We have noted that glycerophospholipids, sphingolipids, and sterols are virtually insoluble in water. When mixed with water, these amphipathic compounds form microscopic lipid aggregates in a phase separate from their aqueous surroundings. Lipid molecules cluster together with their hydrophobic moieties in contact with each other and their hydrophilic groups interacting with the surrounding water. Recall that lipid clustering reduces the amount of hydrophobic surface exposed to water and thus minimizes the number of molecules in the shell of ordered water at the lipid-water interface (see Fig. 4-7), resulting in an increase in entropy. Hydrophobic interactions among lipid molecules provide the thermodynamic driving force for the formation and maintenance of these structures.