|MadSci Network: Chemistry|
Lips chap when they get too dry. This usually occurs in colder weather when the lower temperatures lower the humidity in the air, or in very arid places, like a desert. The dry air pulls moisture out of the top layer of the epidermis, the stratum corneum, faster than it can diffuse in from the underlying layers, causing it to dry out. This is true all over the body in arid weather, however the rate at which skin loses moisture is directly related to the thickness of the epidermis. So a thicker stratum corneum, particularly on the palms of the hands or the soles of the feet, is unaffected by dry air while thinner layers, especially on the lips, dry to the point of becoming brittle and cracking open with the slightest movement. This occurs because the "dead" cells of the stratum corneum are composed primarily of a filamentous protein called keratin which requires water to remain flexible.
So, the most important consideration in treating chapped lips is keeping the skin sufficiently hydrated. The best way to do this is to apply a sealant to the skin which prevents water from escaping. This was traditionally accomplished by applying candle wax or a similar homemade balm to the lips. The modern solutions (including Chap Stick, Carmex, and Blistex) are not very different from the older balms, but they are easier and more convenient to use. They all work by simply covering the lips with a material that is impermeable to water. In all cases, this impermeable material is some form of long-chain hydrocarbons (or fatty acids), the differences in length and saturation of the carbon chains determining whether the balm is waxy or greasy.
In terms of actual molecular interactions, about the only forces at play in lip balms are van der Waal forces and hydrophobicity. Van der Waal's force involves the attraction between non-polar molecules due to transient polarization of their electron clouds. This is the weakest of molecular forces but is still sufficient to hold the long-chain hydrocarbon molecules snugly together so that water molecules have a hard time getting through. The van der Waal forces also help the balm interact with the keratin (which is very hydrophobic) of the cornified cells of the epidermis.
The interactions between the wax/grease molecules help to form a solid barrier to water, but more importantly, water doesn't try to cross the barrier because wax and oils are hydrophobic, meaning they repel water. Hydrophobicity is a strange function of entropy. Basically, water molecules form hydrogen bonds with molecules that are charged or polar, and the more of these bonds each water molecule can make, the more entropy it has. The only way for water to interact with hydrocarbons is to give up its polarity and try to form van der Waal interactions, but this "costs" so much in entropy that instead the water molecules try to form a lattice with other water molecules which is about as successful as making ice in summer. So water molecules actually avoid contact with hydrocarbons wherever possible (that's why oil and water separate, even after vigorous mixing).
Try the links in the MadSci Library for more information on Chemistry.