Naked Science Forum

Life Sciences => Cells, Microbes & Viruses => Topic started by: thedoc on 26/02/2016 01:50:01

Title: Why does hair get soft when it's wet but not become soluble?
Post by: thedoc on 26/02/2016 01:50:01

Cristian asked the Naked Scientists:
   Why is does hair get soft so quickly when wet although it doesn't seem to be soluble.

Same question for nails I guess?
What do you think?

Title: Re: Why does hair get soft when it's wet but not become soluble?
Post by: evan_au on 26/02/2016 09:12:38

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(hair) doesn't seem to be soluble

Hair and nails are made of a protein called keratin.

Keratin with a slightly different structure forms reptile scales, feathers and animal horns.

With all these structures exposed to the weather, a non-soluble material is preferable!

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Why is does hair get soft so quickly when wet although it doesn't seem to be soluble.

Hair consists of very fine fibers, and the high surface tension of water binds it together.
Some birds that are continually exposed to water (eg ducks) have an oily coating on their feathers to repel water, keeping the bird warm and dry (and light enough to fly).

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Same question for nails I guess?

Nails and horn are one integrated piece, don't have such a large exposed area, and so are not as affected by water as hair and feathers are.

The harder versions of keratin have more sulphur bonds between the proteins strands, making them more rigid.

See: http://en.wikipedia.org/wiki/Keratin#Examples_of_uses

Title: Re: Why does hair get soft when it's wet but not become soluble?
Post by: chiralSPO on 28/02/2016 02:02:34

I believe this is similar to other cross-linked polymers, which will absorb solvent and swell without any of the cross-links breaking.

Solids that are composed of small molecules (like sugar, naphthalene, benzoic acid etc.) are held together by relatively weak intermolecular forces. If a liquid solvent can interact with each of the solute molecules about as strongly as the solute molecules interact with each other, then the solid will dissolve. However, the covalent bonds within the molecules are at least 20 times stronger, and so will not typically break when exposed to solvent because the solvent cannot satisfy the molecular fragments. (not the most graceful explanation, but if you need more clarification, I can try again...)

Crosslinked polymers are held together by covalent bonds, so must be chemically digested to break them up. For instance hair can be dissolved by the action of strong acids, strong bases (as found in Nair ®), or strong oxidizers...