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Skin absorption of soap with additives
I am a hobby “formulator” and I have a question. I keep seeing people make soap with clay or botanical additives (like stuff that “makes your stretch marks go away” -ok). Now, I consider myself to be smart and logical, but what do I know? So I questioned someone in a Facebook forum if the “organic silica” she puts in her soap will actually serve any purpose. Apparently, she claims this silica contained in the clay will replenish whatever elements our body stops producing wth aging, and helps with collagen production etc.
Now, soap is a surfactant which will remove things off the skin and not add things into the skin. Right? Anytime I see soap with clay I wonder how much it actually helps, since the soap does not remain on the skin, differently from a clay mask.
So I conclude this is BS. Pure marketing claim. Or am I missing something here? (Of course I know the skin absorbs varying amounts of stuff we put on it.) -
14 Replies
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Skin is pretty good at not allowing various stuff to be absorbed. It’s called barrier function. All claims you mentioned above are marketing nonsense.
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Right! Of course she got all offended so I sent a bunch of scientific articles about cutaneous absorption her way.
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Yes, those are all marketing BS claims. Soap cleans the skin. It removes everything it can from the skin. If you put extra ingredients in your soap, soap will not care if those ingredients are meant to affect the skin. Soap will remove those ingredients too. They will have exactly zero effect on the skin. They may have an effect on the soap by making it less effective at cleaning the skin.
This is also true of putting oils or water soluble ingredients in shampoos and body washes. Those ingredients will not do anything & just get washed away.
The exceptions are things like Silicones and Cationic Polymers. Detergents like soap are not able to solubilize silicones so they can get left behind. And cationic polymers are big enough molecules that they can also get left behind.
But for the vast majority fo ingredients added to soaps and cleansers, they all get washed down the drain and do nothing.
I find that misinformed people are not easily swayed by scientific information. If someone benefits from their customers or fans believing their nonsense, they will not appreciate people correcting them whether it’s based on science or not.
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Sorry @Perry but I have to disagree 😮
Perry said:
Soap cleans the skin. It removes everything it can from the skin. If you put extra ingredients in your soap, soap will not care if those ingredients are meant to affect the skin. Soap will remove those ingredients too. They will have exactly zero effect on the skin. They may have an effect on the soap by making it less effective at cleaning the skin.I agree that most of the soap is washed away in use. This the nature of any wash-off cosmetic. While soap cleans the skin, it really can do so much more.
Soap formulations vary significantly. While soap may cleanse the skin by removing dirt and oil… there will be differences in the amount of removal, the residue left behind, as well as the percutaneous absorbtion of the ingredients in soap depending on the formulation.
A bar of “soap” will be a combination of soap molecules, glycerin, and unsaponified or partially saponified oils. These may remain on this skin after washing. For example, a soap high in glycerin (some oils result is over 15% glycerin as a product of saponification) and oil will leave a film on the skin. As @perry mentioned this may because the soap is less effective at cleaning the skin.
Percutaneous absorbtion varies by a number of factors. Duration of contact, aqueous solubility, lipophilicity, and the presence of percutaneous penetration enhancers.
Surfactants, including soap, can act as penetration enhancers.
see The effects of surfactants on penetration across the skin - WALTERS - 1993 - International Journal of Cosmetic Science
https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1467-2494.1993.tb00572.xOther studies have shown that surfactants can increase transdermal permeation. see
Status of surfactants as penetration enhancers in transdermal drug delivery J Pharm Bioallied Sci. 2012 Jan-Mar; 4(1): 2–9.
While this study is examining drug delivery, there is no reason not to hypothesize that the same may be true for ingredients added to soap.
In my opinion, soap is a sophisticated cosmetic product that is often dismissed as insignificant.
I will get off my soapbox now!
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@Dr_Sara
Won’t excess glycerin wash off with the water used to wash with a soap? Since it’s water soluble and not substantive to the skin even an excess deposition of glycerin should wash off. -
Sadly, there is not really any interest in studying soap so I probably cannot cite any current studies.
If you compare soap without glycerin and soap containing glycerin, there is a difference. You can detect this just holding the two products before washing.
Most of the large soap manufacturers separate the glycerin from soap because they have two commercial products to sell. Compare a bar of Ivory with a bar from a soap maker and see if you can tell the difference.
I suspect that the glycerin does not all wash away. Before I started making soap I would have doubted there was a detectable difference. I am now completely convinced.
Some substances leave a noticeable residue after a brief exposure. Squirt some honey on your hands, rub it around like soap and rinse it off. Although the honey is water-soluble, honey still is emollient following this brief exposure.
Combine the two, glycerin and honey, and you have a fantastic bar of soap!
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Clarification- When I mentioned the glycerin in soap, I am referring to the byproduct of saponification and not “glycerin soaps” (soap dissolved in glycerin).
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@Dr_Sara - It’s ok to disagree. No one here has a monopoly on the truth & my opinion about specific topics are just what I believe based on the research / experience I’ve been exposed to. I’m always willing to update my beliefs when evidence suggests I should.
But as to the delivery benefits of soap, I’m not convinced.
First, I agree with you that surfactants can act as penetration enhancers. And while we can hypothesize that that penetration enhancing will deliver ingredients to the skin, that is not the most likely thing that will happen. To get skin penetration enhancement the ingredient has to remain on the skin for some amount of time. Soap (and body wash) is put on the skin, then rinsed off. There isn’t enough time to leave any significant amount of whatever ingredient you are trying to deliver. At least, I’ve never seen a study to demonstrate that.
Second, if a soap is leaving a film on the skin that means it is not cleaning the skin. Cleaning skin and leaving things behind are conflicting goals. If you are making a soap that works as a cleanser, then it is removing anything that you are hoping to deliver (especially water soluble ingredients). If you use a soap and it leaves a film, that means it’s not working. You would have been better off to use a leave-on skin moisturizer after you use soap that works.
I would be curious though to see studies that demonstrate my beliefs are incorrect. Can you point to a study where a soap with deliverable ingredients was compared to the same soap without the deliverable ingredients and it provided some benefit to the skin?
I don’t believe that it is beyond the realm of possibilities that if you pack a soap full of oils or other nonpolar ingredients that you can get some of that left behind on skin. I would just say that then your soap isn’t cleaning the skin. You would have been better off first cleaning your skin, then using a product afterwards to deliver whatever ingredient you were hoping to have left on the skin.
One area I did a lot of work in was a 2-in-1 shampoo. We were able to show some silicones were deposited and cationic polymers and that there was some slight benefit over just using a shampoo without those ingredients. But the reality was that no 2-in-1 shampoo ever worked as well as shampooing the hair then using a conditioner.
Change My View - If trying to deliver an ingredient to skin or hair, cleansers are not a good way to do that.
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Thank you or your reply @perry.
While I find it interesting that soap and other surfactants can act as penetration enhancers, this is not really my area of interest. I agree the exposure time is likely a very important factor and short exposure may not lead to delivery it would be interesting to investigate. Some chemicals can elicit a reaction with very brief exposure.
An important question might be “how clean is clean?” Personally, I think soap should remove dirt yet not strip all the oils from your skin. Maybe we are trying to be too clean? Perhaps by using soap that is super-fatted and contains glycerin you are cleansing without removing the endogenous oils.
Interesting that you mentioned shampoo. One of my hobbies is reading old pharmacopeias and cosmetic texts. Before detergents became available, shampoo would have been soap based. Today if you asked someone to use soap on their hair, they would not be satisfied with the result. Their hair would not feel clean. Although soap-based shampoo bars are becoming more popular.
Sadly, most of my evidence is anecdotal.
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Some stuffs sure can be left behind. Easiest examples are menthol and fragrance. No scientific study needed. That cooling or scent lasting around is the direct evidence.
But I also don’t believe a wash-off prouduct will have that penetration enhanceing effect that’s supposed to because of the high amount surfactant.
I mean if that’s true, why would ingredients that was limited to use, i.e. preservatives, universally has less regid regulation for wash-offs (usually high in surfactant) than leave-ons? Like you can use kathon cg in shampoo and face wash almost anywhere, but in leave-on it’s banned in EU, China, Japan among other regions. -
Hi @zetein. I think to convince scientists, we like to see studies that support our ideas even when something is as obvious as fragrance transfer from cosmetic to skin.
zetein said:But I also don’t believe a wash-off prouduct will have that penetration enhanceing effect that’s supposed to because of the high amount surfactant.This is something that would have to be investigated. Many biological processes happen very quickly. I mentioned the Walters study because it is an interesting idea, not because I believe every additive will be able to permeate cells in this method.
Perry said:
Change My View - If trying to deliver an ingredient to skin or hair, cleansers are not a good way to do that.@Perry, I appreciate your fact-based opinions. I would love to change your view.
Perhaps I should send you a bar of soap? Many experiments begin with anecdotal observations and maybe that is where I am?I think maybe my message got a bit muddled. I am not trying to deliver ingredients using soap, however, I do believe that there are some ingredients that are left behind. Glycerin for instance (if I had a lab I would look at soap-based glycerol’s ability to enter skin through aquaporins for example).
Soap with excess oil and glycerin may work more like a cleansing cream- removing dirt yet leaving a film behind. The oil in soap solubilizing the dirt and oil yet not completely stripping away all oils or the microbiome.
Question about the hair industry for @Perry; Most shampoos on the market, even inexpensive ones, are not just detergents to clean hair. There are other ingredients added to leave hair in a “desirable condition”. Do you find these additives valuable to hair products? Do they decrease the cleaning ability?
Finally, I appreciate that products like 2-in-1 shampoos may not be as efficient as using separate products (I actually do use both shampoo and conditioner!), but some people are lazy. I wonder how many cosmetic chemists have elaborate beauty routines? I rarely get beyond using a bar of soap (imagined screams of horror from the readers!).
I seem to have crawled back up on the soapbox!
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I have read the discussion and found it a wonderful one !
I too have always wondered how soaps with little contact time claim to deliver benefits. Then recently I have started using shampoos that are sulphate, silicon and paraben free (advisable for curly hair) . I have seen visible difference in my hair. It seems some products moisturize, some strip off moisture (oil?) from skin/hair. I am following this discussion as this is very intriguing.
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Perry You are too objective.!I liked your saying very much
If you use a soap and it leaves a film, that means it’s not working -
Even though I love soaps and cosider them rather complex systems (I’ve seen many phase diagrams during my soap making years, whose phases depend not only on formulation but also on shear during extrussion, for instance, making some glycerol migrate from one phase to another only because of mechanical stress), I also believe that permeation of molecules like glycerol from soap through the skin is very difficult. Deposition of oils is more likely to happen.
Penetrating enhancers would help glycerol if conditions like contact time are met. Also, even though there are some aquaporins that help water soluble substances to go through the skin (there is actually one for glycerol), there are still so many barriers to overcome, like: 1) solvation (glycerol as being so hydrophilic is highly solvated by water molecules), 2) mechanical solvent flux (water will mechanically move solution while bathing), 3) little contact time (because of solvent flux, reducing contact time), 4) osmolarity (high excess solvent outside the skin would not favor skin permeation), 5) detergency (this is more for added oils in soaps, which mostly will be solubilized inside detergent micelles, although some could actually reach and remain on the skin). Things are different when glycerol is used in a cream, serum or gel, frem where it can migrate and permeate more favourably.
The residues that deposit from soap are some oils that escaped from detergent micelles, but mainly, calcium and magnesium salts of soap fatty acids (like calcium stearate), which actually give a soft touch to the skin (that’s why some soap manufacturers use mixtures of free fatty acids in their formulas). But, if you use this as a shampoo, these calcium salts plus natural sebum will leave hair heavy, greasy and dull, and this is why soap (made from tallow) was replaced by synthetic detergents in shampoos.
And in the case of shampoos, the residues are not meant to be unpleasant; for instance, cationic polymers deliver a thin flexible and soft film on the skin. Silicones on the other hand, when not properly formulated (especially non functionalized silicones), can give great conditioning at the start but build up on hair, making it heavy and non “natural”, but not dull as calcium fatty acids, since silicones have a high refractive index.
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