But what kind of soap are we talking about? An Ivory bar made of real soap? Or a Dove bar with no soap? Or a liquid hand soap that has no soap at all, either? Do all of these soaps work the same when confronting the virus? Soap is the original surfactant and the key component of the classic bar. But when we say “liquid hand soap” we normally refer to products that are made with surfactants rather than soap.
This “dissolving” of the fatty membrane comes from the detergent properties that are intrinsic to soap and cleansing surfactants. In fact, detergent comes from the Latin word “detergeo” for cleaning. Therefore, surfactants remove the lipid envelope of viruses the same way they remove the lipids in our skin and hair, or the oily stains in our clothes.
A recently-published paper1 is showing that the action of soaps on virus involves more than this mechanism of detergency. And this “more” results in the finding that not all “soaps” are equally effective in deactivating virus. We must indicate that the study was done before the appearance COVID-19. It uses two other corona viruses (H3N2 and H5N3).
How It Works
In the paper, the interaction of the viruses with solutions of sodium laureth sulfate (SLES), sodium lauryl sulfate (SLS), and soap (potassium oleate) is investigated using isothermal titration calorimetry. The data reveals that these three surfactants interact differently with the virus resulting in different deactivation effectiveness. Soap showed the best deactivation results, followed by SLS. The least efficient was SLES.
Guided by the calorimetric results, the paper identifies an additional mechanism of interaction besides detergency. It involves electrostatic interactions between the surfactant and the virus’ characteristic spikes or corona. The spikes are proteins that mediate the entry into host cells. The strength of the electrostatic interaction affects the efficacy of the products.
Reading the paper, it became difficult not to relate to the similarity of these findings with the two mechanisms of interaction of surfactants with skin and even hair. During the cleansing process, surfactants interact with the lipids and with the proteins in thecorneocytes of the stratum corneum. The removal of lipids results in skin or hair dryness, while the interaction with the corneocytes yields skin irritation.
Therefore, and from this perspective, there should be no surprise that what makes soap and SLS more irritating to the skin than other surfactants; i.e., their strong interaction with corneocytes, is what gives effectiveness in deactivating the spikes of the virus.
We can assure you that surfactants have been forming micelles in water well before micellar cleansers and micellar shampoos arrived on the market. In fact, all you need to make a micellar solution, as per scientific definition, is to dissolve in water as little as 0.1% surfactant. The individual molecules of surfactant will assemble into these physical structures known as micelles. At low concentrations, the micelles will be spherical, about 3nm in diameter. We could visualize the radii of the sphere as the surfactant molecules. And they do this themselves. Therefore, no complicated development research projects are required to make micelles.
The surface of the micelles is electrically charged. This charge, called the Zeta Potential (ZP), can be measured experimentally and even theoretically with quantum chemistry, and it is generated by the charge of each surfactant molecule. It just so happens that soap and SLS have large ZP values. This results in strong interaction with proteins. In our skin, the proteins are in the corneocytes, and in the virus in the spikes. On the other hand, SLES has lower ZP and this makes it “milder” to corneocytes, and as the paper shows, it was also less effective against the virus.
The extrapolation of the paper’s findings, in terms of the electrostatic interaction with the virus, suggest that the so-called sulfate-free surfactants would be less efficient against the virus because of their lower ZP.
There is an issue that the paper did not address and that may have helped to ascertain how much of the superior effectiveness of soap comes from the molecule itself, i.e, the carboxylic end group; and how much from the intrinsic alkalinity of soap solutions, with a pH near 10. This could have been achieved by testing a solution of SLS with the pH adjusted to 10.
Regardless, it is evident from the paper that good old soap remains a very effective cleanser. The specific soap used in the test was potassium oleate but soaps made with different fatty acids neutralized with an inorganic alkali would have done also well in the test.
Do not forget that the Centers for Disease Control advises that washing your hands with plain soap and water is one of the best ways to prevent the spread of infections and decrease the risk of getting sick. We will assume that plain soap means exactly that: soap.
- Kawahara T, Akiba I, Sakou M, Sakaguchi T, Taniguchi H (2018) Inactivation of human and avian influenza viruses by potassium oleate of natural soap component through exothermic interaction. PLoS ONE 13(9): e0204908
Ricardo Diez, PhD
Ricardo Diez, PhD, is an adjunct professor at Rutgers University, New Brunswick, NJ, in the Master of Business and Science, where he teaches a cosmetic science course. He has worked in the cosmetic industry for more than four decades in both consumer product companies (Procter & Gamble, Dial Corp and Chanel) and raw material manufacturers (Miranol, Stepan and Huntsman).
Diez also teaches courses at the Center for Professional Advancement. Until recently, he also gave courses for the IFSCC and the SCC. Contact him at email@example.com