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If it is more appropriate for Chemistry Stack Exchange, please let me know, although it happens when I am traveling.

I am very thorough when washing my hands (I prefer bars of soap to liquid soap), and I noticed that, in some places I have traveled to, I couldn't wash my hands very well: after lathering my hands with the soap, I cannot wash it away. No matter for how long I keep rinsing my hands, they remain soapy/slippery.

This does not happen everywhere I traveled to, but when it does, it appears to happen anywhere I wash my hands in a given city, making me think it might be something with the water: maybe the city's water is rich in minerals, or poor in minerals, or something similar? I should mention that I do not think it is from the soap, as I usually use, at least in the hotels, the soap I bring myself on trips (as I hate hotel's tiny soaps, except as a souvenir); I should also mention that my family thinks I am crazy and this actually is all in my head, so maybe it is about the interplay of the water and my skin's own chemistry? Or it could be because I wash my hands so obsessively, so it is easier for me than for them to notice.

What is going on? Is there a way to avoid that happening?

Community
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AnyDisplayName
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    Just to put some perspective on the last paragraph: You're not alone, I feel exactly the same need to rinse my hands until it does not feel slippery anymore. Sometimes it takes much much more time and it drives me crazy, and while I always thought the reason might just be the "amount of water", now I finally have a satisfying answer! – kopaka May 31 '23 at 11:30
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    When we wash our hands, is it necessary (or even beneficial at all) to keep rinsing until the slipperiness goes away, or is it fine to just rinse for, say, 10 seconds and then stop? – Tanner Swett May 31 '23 at 11:38
  • Had this experience (soap doesn't rinse off) only in one area so far; and I think it helped to use warm/hot water for rinsing. Give it a try and report back! – oliver May 31 '23 at 12:46
  • @kopaka I guess my family just doesn't wash their hands very well... – AnyDisplayName May 31 '23 at 16:33
  • @oliver Will do! – AnyDisplayName May 31 '23 at 16:34
  • @TannerSwett That is a good question as well, but, personally, I just fell very uneasy with the slipperiness, even if my hands would be technically clean after a few seconds. – AnyDisplayName May 31 '23 at 16:35
  • I always assumed this was because a lot of hotels give you really cheap soap. – pacoverflow May 31 '23 at 18:02
  • @pacoverflow Yeah, I thought that was a possible reason as well, until I started carrying around my own soap. – AnyDisplayName May 31 '23 at 18:26
  • Go find the garden hose spigot outside. There's less chance that they're softening that water. Having grown up spoiled in Chicago, now with a summer home in rural MI on well water... there's a second faucet on the laundry sink that is not softened explicitly for washing your hands (and watering plants because it's slightly salty). – Mazura May 31 '23 at 19:36
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    Differences in hardness of water may be one answer, but in my home with its one water supply, I've experienced this difference with different soaps. I've always understood it to be a difference in a moisturizing effect in the soap, as in a feature for some people with dryer skin that may like that feeling of having slippery skin or that it may help them avoid their skin cracking as often. I could be wrong on this, so I'm not providing this as an answer. It's just that I'm not convinced that the current one is sufficient to explain this, given my having only one water supply and still seeing it – JoL Jun 01 '23 at 02:05
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    I got the idea that it's a moisturizing effect from Dove commercials that advertised their soaps as moisturizing and for "delicate" skin. Dove soaps are (or were, idk) some of the more expensive soaps, and I remember hating how slippery they left my skin feeling, how hard it was to completely wash off. I figure that characteristic is what they were advertising. Those commercials and my trying them was possibly in the 90's or early 2000's, so I don't know if they've changed from then. – JoL Jun 01 '23 at 02:26
  • @JoL Funny that you mentioned Dove, those are too the soaps that I also now avoid after experiences in the past where it left slipperiness feeling, which doesn't happen with other soaps. – justhalf Jun 01 '23 at 14:25
  • @JoL different faucets in the house may have strikingly different water quality. – EarlGrey Jun 02 '23 at 08:46
  • @JoL I see, well, different soaps have different compositions. Soft and hard waters are not too dissimilar: they differ only for some 0.1% of their composition (soft water = 0 to 60 mg-CaCO3 per liter, hard water 120 to 180 mg-CaCO3 per liter) so it may well be that different soaps may or may not have excess of stearate ions to build the "slippery" layer. – EarlGrey Jun 02 '23 at 10:05
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    I've noticed this problem specifically with glycerin-based soaps, which I now avoid purchasing for that reason. Oddly, I've not had a problem with Dove which several other people have mentioned. But I usually get Dove in bar-form. The ones that I have a problem with are glycerin-based liquid soaps. – Darrel Hoffman Jun 02 '23 at 20:29

4 Answers4

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Soap is mostly a chemical called sodium stearate that is moderately soluble in water. In pure water it dissociates into positive sodium ions and negative stearate ions, and the negative stearate ions adsorb onto the surface of your skin. This creates a layer of negative charge on your skin, and when you rub your hands together the negative charges repel. The result is a layer of water is trapped in between your hands when you rub them together, and this is what causes the slippery feel.

You can eventually rinse away the layer of stearate, but since it adsorbs strongly to the skin it takes a lot of rinsing.

This is what happens in pure water, but in many regions the tap water contains divalent ions like calcium, and calcium stearate is insoluble. As long as there is an excess of soap the calcium stearate precipitates to form soap scum and plenty of stearate ions are left to produce the slippery feel. However when you start rinsing the stearate concentration falls until enough calcium ions are left to bind to and neutralise the layer of stearate adsorbed on your skin. This neutralises the negative charge so it removes the slippery feel, and if fact the calcium stearate left on your skin has a higher friction than clean wet skin so you get an abrupt transition from a slippery feel to a "grabby" feel.

So the differences you notice are due to the different concentrations of divalent ions in water i.e. the hardness of the water. In soft water regions it takes a long time to completely rinse the soap away while in hard water areas to get a fast transition from slippery to "grabby".

John Rennie
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  • You beat me to it, and with a better explanation. – Peter - Reinstate Monica May 31 '23 at 07:29
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    I used to be a colloid scientist working for Unilever, and Unilever sells a lot of soap :-) – John Rennie May 31 '23 at 07:41
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    Wow, johnny sir here :) – An_Elephant May 31 '23 at 11:59
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    Wow, that is an amazingly thorough answer! Thank you very much! – AnyDisplayName May 31 '23 at 16:44
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    the grabbiness is what some people call "squeaky clean" – Kate Gregory May 31 '23 at 17:45
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    "In soft water regions it takes a long time to completely rinse the soap away" - Interestingly, the wikipedia page claims soft water cleans better: "soft water requires less soap for the same cleaning effort, as soap is not wasted bonding with calcium ions". Those two claims are not mutually-exclusive, but it's surprising if both are true. – BlueRaja - Danny Pflughoeft May 31 '23 at 22:49
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    @BlueRaja-DannyPflughoeft soap is so effective at cleaning precisely because it is hard to rinse away. It adsorbs strongly to the surfaces of dirt and oil drops and lifts them away from whatever you're cleaning. You are left with a monolayer of stearate ions on your skin as a side effect of this. – John Rennie Jun 01 '23 at 05:05
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    @BlueRaja-DannyPflughoeft iow with hard water a lot of the soap is going towards neutralising the hardness of the water, not the cleaning effect of the soap. – jwenting Jun 02 '23 at 06:53
  • What level of hardness is necessary to make the soap easy to wash off? Here in Vancouver, we have soft water (7-11 mg/L), but the soap washes off easily. In my old home, we had a water softener, and the soap became quite slippery. – nneonneo Jun 02 '23 at 19:26
  • @nneonneo It's not just the amount of minerals. Which specific minerals are present makes a difference as well. – bta Jun 02 '23 at 23:04
  • @nneonneo You have to analyse your water to see what metals it contained. I found an article by your water company on the subject and at some of their plants they add lime (calcium hydroxide) to the water. If your plant does this the calcium would be reacting with the soap. – John Rennie Jun 03 '23 at 04:24
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You have an answer to "why" but so far, "what can I do about it" has not been answered.

I have very soft water and the same problem with hands still feeling soapy after rinsing. I've found two things that work for me. After rinsing hands a suitable amount . . .

A few drops of vinegar on the wet hands, rub hands together, then rinse. This instantly eliminates the slippery feel, but then your hands smell like you've been working in a darkroom. I don't mind the odor. You might try using a few granules of citric acid ("Sour Salt") instead of vinegar. I've never tried it but it has no odor.

. . . or . . .

Table salt from a salt shaker. Just a sprinkle, then rub hands together and rinse.

MTA
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  • Will try them! Specially the vinegar one, I wouldn't mind the smell... – AnyDisplayName May 31 '23 at 18:28
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    Hmm… I would assume the vinegar just protonates the stearate to stearic acid, in which case pretty much anything mildly acidic (vinegar, citric acid, lemon juice, etc.) should indeed work to get rid of the slippery feel. And conversely, I'd assume that adding a bit of baking soda (i.e. sodium bicarbonate, which is basic, and also precipitates out calcium ions from water as calcium carbonate) to the water ought to work for making your hands slippery after washing, if you wanted that. And I just tried both, and they do seem to work. – Ilmari Karonen May 31 '23 at 23:01
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    … But I don't get how the table salt (sodium chloride) is supposed to help: it seems like, if anything, it should increase the slippery feel by adding more sodium ions. And it didn't seem to have any effect when I tried it, either. Maybe your table salt contains some additives (like anti-caking agents) that mine doesn't? (Mine apparently contains some potassium iodide and sodium ferrocyanide.) – Ilmari Karonen May 31 '23 at 23:12
  • @IlmariKaronen I can't explain the chemistry, but my "kitchen chemistry" rationale in trying table salt was based on the fact that bar soap does not lather well in sea water, so perhaps using table salt on hands would cancel any remaining surfactant effect on slippery hands. The vinegar rationale was based on knowing that alkali such as dilute lye solution on hands is slippery and takes forever to wash off, so a weak acid should neutralize that effect. – MTA Jun 01 '23 at 00:47
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    Bar soap doesn't lather well in sea water because sea water contains high concentrations of calcium and magnesium. – John Rennie Jun 01 '23 at 04:28
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    @IlmariKaronen Sodium bicarbonate cannot precipitate calcium carbonate, you'd need to dry/heat the solution to convert bicarbonate to carbonate. In fact, water hardness is often due to dissolved calcium bicarbonate in the first place. – Dmitry Grigoryev Jun 01 '23 at 08:09
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Is there a way to avoid that happening?

Toothpaste typically contains soluble calcium ions (needed to remineralize your teeth) and is commonly available in the bathroom. Rubbing even a tiny speck of it into your hands will instantly eliminate any remaining soap, same as hard water would do.

Dmitry Grigoryev
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My late answer is from practical experience.

As others have said, with hard water (lots of dissolved minerals) it is difficult to raise a lather from soap, but with soft water it is easy to use too much if you are accustomed to hard water.

My way to kill the excess soap is with detergent. Washing the hands with a drop or two of washing-up liquid (aka dish 'soap') will get rid of the excess soap.

Weather Vane
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