Chemistry in your cupboard

Content

Application of Veet to the skin

To get a high concentration of thioglycollate anions, we need to increase the concentration of hydroxide ions. This poses a problem as OH^- ions can damage the skin. The answer to this is to minimise the time the alkaline solution is in contact with the skin.

Skin also contains proteins - including another type of keratin. Fortunately the ‘soft keratin’ of skin has less cysteine than the ‘hard keratin’ of hair (less than 5% cysteine compared with 14% for hair keratin). It therefore has fewer S-S bridges and is less susceptible to attack by thioglycollate.

^-S-S-R + 2^-SCH₂COO^- 2R-S^- + ^-OOCCH₂-S-S-CH₂COO^-

Notice that in the product a sulfur-sulfur bridge is formed between two thioglycollate ions, Figure 15.

The breaking of the sulfur-sulfur bonds loosens the links between the coiled protein chains of keratin and makes it less tightly held together.

The strongly alkaline solution has two further functions. Firstly it opens up the structure of the hair to allow the aqueous solution to penetrate and ‘get at’ the S-S bonds. Secondly it also affects other bonds within the keratin:

• Hydrogen bonds. These are broken because the strongly alkaline solution removes protons (H⁺ ions) from the -NH groups of amino acids along the protein chain so that they can no longer form hydrogen bonds (see question 4), thus ‘loosening’ the protein helix. In chemists’ jargon, the -NH groups are ‘deprotonated’.

• Salt bonds. These are disrupted by removal of H⁺ ions from -NH₃⁺ groups leaving neutral NH₂ groups which are not attracted to -COO^- groups.

• Amide (peptide) bonds. Peptide bonds can be broken (hydrolysed, i.e. reacted with water) under alkaline conditions, thus breaking apart the protein chains themselves. In fact the keratin may be broken into strands as short as just a few peptides.

In another piece of jargon, chemists often refer to concentrated alkaline solutions as having a ‘high pH’. This can be confusing at first until you remember that alkaline solutions have pHs above 7.

The overall effect of all these processes is to solublise the hair keratin.

Veet is applied to the skin as a cream or mousse, Figure 15, which is left on for a few minutes and then removed with a tool provided, usually a spatula or sponge. The physical action of the tool removes the dissolved hair from the surface of the skin. Any remaining product is then rinsed away with water.

Potassium thioglycollate and potassium hydroxide are not the only ingredients, of course. Aqua is water and parfum is fragrance which is added to make the cream smell nice. Most of the other components are used to produce a cream that will dissolve the active ingredients, hold them on the skin for the requisite time and then be soluble enough to be rinsed off. Other ingredients include colours and moisturisers.