Eli5 why does licking your lips make them more dry
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Our skin coatings are a mixture of many different compounds
Sebum is an oily, waxy substance produced by your body’s sebaceous glands. It coats, moisturizes, and protects your skin.
It’s also the main ingredient in what you might think of as your body’s natural oils.
So, what exactly is sebum made up of? As an article from Harvard Medical School explains, “sebum is a complex mixture of fatty acids, sugars, waxes, and other natural chemicals that form a protective barrier against water evaporation.”
To be more specific, sebum contains triglycerides and fatty acids (57%), wax esters (26%), squalene (12%), and cholesterol (4.5%).
Now that we understand there is a mixture, next we need to understand that Yes oils evaporate. It is slower than water. But it has to do with the composition of the mixture. More specifically:
All liquids have a certain vapour pressure, i.e. the tendency to release molecules into the air. This process is called evaporation. Even solids have a vapour pressue, in case of sublimation. If the vapour pressure is higher (or equal) the ambient pressure the liquid boils.
In case of some liquids, this vapour pressure is (at normal temperatures) rather low, so it will take ages to have a substantial amout evaporating.
In case of a binary mixture between water and alcohol (ethanol) usually the ethanol will evaporate first, as a anybody distilling spirits will tell you. Though in this case there is one speciality, at aroung 96% alcohol you have what is called an azeotrope, a mixture whose concentrations you cannot alter by destilation any more.
In case of mixture between oils, skin, acids,waxes, sugars, and water, the vapor pressure of the entire mixture is dependent on the overall composition of the mixture.
Hence, adding saliva to the skin changes it’s overall vapor pressure towards a more rapid evaporation.
TLDR: adding saliva makes the oils of the skin evaporate faster.
Our lips do not produce oil as they don’t have sebaceous glands. Repeatedly licking our lips coats them with the enzymes in our saliva which can cause increased dryness. Wind, cold, sun, friction – can also cause dryness.
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