λίπος "fat" and "friendly"), refers to the ability of a chemical compound
to dissolve in fat
s, and non-polar solvent
s such as hexane
. These non-polar solvents are themselves lipophilic (translated as "fat-loving" or "fat-liking" Compendium of Chemical Terminology
, accessed 15 Jan 2007.)—the axiom that "like dissolves like" generally holds true. Thus lipophilic substances tend to dissolve in other lipophilic substances, while hydrophilic
("water-loving") substances tend to dissolve in water and other hydrophilic substances.
Lipophilicity, hydrophobicity, and non-polarity can
describe the same tendency towards participation in the London dispersion force
as the terms are often used interchangeably. However, the terms "lipophilic" and " hydrophobic
" are not synonymous, as can be seen with silicone
s and fluorocarbons
, which are hydrophobic but not lipophilic.
Lipophilic substances interact within themselves and with other substances through the London dispersion force
. They have little to no capacity to form hydrogen bond
s. When a molecule of a lipophilic substance is enveloped by water, surrounding water molecules enter into an "ice-like" structure the greater part of its molecular surface, the thermodynamically
unfavourable event that drives oily substances out of water. Being "driven out of water" is the quality of a substance referred to as hydrophobe (water-"avoiding" or "water-fearing"). Thus lipophilic substances tend to be water-insoluble. They invariably have large o/w (octanol/water) partition coefficient
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 o/w 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, fluorosurfactant
s are not amphiphilic or detergents
because fluorocarbons are not lipophilic.
, a common cosmetic ingredient often used in sunscreens, is particularly penetrative because it is not very lipophilic.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 Anywhere from 0.4% to 8.7% of oxybenzone can be absorbed after one topical sunscreen application, as measured in urine excretions.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.