Why can't I wash away soapy hands?

Question

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?

Answer 1

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".

Answer 2

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.

Answer 3

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.

Answer 4

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.