Hydrophobic or Nonpolar molecules do not have ionic groups and can not form hydrogen bonds. Also, the presence of hydrophobic molecules in water causes the water molecules to organize around the hydrophobic molecule in the form a Clathrate . Clathrate formation causes the order of the water molecules to increase. This corresponds to a decrease in Entropy , an unfavorable process. The combination of the lack of favorable hydrogen bonding interactions between hydrophobic molecules and water, and the unfavorable entropy combine to explain the very low solubility of hydrophobic molecules in water.
Amphipathic molecules have a hydrophilic polar head group and a hydrophobic nonpolar tail.
Such molecules have an identity crisis - the polar head group interacts favorably with water but the hydrophobic tail does not. This crisis is solved by the formation of a Monolayer , a Micelle or a Bilayer . These self-assembled structures remove the nonpolar tail from interaction with water while allowing the polar head group to interact with water. Amphipathic molecules with a single tail, e.g., detergents or fatty acid salts, usually form micelles, while molecules with two tails, e.g., phospholipids, form bilayers, which, in turn, form closed structures like vesicles. Two-tailed amphipathic molecules are the basis for the formation of Cellular Membranes .