Membranes and Films

Place a drop of liquid detergent on water and it will rapidly spread out to form a thin film on the surface. Detergents and soaps are made from special molecules comprising a hydrocarbon chain (which is hydrophobic) with a polar 'hydrophilic' headgroup. Water is a polar solvent; so the polar end of the molecule buries itself in the polar solvent. The hydrophobic end of the molecule sticks out to avoid contact with the water; a film of molecular thickness is formed.

Because the surface of the water is coated with hydrophobic molecules, it behaves very differently. Pure water has a high surface tension; it behaves as though it has a skin (as can be shown by carefully placing a steel needle on its surface; the needle will be held by the skin, but it is not 'floating'; as soon as it penetrates the skin, it will sink). This high surface tension is a consequence of the strong interaction between the molecules; these interactions mean that the liquid is driven to reduce its surface area as much as possible, because when molecules are at the surface they experience less attraction from neighboring molecules. We cannot therefore blow bobbles of pure water; the high surface tension will make them collapse. But we can blow soap bubbles, because the films of soap molecules that form at the surfaces have a dramatically lower surface tension; the hydrophobic molecules interact with each other much less than do the water molecules. They can easily form structures like soap bubbles with high surface areas. It is the relative sizes of the molecular forces between the molecules in soaps and detergents and those of water that allow us to blow soap bubbles.

The other response which our two ended (or amphiphilic) molecules can make in solution is to cluster together, with the hydrophobic component of the molecule pointing inward, and the hydrophilic pointing outward. Formation of these micelles may also help to capture other molecules, for example the molecules of fats and greases which are hydrophobic; they play an important role in detergency actions.

Another way in which the hydrophobic interaction can express itself is by forming double layers in which the hydrophobic parts of the amphiphilic molecules on neighboring layers point inwards, but instead of rolling up into a ball as in the micelles, they line up in a bi-layer. By far the most important context in which such layers are formed is in living matter. Membranes are vitally important structures in living organisms. Cells, which are the most important structural entity in all sophisticated life forms are bounded by membranes, as is the nucleus in their center (in which all the genetic material is stored). These membranes are essentially double layers. The amphiphilic molecules from which they are constructed are known as lipids. Membranes, as well as playing a structural role, also must allow certain molecules to pass through them.