Soap, syndet and shower gel all cleanse the skin thanks to surfactant molecules, but their pH differs. Soap is alkaline, most often between 9 and 10, while syndets and shower gels can be formulated at a pH close to that of the skin.
Understanding this difference helps when reading a label. But first, it is worth returning to what “cleansing” actually means.
How the skin is cleansed
Water alone rinses away dust and sweat. It removes sebum and other fatty substances poorly, because water and oil do not mix.
Surfactants solve this problem. They are molecules with one part attracted to water and one part attracted to fats. They surround the fatty droplets and keep them suspended in the rinsing water. In other words, they emulsify the dirt.
This mechanism has a downside. Surfactants do not perfectly distinguish excess sebum from the lipids and proteins of the stratum corneum. All interact to some extent with the skin's lipids. Their interaction with proteins varies greatly according to their chemical structure (Ananthapadmanabhan et al., 2004). This is one of the criteria that distinguish a mild cleanser from a harsher one.
Soap
Soap is the oldest of cleansers. It results from saponification: a fatty substance, oil or fat, reacts with a strong base such as sodium hydroxide. The molecules obtained are fatty acid salts, which cleanse effectively.
This chemistry fixes its pH. Soap can only exist in an alkaline environment. An Indian study measured the pH of many commercial toilet soaps: most were between 9 and 10 (Tarun et al., 2014).
This high pH has consequences for the skin's surface. After washing with soap, the skin's pH rises, then gradually returns to its usual value within a few hours (Korting and Braun-Falco, 1996). Laboratory work also shows that a solution at pH 10, even without surfactant, makes the stratum corneum swell and modifies the rigidity of its lipids (Ananthapadmanabhan et al., 2004).
Syndet
The word comes from the English synthetic detergent. A syndet cleanses with surfactants other than soap, of synthetic or plant origin. It comes in bar form, like soap, or in liquid form.
Its main advantage lies in the freedom of formulation. Because it does not need an alkaline environment to exist, its pH can be adjusted. Syndets are generally neutral or slightly acidic, depending on the formula (Mukhopadhyay, 2011). Authors who study the mildness of cleansers consider that those with a neutral or acidic pH, close to 5.5, are potentially less aggressive for the skin (Ananthapadmanabhan et al., 2004).
pH is not the only criterion. The nature of the surfactants, their concentration and the presence of relipidising agents also matter.
Shower gel
Shower gel is, in the vast majority of cases, a liquid syndet. It therefore shares its flexibility: depending on the formula, its pH can range from slightly acidic to neutral, or even beyond.
The term “shower gel” gives no information about pH. Only a value stated by the manufacturer makes it possible to know it. The mention “pH neutral” in principle designates a pH of 7, that of pure water. The skin's surface, for its part, is on average around 4.7 (Lambers et al., 2006).
Three cleansers at a glance
| Cleanser | Cleansing principle | Usual pH |
|---|---|---|
| Soap | Fatty acid salts from saponification | Alkaline, most often 9 to 10 |
| Syndet (bar or liquid) | Surfactants other than soap | Adjustable, generally neutral or slightly acidic |
| Shower gel | Most often a liquid syndet | Variable according to the formula |
| For reference: skin surface | On average 4.7 |
Beyond the product
The water itself matters: in Europe, tap water often has a pH close to 8 (Lambers et al., 2006). The length of the shower, the water temperature and the amount of product used also influence the result.
Soap remains an effective cleanser, used for centuries. Its particularity is chemical: it can only be alkaline. Syndets and shower gels offer more choice, provided you read the label.
This article is part of the PHYTO 5 Library.
Sources
- Ananthapadmanabhan KP, Moore DJ, Subramanyan K, Misra M, Meyer F. Cleansing without compromise: the impact of cleansers on the skin barrier and the technology of mild cleansing. Dermatologic Therapy. 2004;17(Suppl 1):16-25. doi:10.1111/j.1396-0296.2004.04s1002.x
- Korting HC, Braun-Falco O. The effect of detergents on skin pH and its consequences. Clinics in Dermatology. 1996;14(1):23-27. doi:10.1016/0738-081X(95)00104-N
- Lambers H, Piessens S, Bloem A, Pronk H, Finkel P. Natural skin surface pH is on average below 5, which is beneficial for its resident flora. International Journal of Cosmetic Science. 2006;28(5):359-370. doi:10.1111/j.1467-2494.2006.00344.x
- Mukhopadhyay P. Cleansers and their role in various dermatological disorders. Indian Journal of Dermatology. 2011;56(1):2-6. doi:10.4103/0019-5154.77542
- Tarun J, Susan J, Suria J, Susan VJ, Criton S. Evaluation of pH of bathing soaps and shampoos for skin and hair care. Indian Journal of Dermatology. 2014;59(5):442-444. doi:10.4103/0019-5154.139861