5.25.2020

Good hygiene advice: on the science of soap and disinfectants

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Editor's Note: The opinions expressed in this article are those of the author as published by our content partner and do not represent the opinions of MSN or Microsoft.

Wash your hands with soap and water to best limit the spread of Covid-19. If this is not possible, hand disinfectants can be used. However, it is important to know what ingredients the disinfectants should contain.

There has been a plethora of social media videos about hand sanitizer manufacturing at home in recent days. In fact, in the fight against Covid-19, we are always recommended to practice good hygiene, especially regular hand cleaning with soap or disinfectants to keep them germ-free.

There are many misunderstandings about hand sanitizers that exploded during this pandemic. Before we examine these misunderstandings and the risks involved, let's take a look at the science behind soap and disinfectants. To understand why something simple like soap or disinfectant works, we need to look at the expression "as dissolved as".

We know from practical experience (e.g. washing dishes) that oil or fat cannot be mixed well with water. Based on the "as solved as" principle, fats do not dissolve in water since fat molecules and water molecules are not the same.

When washing up, we add soap to the water to remove fat from food residues. It works because soap has a special molecular structure. Soap is made from fat in a process called saponification. During this process, the fat molecules are changed so that one end of the molecule is much more like water.

This structural change gives soap molecules a double personality: they can behave like fat and like water. In scientific terms, we say that it is polar (like water) and non-polar (like fat). Because of this interesting quality / dual personality, we can use soap to dissolve the fat in water. Fat interacts with the non-polar end of the soap molecule, while water interacts with the polar end.

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

Antibacterial soap contains special molecules that can break down bacteria, but Covid-19 is a virus, not a bacterium (which is why antibiotics cannot be used to treat patients with Covid-19). While hand washing 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 disinfectants can be used.

Hand disinfectants contain alcohol molecules that also dissolve the protective fatty layer of the virus. There are many different types of alcohol molecules, and the simplest are called methanol, ethanol and propanol. The atoms (that is, the building blocks) that make up propanol can rearrange and form another alcohol molecule known as isopropanol. All of these alcohols have different molecular structures and therefore behave differently.

For safety reasons, ethanol and isopropanol are only used in hand disinfectants. When reading the disinfection labels, it can be confusing to know which alcohols have been used because common names or synonyms are often used. For example, ethanol can be referred to as ethyl alcohol. Hand disinfectants obtained from trusted sources must contain ethanol or isopropanol, as manufacturers must test and test their products before they can be marketed.

Ethanol has the same chemical structure as alcohol, while isopropanol is often used in spirits or cleaning alcohol. Because ethanol or isopropanol can leave the skin dry, hand disinfectants add glycerin (the chemical name for what we call glycerin). Glycerin is non-toxic and can soften or soothe the skin. From a scientific point of view, we say that it is a plasticizer. Interestingly, glycerin is a by-product of the soap making process. Glycerin is also responsible for the sticky feeling on the hands after using disinfectants.

You should never add water to hand sanitizers to make them less sticky or to last longer. This reduces the alcohol content and makes hand disinfectants less effective or completely ineffective.

It's not as easy to make your own hand sanitizer at home as some social media videos suggest. Security is the main concern. You must be trained to use pure chemicals and take special precautions. You may also find it difficult to find some of the ingredients listed in the myriad of recipes available online.

To be effective, hand sanitizers must contain at least 70% alcohol, which makes them highly flammable. Therefore, you should never use or keep hand sanitizers near an open flame or heat source, and never attempt to light them on purpose.

One of the many misunderstandings about hand sanitizers is that the color of the flame that is created when lit indicates whether it is safe to use. This is extremely dangerous and should never be attempted. When using a hand sanitizer, rub your hands together until they are completely dry. At this point, all alcohol molecules should have evaporated and you can safely continue.

When making a hand sanitizer, the goal is to get a product that works as expected but is also safe to use. It's not as easy to make your own hand sanitizer at home as some social media videos suggest. Security is the main concern. You must be trained to use pure chemicals and take special precautions. You may also find it difficult to find some of the ingredients listed in the myriad of recipes available online.

When cooking or baking, we often become creative or innovative when replacing ingredients. Unfortunately, when we do chemistry, replacing ingredients with something "similar" can result in a dangerous and ineffective disinfectant, or both. At home, washing your hands with normal soap is much safer than trying to make your own hand sanitizer.

There is a simple experiment to show young children how important it is to wash their hands with soap. Put some water in a bowl first. Sprinkle some dried herbs in the bowl (most float on water, do not mix). Tell your child that herbs are "germs". Ask your child to put a finger (clean and dry) in the mixture. Some of the "germs" stick to this finger. Now use your other hand (always clean and dry) and put your finger in liquid soap (e.g. liquid detergent or hand washing liquid) before adding it to the water and herb mixture.

Herbs will now drift off your finger! The science behind it is a little different, but also interesting. In this case, soap breaks the surface tension of water (surface tension is the scientific term that can also describe why a stomach bump really hurts in a pool).

Stay safe and remember to wash your hands regularly. DM

Dr. Elize Smit is a professor in the Department of Chemical Sciences of the Faculty of Science at the University of Johannesburg.

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