They're very convenient, but why are plastic containers so hard to clean?

Plastic is hydrophobic, meaning it repels water, which in turn makes it harder to both clean and dry. Picture: Shutterstock.

Plastic is hydrophobic, meaning it repels water, which in turn makes it harder to both clean and dry. Picture: Shutterstock.

Why do plastic containers take so long to dry in the dish rack; and why is it so hard to wash them when they are greasy? Asked by Ian.

If you've ever stored a well-dressed salad in a reusable plastic container, you'll know how difficult it is to get all that oil cleaned off - and, if you succeed, how long it takes the plastic to dry afterwards.

This is because plastic is hydrophobic: it repels water. Its water-repelling properties are what makes it such a desirable food storage material.

Unlike wood or carboard, it's not going to let water leach in from the outside (allowing the food to go off faster), nor is it going to let water on the inside leak out.

Unfortunately, being hydrophobic makes it harder to both clean and dry.

More science:

If you add a droplet of water to both a flat plastic and flat ceramic surface, the two drops will probably form different shapes: on ceramic, the droplet will spread out further and seem flatter, while on plastic, it will bunch up and remain high.

The water will avoid interacting with the plastic as much as possible.

Take a look at each of these substances dropped on either parafilm or teflon. Water resists the hydrophobic surface the most, while castor oil sinks into it the most. Credit: By DaraDaraDara Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=84469926

Take a look at each of these substances dropped on either parafilm or teflon. Water resists the hydrophobic surface the most, while castor oil sinks into it the most. Credit: By DaraDaraDara Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=84469926

If the water isn't spreading further across the surface, it takes longer to evaporate and thus the plastic takes longer to dry.

Plastic's hydrophobic character is also what makes it stick to oil so easily: they're made of similar stuff.

Plastic food containers are usually made of a substance called polypropylene. It's a polymer made of carbon and hydrogen atoms.

Cooking oil, similarly, contains fat, which is also made mostly from carbon and hydrogen (and a tiny bit of oxygen), bonded together in a similar manner.

Carbon and hydrogen atoms form what chemists call a non-polar bond. These non-polar molecules are very attractive to one another, so they're much more likely to mix and stick together.

(Water, on the other hand, is a polar molecule, with polar hydrogen-oxygen bonds in it. This is why it repels plastic and oil.)

If you repeat your droplet experiment with oil, you'll see it sinks much more easily into the plastic surface than it does the ceramic.

Because oil is hydrophobic like plastic, it resists rinsing. And because it's got a boiling point higher than water, it doesn't evaporate as the plastic dries - instead, it hangs around and spreads itself further.

If you're struggling to clean your containers, try scrubbing with a blob of pure detergent before washing with water.

Detergent molecules have oil-attracting and water-attracting qualities (or, non-polar and polar sections, respectively) - this is what makes them so good at cleaning.

If you're attacking the oil with something less watery, you're more likely to get it off.

  • This article was published in partnership with Cosmos Magazine. Cosmos is produced by The Royal Institution of Australia.

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