Why is the effect of washing hands with clean water not good for sterilization?
The skin of our hands is actually very rough. Even the delicate palm of a child is covered with fingerprints and palm prints and other epidermal grooves on the epidermis. Of all mammals, only primates and koalas have these grooves on their hands and feet. Their function is to increase the friction when gripping smooth objects. In order to avoid a dry environment that reduces the friction of fingerprints, the sweat glands of our hands and feet secrete sweat to keep the epidermis moist. However, fingerprints and other grooves can easily become places for viruses or bacteria to attach and hide, and sweat provides ideal conditions for the reproduction of these pathogens.
Fingerprints and sweat under the microscope. Our hands are an ideal place for pathogens to multiply.
To wash away the pathogens on the hands, the effect of rinsing with water is not ideal. Take viruses as an example. Most viruses are enveloped viruses, and their outermost layer has an envelope structure composed of lipid molecules and glycoproteins. The composition of the envelope is similar to that of human cell membranes, which can help the virus to quickly enter the human cell. The glycoprotein on the envelope can also mutate quickly to help the virus trick the human immune system. Influenza virus, hepatitis B virus, and coronavirus are all enveloped viruses.
If you only wash your hands with water. Then when water envelops the envelope of the virus, the hydrophobicity of the lipids in the envelope makes the lipid molecules in the envelope more compact. As the envelope becomes denser, the glycoprotein in the envelope not only maintains its shape, but also becomes more firmly embedded. The cell membrane of bacteria is also composed of lipid molecules, so it cannot be washed away by running water alone.
What is the secret pot of soap?
Schematic diagram of the process of soap disintegrating and removing the virus.
This structure of the envelope also has its fatal weakness, that is, the envelope is easily pried apart by certain substances (such as sodium stearate, the main component of soap). The sodium stearate molecule is like a pin—the round head is hydrophilic, and the fine tail is oleophilic. When we wipe soap, the tail of the sodium stearate molecule can be easily embedded in the lipid envelope that forms the outer shell of the virus. However, the head of the sodium stearate molecule is very different from the lipid molecule in the envelope. This structural difference will make the envelope unable to maintain a stable shape and break, and the internal parts of the virus envelope will also enter the water. In this way, sodium stearate can easily complete the task of destroying the structure of the virus, but their cleaning work is not over.
When the sodium stearate molecules in the water reach a certain concentration, they will gradually combine into tiny spheres called micelles. The sodium stearate molecules in the micelles push the hydrophilic tail outwards, while the virus and bacterial residues are wrapped in the micelles. In most cases, when we rub our hands with soap for 30 seconds, sodium stearate will directly wrap the oily dirt containing pathogens such as viruses or bacteria. After washing with running water, these pathogens will be packaged and taken away from our hands.
Of course, soap is also powerless against certain viruses. For example, the outer layer of rhinovirus that causes some colds has no lipid envelope, only a "capsid" made of protein. The surface of the capsid does not have so many "flaws" that can pry open the sodium stearate molecules. Therefore, in the limited hand washing time, soap cannot fully destroy and take away such pathogens. Therefore, doctors recommend washing hands for at least 20 seconds, precisely because of the complex lines on the hands. Only by allowing soap to fully cover the palm, fingertips, back of the hand and the area between the fingers can we better protect our health.
Soap is very effective in removing pathogens on the hands.
How was hand sanitizer invented?
In the 1940s, Ms. Goldie worked in a rubber factory in Ohio, USA, where she was responsible for adding carbon black to rubber. Carbon black can not only give light-colored natural rubber a dark color, but also improve the wear resistance of the rubber and extend the service life of the rubber. Before the lunch break, the workers immersed their hands in a strong cleaning solution. After a simple scrubbing, the carbon black that was difficult to remove with water was immediately washed away. But the workers did not know that the strong cleaning fluid contained benzene. Although benzene has excellent degreasing ability, it is toxic.
Goldie and her husband Jerry both work in this factory. Although Goldie did not know the toxicity of benzene, in actual use, she still found that this benzene-containing hand sanitizer would harm the skin of the hands. Therefore, she wanted to find a safer cleaning fluid. After several years of exploration, the couple finally produced a new type of hand sanitizer with ethanol and paraffin oil as the main ingredients. They mixed the first batch of finished products in the washing machine of Gordy's parents' home, and then finished the packaging in the garage.
With this batch of products, Jerry confidently began to visit the large and small auto repair shops in the city. In order to persuade the customer, he thought of a surprisingly winning trick: before shaking hands with the customer (the auto mechanic whose hand is full of motor oil) to introduce himself, he hid a small group of hand sanitizer in his palm, and put the hand sanitizer when shaking hands. Wipe it firmly on the palm of the auto mechanic. Normally, a shocked mechanic will quickly withdraw his hand and wipe his hand with a rag. After wiping off the hand sanitizer, the auto mechanic will find that the dark palms that were originally covered with oil have become clean. Relying on this simple and direct marketing method, the Jerry couple's hand sanitizer quickly occupied a large market. This kind of hand sanitizer is the prototype of the hand sanitizer we commonly use today, but today’s hand sanitizer has its decontamination component reduced to a surfactant, and the main sterilizing component has been simplified to a high concentration of ethanol or isopropanol (these components can be Dehydrate and denature the protein of bacteria or viruses, thereby coagulating and becoming invalid).
The rubber factory uses benzene solution as a hand sanitizer for workers, but in fact it is more out of frustration. Because that period coincided with the "Second World War", at that time one of the main raw materials of soap-animal fat was used to extract glycerin (glycerin can be used to make explosives, lubricants, military equipment paint, hydraulic oil, etc. Important military supplies). Therefore, during the "Second World War" animal fats were in short supply, and even lard and butter left over from cooking in American households were recycled by the authorities.
