Phospholipid bilayer I'm a high school student. The head is hydrophilic, the tail is fatty acid, in other words hydrophobic. Here is the thing I don't understand, all textbooks state that water is repelled by hydrophobic tail, why? The hydrophilic head is composed of polar molecules, and water is polar, so they will attract each other. But why hydrophobic tail will repel polar molecule? Isn't hydrophobic tail is just composed of non-polar molecules?
 A: The repelling is another way of saying that owing to the strength of the hydrogen bonding between water molecules, the water molecules are better off with themselves alone as compared to with non-interacting non-polar molecules within. A substance dissolves only in a solvent, where the solvent-solute interaction is as strong (or stronger) than the solvent-solvent interaction and therefore the solvent finds it better (Energetically and thermodynamically favourable) to allow the solute molecules to dissolve, i.e. take up spaces between the molecules. But if the solute-solvent interaction is poor, (as in the case of non-polar/hydrophobic molecules), the solvent finds it better to be among itself and not allow the hydrophobic molecules to take up spaces between the molecules, which is equivalent to having repelled the non-polar substances, i.e. their mixing with water is energetically opposed. This is also the reason why polar substances do not dissolve in non-polar solvent.  
There are hydrophobic surfaces which depend on the surface energy or the contact angle of water on that surface, but the same argument cannot be extended to molecules where there is no surface to account for the surface energy. 
