Good hygiene tips: On the science of soap and sanitiser

Date: May 25, 2020 | News

Dr Elize Smit is a lecturer in the Department of Chemical Sciences, Faculty of Science, at the University of Johannesburg. She recently penned an opinion article published by the Daily Maverick on 25 May 2020.

”Washing your hands with soap and water is the best way to limit the spread of Covid-19. If this is not possible, hand sanitisers can be used, but it’s important to know what ingredients the sanitisers should have.”


In recent days, there has been a proliferation of videos on social media about making hand sanitiser at home. This is because in the fight against Covid-19, we are continually told to practise good hygiene, especially the regular cleaning of hands using soap or sanitisers to keep them germ-free.

There are many misconceptions around hand sanitisers that have flared up during this pandemic. Before we look at these misconceptions and the associated risks, let us look at the science behind soap and sanitisers. To understand why something simple like soap or sanitiser works, we need to consider the phrase “like dissolves like”.

From practical experience (e.g. washing the dishes), we know that oil or fats do not mix well with water. Based on the principle of “like dissolves like”, fats will not dissolve in water, because fat molecules and water molecules are not alike.

When washing the dishes, we add soap to the water to dissolve the fat from food residues. This works because soap has a special molecular structure. Soap is made from fats in a process known as saponification. During this process, the fat molecules are changed so that one end of the molecule looks a lot more like water.

This change in structure gives soap molecules a dual personality – it can behave like a fat and it can behave like water. In scientific terms, we say it is polar (like water) and also non-polar (like fats). Because of this interesting quality/dual personality, we can use soap to make fats dissolve in water. The fat will interact with the non-polar end of the soap molecule while the water will interact with the polar end.

Covid-19, like many other viruses, has a fat-like protective layer on the outside. When we wash our hands with soap and water, this fat-like layer is destroyed, and the virus is killed. Any soap can be used, it does not have to be antibacterial soap.

Antibacterial soap contains special molecules that can break down bacteria, but Covid-19 is a virus, not a bacteria (this is also why antibiotics cannot be used to treat patients with Covid-19). Even though washing your hands is the best way to limit the spread of Covid-19, it is not always possible or practical to wash your hands with soap and water. In these cases, hand sanitisers can be used.

Hand sanitisers contain alcohol molecules, which also dissolve the fat-like protective layer of the virus. There are many different types of alcohol molecules, and the simplest ones are called methanol, ethanol and propanol. The atoms (i.e. the building blocks) that make up propanol can be rearranged to give a different alcohol molecule, known as isopropanol. These alcohols all have different molecular structures and therefore behave differently.

For safety reasons, only ethanol and isopropanol are used in hand sanitisers. When reading the labels on sanitisers, it may be confusing to know which alcohols were used since common names or synonyms are often used, e.g. ethanol can be called ethyl alcohol. Hand sanitisers purchased from reliable sources should contain either ethanol or isopropanol, since manufacturers have to analyse and test their products before they can be commercialised.

Ethanol has the same chemical structure as the alcohol in liquor, while isopropanol is often used in surgical spirits or rubbing alcohol. Since ethanol or isopropanol can leave your skin feeling dry, glycerol (the chemical name for what we know as glycerine), is added to hand sanitisers. Glycerol is non-toxic and can soften or soothe the skin. In scientific terms, we say it is an emollient. Interestingly, glycerol is a by-product of the soap-making process. Glycerol is also responsible for the sticky feeling on your hands after using sanitisers.

You should never add water to hand sanitisers to make it less sticky or to make it last longer. Doing so will reduce the alcohol content and make the hand sanitisers less effective or completely ineffective.

”Making your own hand sanitiser at home is not as simple as some videos on social media may suggest. Safety is the main concern. You have to be trained to work with pure chemicals and take special precautions. You may also struggle to find some of the ingredients listed in the plethora of recipes available online. ”

To be effective, hand sanitisers must contain at least 70% alcohol, which makes it highly flammable. As such, you should never use or store hand sanitisers near an open flame or heat source and you should never try to ignite it on purpose.

One of the many misconceptions around hand sanitisers is that the colour of the flame produced when igniting it will reveal if it is safe to use. This is extremely dangerous, and should not be attempted under any circumstances. When using hand sanitisers, rub your hands until completely dry. At this point, all the alcohol molecules should have evaporated, and you are safe to proceed.

When making a hand sanitiser, the aim is to end up with a product that will work as intended, but will also be safe to use. Making your own hand sanitiser at home is not as simple as some videos on social media may suggest. Safety is the main concern. You have to be trained to work with pure chemicals and take special precautions. You may also struggle to find some of the ingredients listed in the plethora of recipes available online.

When cooking or baking, we often become creative or innovative by substituting ingredients. Unfortunately, when we are doing chemistry, substituting ingredients with something “similar” could result in a sanitiser that is either dangerous, ineffective or both. At home, it is much safer to wash your hands with regular old soap than trying to make your own hand sanitiser.

There is a simple experiment you can do to show small children the importance of using soap when washing their hands. First, put some water in a bowl. Sprinkle some dried herbs into the bowl (most will float on top of the water – do not mix). Tell your child that the herbs are “germs”. Ask your child to put a (clean and dry) finger in the mixture. Some of the “germs” will stick to this finger. Now use the other hand (still clean and dry) and place a finger in some liquid soap (e.g. dishwashing liquid or hand wash liquid) before placing it in the water/herb mixture.

The herbs will now move away from your finger! The science behind this is a little bit different, but interesting as well. In this case, the soap breaks up the surface tension of the water (surface tension is the scientific term that can also describe why a belly flop in a swimming pool really hurts).

Stay safe and remember to wash your hands regularly.

hand wash

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