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Discussion around the question Why does bleach feel slippery? has started me thinking about the saponification explanation for the slippery feeling of basic solutions.

According to Wikipedia:

Alkaline drain openers can dissolve hair (containing proteins) and fats inside pipes via alkaline hydrolysis of amide and ester respectively:

$$\ce{RCONH2(amide or proteins)+ OH− → NH3 + RCOO−}$$

$$\ce{RCO2R’(ester or fats)+ OH− → R'OH + RCOO−}$$

and just above that:

Essentially, the hydroxide ions from the basic lye attack the carbonyl carbons of the fat, which eventually kicks off the hydrophobic tails of the triglyceride (tristearin/fat) to isolate glycerol.

So I am not sure if the "saponification" explanation for why a small amount of a fairly strong base feels slippery between thumb and forefinger really says that the reaction has produced a true soapy surfactant or that it should really say that it is lubrication by glycerol-like compounds that generate the intense slippery feeling.

Wikipedia also says:

The Stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis, consisting of dead cells (corneocytes). This layer is composed of 15–20 layers of flattened cells with no nuclei and cell organelles. Their cytoplasm shows filamentous keratin. These corneocytes are embedded in a lipid matrix composed of ceramides, cholesterol, and fatty acids. (emphasis added)

Corneocytes are keratinocytes and named as such because of the abundance of keratin protein filaments they contain.

I seem to remember my very smart high school chemistry teacher telling us that the breakdown of protein was also important, but I don't remember the production of soap as being so central. So I'd like to ask if there is any actual empirical evidence - ideally with a scientific reference, that demonstrates which molecules present between the fingers in this situation is primarily responsible for the slippery feeling. Potential candidates might include:

  1. Soap/surfactant,
  2. Glycerol produced from hydrolysis of "finger lipids",
  3. Broken down protein products, or
  4. the pre-existing inter-cellular lipid layers that would be exposed by the removal of successive layers of cells within the stratum corneum.

below: Epidermal layers, from here.

enter image description here

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"Slippery" is vaque. When dealing with 0.5 to 5 M NaOH and gloves I had not really the impression it was slippery by itself. For sure the solution becomes slightly more viscous at higher molarity. Similiar to making a higher molarity Urea solution. If at all the soap like effect is weak.

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  • $\begingroup$ Wearing gloves makes your answer moot. "So I am not sure if the 'saponification' explanation for why a small amount of a fairly strong base feels slippery between thumb and forefinger really says that the reaction has produced a true soapy surfactant or that it should really say that it is lubrication by glycerol-like compounds that generate the intense slippery feeling." Note also the proteins and lipids tags, and the large image of Epidermal layers. $\endgroup$ – uhoh Nov 15 '18 at 10:25
  • $\begingroup$ on skin, I also had not really the impression it is soapy. maybe you should wait until it dissolves your layers :D But then it is because of the dissolved keratin and not NaOH :/ $\endgroup$ – dgrat Nov 15 '18 at 10:27
  • $\begingroup$ I hope you haven't put several Molar NaOH solutions on your skin, nor anyone else's. Chemistry SE may actually have some rules about discussing dangerous activities in posts or comments. Here I'm just quoting Wikipedia and my beloved high school chemistry teacher. $\endgroup$ – uhoh Nov 15 '18 at 10:29
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Potential candidates might include:

  1. Soap/surfactant,
  2. Glycerol produced from hydrolysis of "finger lipids",
  3. Broken down protein products, or
  4. the pre-existing [inter-cellular lipid layers][5] that would be exposed by the removal of successive layers of cells within the stratum corneum.

Very good guesses but none of the above actually (i think, what does "finger lipids" mean?). But the reason bases feel slippery to bare skin no matter how much you scrub first, is bases attack the phospholipid bilayer of your cells quite readily not only saponifying them, but breaking the inter-molecular bonds of molecules that already resemble a soap. Take for example phosphatidylcholine:

Phosphatidylcholine

The phosphochloline already has all of the elements to make a soap and when base is added, the molecule does not need to interact with other molecules to achieve local charge balance, but now have the ions from the base to balance, which breaks intermolecular bonding creating a small layer of soap, making the base feel slippery.

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    $\begingroup$ I see; bases allow us to "release our inner soap". This is a very helpful and teaching answer, thank you very much! $\endgroup$ – uhoh Nov 16 '18 at 0:31
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As a chemist, I have no idea what "glycerol-like" means. I guess erythritol, sugars, sugar alcohols, sugar acids, amino sugars, and other polyols qualify. I just dissolved some sucrose on my fingers, and didn't notice any significant slip. Even more significant US Patent Application US 5709849 A claims that polyhydrics are "sticky", not slippery. So, your first task is justifying (to yourself, at least) your claim that polyhydrics like glycerol are slippery. Also, your first Wikipedia link claims OH- "reoxidizes" Al2O3. That is ludicrous. Coming across some nonsense like that is a good reason (imho) to stop reading. Or, at best, take anything else claimed as dubious. Now your question is imho an interesting question but isn't clear enough. You seem to acknowledge that having a slippery feel is different than actually being slippery. We can measure the coefficient of friction between two surfaces, we can't objectively measure the feeling we get when pressing and rubbing thumb to forefinger. Broadly, I'd mechanistically reduce your question to:1. does a strong base change the physical structure of the skin? 2. does a strong base change the composition of the skin? and 3. does a strong base change the lubricants present between the two (skin) surfaces? (Assuming that we've established that the coefficient of friction does change in the presence of a solution of a strong base). There are some experiments you could do to determine whether it was a permanent change in structure and/or composition causing it. Water makes things slippery (in general). Soaps are well known lubricants. The use of soaps, surfactants, and oils is well known in the cosmetics formulator's repertoire, but it is mostly an area of empirical science. Feel and tactility aren't easily reduced to molecular effects. Distinguishing between mechanical (physical, structural) effects and chemical effects isn't easy when both occur together. It's certainly the case that hydrogen bonding is important for the adhesion/slip of skin. While soaps might orient at the surface, it's possible that the reduction in the number of 'available' h-bonding sites is what reduces friction/adhesion between skin and another polar surface. A strong base will provide more opportunity for chemical groups on the skin's surface to h-bond to it instead of to polar groups on the other surface. Based on the slippery feel my old finger tips have, another aspect to this is the ability of base to extract (solubilize) oils present on the skin surface which may be there to improve friction/tactility. My best guess, and it is just a guess, is that since surfactants at neutral pH feel slippery, that the main tactile effect of base on skin is the creation of soaps. But a definitive answer this is not.

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    $\begingroup$ The title: Is it known for sure that bases feel slippery because of the production of soap/surfactant? and a lengthened re-statement without the space constraint is So I'd like to ask if there is any actual empirical evidence - ideally with a scientific reference, that demonstrates which molecules present between the fingers in this situation is primarily responsible for the slippery feeling. I am absolutely not asking for what you or anyone thinks is happening, I'm asking for data. While I know condescension can be fun, there's no need for any more of it in SE than there is already. $\endgroup$ – uhoh Mar 14 '17 at 14:55
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    $\begingroup$ I've done the best I can to show some independent research as defined by stackexchange (take a moment and enjoy the tour!), and stated my reasoning clearly so that if there are errors or misconceptions, they can be cleared up or corrected. The problem with the Aluminum in the Wikipedia article may be just a wayward drive-by edit, one or two words changed. It is Wikipedia after all. $\endgroup$ – uhoh Mar 14 '17 at 14:55
  • $\begingroup$ Re: Also, your first Wikipedia link claims $\ce{OH-}$ "reoxidizes" $\ce{Al2O3}$. That is ludicrous. // Actually that is exactly how it works... All Al metal has a coating of $\ce{Al2O3}$. $\endgroup$ – MaxW Mar 14 '17 at 17:04

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