If you have ever made salad dressing then you know that oil and vinegar don't mix. Oily substances are hydrophobic, while vinegar is mainly water with a small percentage of acid. Similarly, sickle cell hemoglobin doesn't want to mix with the cellular environment and associates with itself instead. The insides of most proteins are hydrophobic, while the outsides or surfaces contain more charged or polar amino acid residues that are compatible with water.
Learn more about Hydrophobicity:
Kevin
Lehmann's Bad Chemistry webpage
-clarifies what is and what is not hydrophobicity.
http://www.princeton.edu/~lehmann/BadChemistry.html