt to add the same amount of flavour.

More importantly, we may be able to reduce doses of medicines if they are produced in nanoscale.

Sun-block is often made of Zinc Oxide - but bulk Zinc Oxide is white, while nanoparticle Zinc Oxide is invisible.

Bulk Zinc Oxide makes a good Sun-block - but who wants to look like an Australian cricketer?

And invisible nanoparticles would be equally useful in deodorants and cosmetics.

Sometimes, the increased surface area to volume ratio means that nanoparticles will react completely different;y to normal sized grains - being smaller has changed the chemical properties from that of the bulk material.

For instance, catalysts are substances that speed up reactions without being used up.

Often they do they by having other substances stick to their surfaces - so a substance may be a much more effective catalyst in nano-size particles than as a bulk material

Atoms, Molecules and Order of Magnitude

Recall that the diameters are:

Atoms and small molecules ---- around 0.1 nm

Nanoparticles ----- around 1 to 100 nm

An exam question might be:

If a Copper Oxide nanoparticle has a diameter of 31 nm.

The diameter of Copper atom is 0.29 nm.

Estimate how many times larger the nanoparticle is compared to a Copper atom.

Round each number to 1 significant figure:

Nanoparticle = 30 nm and Copper atom = 0.3 nm

Answer:

30/0.3

Number of times larger ≈ 100

The nanoparticle is about 100 times larger than the Copper atom.

Risks

Nano particles are so small that they may be able to get inside cells.

If they do this they may damage the cells and this could cause medical problems.

No one knows if they might damage DNA, but damaged DNA often causes cancer.

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