In aqueous solutions, the polar head groups of lipids like to stay in water, the hydrophobic hydrocarbons tails try to avoid it. There are two ways to arrange a form that lowers the Gibbs energy, micelles or membranes, both shown schematically in Fig. The usual structure is the membrane. In water, lipids assemble and form membranes spontaneously and rapidly. The driving forces are the hydrophobic interaction and van der Waals interaction between tails.
Membranes thus form non-covalent cooperative structures.
7.4 shows that membranes form essentially two-dimensional systems. The physics of membranes, still in its infancy, displays many phenomena similar to those in fluids and solids. The basic structure of a membrane is shown in Fig. Real membranes are obviously far more complex. The detailed arrangement of the lipids and proteins depends very much on characteristics of all components and the phase diagram of a membrane system can be complicated.