Soap and rain water, why does it not rinse off?

Question

I am in Papua New Guinea at the moment, where rain water harvesting is the most common way of getting water. When washing hands, the soap feels hard to rinse off, and my skin feels "greasy".

I read around a bit here and here and I think I understand what is said: essentially, sodium stearate in the soap separates into sodium and stearate ions, which then precipitate with other ions (like Calcium, into calcium stearate) and get rinsed away. But in very soft water, there are sodium ions rather than calcium ones, so this reaction is slowed down a lot, resulting in the greasy feeling. As it turns out, I have access to hard ground water at the same time, and the difference is very obvious: with hard water, soap is rinsed off instantly.

My question now: I assume rain water has in fact almost no ions in it (I'm thinking Sodium ions mostly), and I would therefore assume that the Sodium stearate dissolves very easily, but has no replacement ion to precipitate with, therefore sticks to the skin.

Is this the right explanation or are there other factors at play in the explanation about that greasy feeling?

Answer 1

Yes, this is a good explanation. Rain water is almost pure water, it lacks of bivalent ions such as $\ce{Ca^{2+}}$ and $\ce{Mg^{2+}}$ which helps soap to rinse of. You will get the same effect when a water softener is installed on water distribution.

Answer 2

The inefficiency of soft, essentially pure, water for rinsing soap from skin is well recognized. However, a broader explanation includes the difficulty in removing the last traces of anything from anything else.

I estimate that the wash liquid from bar soap results in about 2% soap, perhaps more. This concentration will reduce the surface tension of water dramatically, and give you that slippery feeling. Rinsing is generally not as energetic as soaping up, so the soap solution you forced into your skin doesn't usually get the same rough treatment during the rinse cycle. Rinsing might cut the concentration of the soap in that viscous layer next to your skin by half, maybe a little more; do it again, cut the soap by another half, perhaps reducing the sodium stearate concentration to 0.1% or 1000 parts per million. Yet the concentration of sodium stearate can be reduced by another factor of three before its surface tension is reduced at all (see picture, Ref 1)!

Since soap solutions have very low surface tensions even at very low concentrations, and skin is so porous and irregular (fingerprints), some soap may continually exude from the skin as molecules. Rinsing with pure (soft) water will eventually remove these soap molecules (as molecules, not micelles) at very low concentrations (but still surface-tension-reducing). The benefit of hard water during the rinse is that the molecules of soap are deactivated as the rinsing proceeds, and even before it is completely accomplished. So the rinse seems to be finished quicker.

Strong mechanical action will do much the same as hard water in squirting the last of the soap molecules out of the skin. It is interesting that some people like the "grabby" feel of non-soapy skin while some prefer the silky feel of soft water washing.

Ref 1. https://www.researchgate.net/figure/Effect-of-sodium-benzoate-and-sodium-stearate-concentration-on-surface-tension-in-water_fig1_336817830