Question

Why do phospholipids form a lipid bilayer rather than micelles in water? What are the thermodynamic driving forces (considering enthalpy and entropy) for this structure?

Answer 1

It is the shape and amphipathic nature of the lipid molecules that cause them to form bilayers spontaneously in aqueous environments. hydrophilic molecules dissolve readily in water because they contain charged groups or uncharged polar groups that can form either favorable electrostatic interactions or hydrogen bonds with water molecules. Hydrophobic molecules, by contrast, are insoluble in water because all, or almost all, of their atoms are uncharged and nonpolar and therefore cannot form energetically favorable interactions with water molecules. If dispersed in water, they force the adjacent water molecules to reorganize into icelike cages that surround the hydrophobic molecule.

Because these cage structures are more ordered than the surrounding water, their formation increases the free energy. This free energy cost is minimized and change in enthalpy also increases and the entropy chage become negative due to regular arrangement of molecules.