Lipophilicity

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Lipophilicity, fat-liking, refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene.^[1] These non-polar solvents are themselves lipophilic — 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 (the latter as used to describe intermolecular interactions and not the separation of charge in dipoles) all essentially describe the same molecular attribute; the terms are often used interchangeably.

Chemical bonding

Lipophilic substances interact within themselves and with other substances through van der Waals forces. They have little to no capacity to form hydrogen bonds. When a molecule of a lipophilic substance is enveloped by water, surrounding water molecules enter into an 'ice-like' structure over 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 hydrophobic (water-avoiding or water-fearing). Thus lipophilic substances tend to be water insoluble. They invariably have large o/w (oil/water) partition coefficients.

Surfactants

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 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.