Surfactant molecules (e.g., phospholipids, soaps, detergents, and block copolymers) self-assemble15 in selective solvents (e.g., water) to form bilayer membranes and micelles of several structures (Figure 2). The membranes also organize in a variety of patterns. Such self-assembled structures may be swollen by adding an organic solvent to the system, which remains stable as a clear single phase. Such two-solvent composites are called microemulsions. Surfactant bilayers have been investigated for many years as model systems for biological membranes.
Repulsive forces between the surfactant head groups organize the bilayers on longer length scales. This process may form lamellar stacks, sponge phases, or microemulsions, depending on whether there are one or two solvents. Even more relevant to biomolecular systems is the formation of closed-film structures called vesicles (Figure 3), which are already being employed as simple containers to transport drugs within the blood system.
The self-organizing micelles may form in different shapes depending on the specific chemistry and on solvent conditions such as pH, ionic strength, and temperature. The most common form is spherical, but cylindrical micelles also occur. In fact, entropic optimization drives long cylindrical micelles into the form of flexible polymer-like chains. These polymeric micelles have many of the rheological properties of polymers with covalently bonded backbones, but their lengths are determined by equilibrium thermodynamics rather than being fixed.
An interesting example of a self-assembling structure is the tubule, a hollow phospholipid bilayer cylinder morphologically similar to a soda straw (Figure 4). The length of these ultrasmall cylinders is
S.A. Safran, Statistical Thermodynamics of Surfaces, Interfaces, and Membranes (Addison-Wesley, New York, 1994).