text: Lipid molecules in the HII phase pack inversely to the packing observed in the hexagonal I phase described above. This phase has the polar head groups on the inside and the hydrophobic, hydrocarbon tails on the outside in solution. The packing ratio for this phase is larger than one, which is synonymous with an inverse cone packing.
  Extended arrays of long tubes will form (as in the hexagonal I phase), but because of the way the polar head groups pack, the tubes take the shape of aqueous channels.  These arrays can stack together like pipes. This way of packing may leave a finite hydrophobic surface in contact with water on the outside of the array. However, the otherwise energetically favorable packing apparently stabilizes this phase as a whole. It is also possible that an outer monolayer of lipid coats the surface of the collection of tubes to protect the hydrophobic surface from interaction with the aqueous phase.
  It is suggested that this phase is formed by lipids in solution in order to compensate for the hydrophobic effect. The tight packing of the lipid head groups reduces their contact with the aqueous phase. This, in turn, reduces the amount of ordered, but unbound water molecules. 
  The most common lipids that form this phase include phosphatidylethanolamine (PE), when it has unsaturated hydrocarbon chains. Diphosphatidylglycerol (DPG, otherwise known as cardiolipin) in the presence of calcium is also capable of forming this phase.