Lipophilicity

Surfactants

Hydrocarbon-based surfactants are compounds that are amphiphilic (or amphipathic), having a hydrophilic, water interactive "end", referred to as their "head group", and a lipophilic "end", usually a long chain hydrocarbon fragment, referred to as their "tail". They congregate at low energy surfaces, including the air-water interface (lowering surface tension) and the surfaces of the water-immiscible droplets found in oil/water emulsions (lowering interfacial tension). At these surfaces they naturally orient themselves with their head groups in water and their tails either sticking up and largely out of water (as at the air-water interface) or dissolved in the water-immiscible phase that the water is in contact with (e.g. as the emulsified oil droplet). In both these configurations the head groups strongly interact with water while the tails avoid all contact with water. Surfactant molecules also aggregate in water as micelles with their head groups sticking out and their tails bunched together. Micelles draw oily substances into their hydrophobic cores, explaining the basic action of soaps and detergents used for personal cleanliness and for laundering clothes. Micelles are also biologically important for the transport of fatty substances in the small intestine surface in the first step that leads to the absorption of the components of fats (largely fatty acids and 2-monoglycerides).

Cell membranes are bilayer structures principally formed from phospholipids, molecules which have a highly water interactive, ionic phosphate head groups attached to two long alkyl tails.

By contrast, fluorosurfactants are not amphiphilic or detergents because fluorocarbons are not lipophilic.

Oxybenzone, a common cosmetic ingredient often used in sunscreens, penetrates the skin particularly well because it is not very lipophilic. Anywhere from 0.4% to 8.7% of oxybenzone can be absorbed after one topical sunscreen application, as measured in urine excretions.

References

References

1. [[Compendium of Chemical Terminology]], [http://goldbook.iupac.org/L03572.html lipophilic], accessed 15 Jan 2007.
2. Alyn William Johnson. (1999). "Invitation to Organic Chemistry". Jones & Bartlett Learning.
3. Hanson KM, Gratton E, Bardeen CJ. (2006). "Sunscreen enhancement of UV-induced reactive oxygen species in the skin". ''Free Radical Biology & Medicine'' 41(8): 1205–1212
4. H. Gonzalez, H., Farbrot, A., Larko. O., and Wennberg, A. M. (2006). "Percutaneous absorption of the sunscreen benzophenone-3 after repeated whole-body applications, with and without ultraviolet irradiation". ''British Journal of Dermatology'', 154:337–340.