Soaps: the story of ashes, pH and sensitive skin August 31, 2015 17:59

Have you ever wondered how the natural, handmade soaps are different from the commercial bars, what the soap pH is all about and why some washing gels insist that they contain no soap? So have I! Which is why I decided to research this topic.

Soap was probably invented independently in various areas of the world. One story of soap origin tells us about the animal sacrifice performed by Romans on Mount Sapo. After they burned parts of the animals for the gods, the Romans discovered that the animal fat mixed with ashes and then with rain water formed a substance which could clean surfaces – soap. It is this story that gave the name to the chemical reaction of soap making – saponification.

Soap is a product of mixing fats (animal or vegetable) with a strong base, such as sodium hydroxide, or lye. Traditionally lye was made by leaching ashes with water. When fats react with lye, they form fatty acid salts and glycerol. It is the structure of the fatty acids that enables soap to clean: they have the ‘acid’ end, which mixes well with water (it is hydrophilic, or it ‘water loving’) and the ‘fatty’ end (a long chain of carbons), which mixes well with fats (it is hydrophobic, or ‘water hating’).

When I was plagued by eczema, my GP advised me to wash myself only with water. If you ever tried it, you know that water on its own is not that effective at cleaning! This is because the particles of water (H2O) tend to ‘stick’ together. Since they are very attracted to each other, it is difficult to pull them apart – they have a high a surface tension. Due to the structure of the fatty acids soap is a surfactant - it can reduce the surface tension of the water and suspend the oil particles among the water particles, so they can be rinsed off.

 

 

A soap micelle: a grease particle surrounded by the fatty acids. The tails of the fatty acids are attracted to the grease, while the heads are attracted to the water.

 

These days we have a wide choice of soap, not only manufactured by big companies, but also handcrafted by small artisans. Such soap is usually completely natural- it contains only vegetable oils and base, sometimes herbs, flower petals of essential oils, but no synthetic additives such as sodium lauryl sulphate (SLS), which can be very irritating to the skin. You might have heard about handmade soaps produced by ‘cold process’ and by ‘hot process’ method. How are these two different? Part of the answer is in the name: cold process of soap making relies on the chemical reaction taking place in the room temperature. As the oils and base mix, the mixture becomes thicker (this is the ‘trace’ stage of soap making) until it acquires the consistency of a pudding. The mixture is then poured into a mould and left to continue to saponify for 12-48 hours. When the soap is firm, it is cut into bars and left for another two weeks to mature. On the other hand, in the hot process the artisan encourages the saponification by heating up the mixture of base and oils. The reaction is completed within a few hours and the soap can be poured into moulds.

So what are pH of soaps and the cleansers without soaps all about? The natural pH on the surface of the healthy skin is around 5.6 (slightly lower in men). Soaps are often alkaline, i.e. their pH is higher (according to some studies, commercial soaps have pH around 10 and shampoos around 6 (Tarun et al., 2014)). Since rise in the skin surface pH can be associated with increased water loss and dehydration, commercial cleansers are often advertised as ‘acidic’ or with ‘neutral pH’. However, a recent study compared pH on the skin of two groups of people: one used acidic cleanser and the other normal soap (with pH around 10) for over 5 years (Takagi et al., 2015). The researchers found that pH increased in both groups. This suggests that soap does not affect the balance of healthy skin anymore than a mild acidic cleanser.

 

 "PH scale 2" by Original uploader was Piercetheorganist at en.wikipedia - Transferred from en.wikipedia; Transfer was stated to be made by User:LeaW.. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:PH_scale_2.png#/media/File:PH_scale_2.png

The drying of the skin could also be caused by the fact that the soap removes the natural oils – sebum – from the surface of the epidermis (the superficial layer of the skin). To test this, a study compared the skin of babies washed with water, mild acidic cleanser and alkaline soap (Gfatter et al., 1997). All three cleansers, including water, removed the sebum and increased pH, although these effects were greater in the group washed with alkaline soap. However, there were no differences in the skin hydration.

Therefore, it seems that even quite alkaline soaps do not disturb the healthy skin. This is because the epidermis produces an acidic mantle from its own free fatty acids. Bacteria, sweat and sebum also contribute to the slightly acidic pH of the skin surface. This is why the skin can neutralize the post –cleansing increase in pH within a few hours. And if you really want to speed up the neutralization of the soap, running a few hundred meters will do just that!

However, the effects of the alkaline soaps can be different on sensitive skin. If you have allergies, tendency for eczema and your skin feels tight, dry or stings and burns after washing, you probably have sensitive skin. This means that the natural protective barrier – the striatum corneum – does not function properly, it might be too thin and ‘leaky’ (Berardesca et al., 2013). Hence, water, soap and cleansing gels get in between the layers of the epidermis and disturb the skin balance, causing dryness. In addition, your immune system might be hyper-responsive and react with inflammation.

Still, natural soap could be an opiton. These products are free of the synthetic additives, which exacerbate the irritation experienced by those with senstive skin. Compared with most commercial soaps they contain greater amounts of glycerine, which moisturises the skin, and their pH can be lower. And if you really cannot tolerate even the natural soap, your best option if the micellar water – but this is another story!

 

Jadwiga Nazimek

 All images and contents are subject to copyright.

References:

http://science360.gov/obj/tkn-video/81074969-11e0-4a2e-b674-8fc8886fd9c3 accessed 30th August 2015

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p096.shtml#background accessed 30th August 2015

Berardesca, E., M. Farage, and H. Maibach, 2013, Sensitive skin: an overview: Int J Cosmet Sci, v. 35, p. 2-8.

Gfatter, R., P. Hackl, and F. Braun, 1997, Effects of soap and detergents on skin surface pH, stratum corneum hydration and fat content in infants: Dermatology, v. 195, p. 258-62.

Takagi, Y., Kaneda, K., Miyaki, M., Matsuo, K., Kawada, H. and Hosokawa, H. (2015), The       long-term use of soap does not affect the pH-maintenance mechanism of human skin. Skin Res Technol, 21: 144–148. doi:10.1111/srt.12170

Tarun, J., J. Susan, J. Suria, V. J. Susan, and S. Criton, 2014, Evaluation of pH of Bathing Soaps and Shampoos for Skin and Hair Care: Indian J Dermatol, v. 59, p. 442-4.