Amphiphilic Membranes

Amphiphilic molecules (the word derives from the Greek αμφὶ φιλία, meaning “love on both sides”) are molecules which both love and hate water. They are formed by two parts with very different tastes, which are covalently bound together: one, the hydrophilic head, is polar or even ionized, and tends therefore to be close to the small, polar water molecules; the other, the hydrophobic tail, is usually a hydrocarbon chain, which perturbs the high order of water, and has therefore the tendency to pack close to similar chains. Amphiphilic molecules in solvents (water and/or oil) may form several structures (micelles, hexagonal phases, cubic phases. . . ), but we shall discuss mostly the cases in which they form a bilayer, i.e., a sheet made up of two layers of amphiphilic molecules: in water, the hydrophilic heads stem out of the bilayer on both sides, while the corresponding tails remain at the interior. Membranes can be formed by an isolated bilayer or by several bilayers stuck one on top of another: in this case one speaks of multilayers. Amphiphilic membranes are a physical realization of fluctuating surfaces. They are thin sheets (50–100Å) of amphiphilic molecules immersed in a fluid, usually water or brine (water and salt). They can be of natural or artificial origin: the most important example of natural membranes is the cell membrane, which separates the interior of all living cells from its exterior. Living cells, and in particular eucaryotic ones, possess a large number of membranes, like the nuclear membrane, which separates the nucleus from the rest of the cell, allowing, e.g., mRNA to pass from the inside to the outside, and several chemical signals in the reverse direction, or the Golgi apparatus, which acts as a sort of “chemical factory” for the cell. Mitochondria are also organelles essentially formed by a membrane, folded on itself several times. Artificial amphiphilic membranes have recently become a lively research field stimulated by their applications in the industry, in medicine and in cosmetics. One can form with them tunable or “active” filters, simulating, as it were, the action of the cell membrane. They are also able to close on themselves, forming vesicles (small closed surfaces), which may act as drug carriers, designed to open up and release their load when the