When we take baths, how does the water in the bathtub absorb only the dirtiness on our skin?

Question

I'm 16. I study chemistry in college along with biology, physics, and maths, and I have a general interest in all of them.

I always found this interesting: sometimes when I left the bathtub, even when I forgot to use soap, the bath would still have a somewhat noticeable, murky tint in the water, indicating that something had dissolved in it, even though I didn't take the time to rub it off manually. When we get in the bath, how does the dirtiness (small amounts of mud, bacteria, pen markings etc.) dissolve in the bath alone without any of our skin dissolving as well so that we can leave clean? Maybe dead skin might dissolve, that would make sense, is there an adaptive mechanism on the skin involved in this sort of process of hygiene? The same thing would happen if I put any other object in the water with something on its surface and the object would come out clean when the thing on the surface dissolved.

How does the water break the attraction? Some sort of cohesive force, between the atoms/molecules of the thing (dirtiness) on the surface of the object and the atoms/molecules of the object (our bodies) and separate them to make it dissolve? And how can it differentiate between the cohesion of the skin and the dirtiness (what determines solubility, if both polar and non polar molecules on our skin will dissolve in the water)?

Answer 1

Water is called a universal solvent, and perhaps it is rather a miraculous solvent with unusual properties. It is able to "wet" a lot a surfaces and thus able to disrupt the adhesive forces. (Cohesion refers to sticking of like molecules only.)

However, the murkiness in water is not due to dissolution of dead skin or dirt dissolving in water, but rather it is a suspension (hence murky). If you had a microscope, you would rather see particles of dust, mud, soil, oils, dead skin etc if someone has not taken a shower for a while. Just pouring water does not remove these things from objects, mechanical action is also needed to dislodge them from the surface (e.g., dishwashers and washing machines use mechanical forces to clean items with water).

Since you appear to like science, next time do a Tyndall light experiment. Take that murky tinted water (as you call it), and fill it in a transparent glass tumbler. Pass light through it, and view that water at ninety degrees (to the light beam). If you see a light beam, it indicates really small particles floating in water. It is not a true solution.

Answer 2

The outer layer of skin is water-insoluble. According to this paper, there is a structure specifically made in skin cells before they die and become part of the outermost layer of the skin:

The outermost, cornified, layers of the epidermis are composed of terminally differentiated keratinocytes known as corneocytes. Corneocytes lack a plasma membrane and instead are encased in a structure known as the cornified envelope (CE). CEs consist of highly insoluble, cross-linked proteins with covalently attached lipids and are essential for the mechanical integrity and water impermeability of the skin.

So not dissolving in water is one function of the skin. It also protects the inside of the body from dehydration by preventing excess evaporation of water on the skin (unless you are hot, when sweating helps to maintain a lower body temperature, also important for survival).

[OP] How does the water break the attraction?

That depends on what is on the skin. If you are muddy, the components of the mud are easy to wash off with water because the components are polar, and water is polar. If you are covered in motor oil (which is non-polar), water alone will not be efficient to wash it off, but soapy water will work (with some scrubbing).

[OP] When we get in the bath, how does the dirtiness (small amounts of mud, bacteria, pen markings etc.) dissolve in the bath alone without any of our skin dissolving as well so that we can leave clean?

While the layers of dead skin cells are not water-soluble (or soapy water soluble), we nevertheless continually shed these former cells, and continually replace them. In fact, much of household dust is made of material we shed (e.g. it contains quite a bit of cholesterol, as described here).

When we take baths, how does the water in the bathtub absorb only the dirtiness on our skin?

It doesn't. When you take long bubble-baths, your skin will also lose oils and other substances (giving you "dry skin"). Some people use oils or lotions ("moisturizer") to replenish these after a long soak.