How Soap And Shower Gel Dry Your Skin

How Soap And Shower Gel Dry Your Skin

I think I can claim to be something of an expert about soap and skincare, but the trouble with getting really immersed in a subject is that you begin to forget that not everybody knows what you know.  I spend a lot of time talking to people in the beauty industry who know a great deal about cosmetic products.  And when I am not talking to them I am often talking to beauty bloggers who know even more.  So when I gave a talk the other day to some people with no particular connection to the business I was struck that the questions they asked were ones that I would have thought everybody already knew the answer to.  Here’s one interesting one.

“My doctor has told me that because my skin is dry I should avoid soaps because they are now made of detergents.”

I was at a bit of a loss to answer that one.  As framed it made no sense at all to me.  Several minutes of confusion followed and I don’t think my attempt to explain what all the terms she was using meant helped much.  So lets see if I can do a better job now.

Is Soap Made From Detergent?

First off, tablet soap is still very much soap.  Soap has been manufactured at least since the time of the Roman Empire and has not changed much in chemistry in that time. To make it you get hold of some fat or oil, and boil it with an alkali.  The fat breaks down and reacts with the alkali to give you soap.  Tallow from beef  used to be the big source of fat, but since the BSE crisis palm oil and coconut oil have taken over.  (From a chemical point of view oil and fat are much the same thing.  The only difference is that fat is solid and oil is liquid.)

The Romans noticed that soap was a good cleaning agent but didn’t know how it worked.  But in the twentieth century chemists have discovered that it is due to the way that soap molecules are partially soluble in water and partially soluble in fat, and can form a bridge between the two.  This explains how soap can shift fat and grease in a way that water alone cannot.   Not only that, we can now create other similar and better cleaning materials where we have much more control over the final product.

This new class of materials are referred to as synthetic detergents, or just detergents.  The strict definition of a detergent is a cleaning agent that is soluble in both water and oil.  So if you want to be pedantic about it, soap is just as much a detergent as any other.   But most people would distinguish soap from other detergents purely on the grounds of our familiarity with it.  It is often called natural soap,even though there is nothing particularly natural about it.  We’ve just been using it for a long time so it seems natural.

Is Soap Better Than Detergent?

Detergents have displaced soap from many applications – but it remains the most popular choice for everyday use.  This isn’t just conservatism on the part of consumers.  Soap is, as it happens, a pretty good choice.  It isn’t a very efficient cleaning agent but that turns out to be a good thing.  It certainly does remove dirt and grime from your skin.  And it also removes the skins own fats.  But it does so only to a limited extent.

Let’s probe a bit deeper into the way soap and detergents interact with the skin.

The skin protects itself by secreting a greasy material called sebum from the glands onto the surface of the skin.  This stops stuff getting into your skin, and it stops water in the skin from being lost too quickly.   Cleaning the skin removes some of this natural protective layer and allows the skin to dry out more quickly.  The skin does replace what is lost, but for a while after washing or showering you are more vulnerable to your skin drying out.

The second layer of defence of the skin is its upper layer, which contains some very specific oils that are arranged in ordered layers which provide a very effective barrier to things getting in from the outside and again to water escaping.  In fact this layer is so effective it works even though it is only half the thickness of photocopy paper.  Detergents can penetrate into the skin and temporarily disrupt this ordered structure.  Again the effect is to let water get out from the skin more quickly than usual leaving your skin dry.

All Soap Is Irritating

So getting back to soap, it happens to be a pretty good compromise between cleaning the skin and damaging it.  It does take off some of the oils from the surface of the skin, but it doesn’t really do very much to the oils inside the top layer of the skin.  So all soap is drying – even home made soap made by crafters like these rather fun soaps on sale on Etsy.  Don’t write in and tell me yours isn’t.  It is.  But most soap is not too drying.

You can come up with a synthetic detergent bar which has a similar or even a superior level of mildness.  But very few of these have ever taken off much in popularity.  Dove is probably the most successful.  Instead of soap it is based on sodium lauroyl isethionate, a very mild synthetic detergent.  

This is much more popular in the States than it is in Europe, for reasons that nobody has ever been able to explain least of all Unilever the Anglo-Dutch company who makes it.  It certainly isn’t due to them not trying to get sales to take off – at one point in the nineties they delivered a free bar to most of the population of the UK in the hope that if they tried it they would fall in love with it.  It was a brave try, but they didn’t.  Detergent bars remain very rare in Europe, and even in the states they are much less popular than the traditional soap that has been used for centuries.

The Difference Between Liquid Soap and Tablet Soap

But while natural soap still rules the tablet category liquid soaps are almost always composed of detergent.  The only big corporation exception to this is Dr Bronner’s Magic Soap which is chemically speaking a soap.  Some small and natural companies also produce them.  But the chances are that the liquid soap you are most likely to come across is detergent based.  

The reason for this is largely because people expect liquid soaps to foam, and natural soap is not great for foaming.  People like bubbles.  The price we pay for our love of a rich creamy foam is that the drying effect on the skin is somewhat greater. They probably remove sebum from the surface of the skin at about the same rate as soap does, but they also penetrate the skin and disrupt its structure enough to allow water to escape more quickly.

The same process goes on with shower gels, which again are primarily detergent based and generally prioritise foaming over mildness.  Formulators know very well how to make them milder, but also know very well that they won’t sell if they don’t deliver plenty of bubbles.  And it just so happens that kinds of formulation that create bubbles also have a tendency to disrupt the ordered structure of the oils in the top surface of the skin.

So that I think is what the doctor was saying.  Liquid soaps do tend to be drying because of the detergents they contain.  And this is the result pretty much of a choice on the part of consumers to sacrifice mildness for foaminess.  If you are troubled by dry skin you might well want to stick to ordinary soap bars, and if it is really bad even to look for alternatives to soap itself.  But that is probably another blog post.

5 thoughts on “How Soap And Shower Gel Dry Your Skin”

  1. It’s good to go back to basics. Thanks.

    Where does pH come into this? Is it a separate issue, or does the pH of a soap or shower gel affect its level of the disruption to sebum and the deeper layers?

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  4. This is great. I personally don’t have time to do a lot of reading so this route of teaching works so much better for me. I listen while driving or while at home doing other things. So convenient! Look forward to more! Make up chemistry is my favorite subject!

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