Good hygiene tips: On the science of soap and sanitiser

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.