Understanding Lipid Hydrophobicity: Why Fats and Oils Repel Water

If a substance is hydrophobic, it does not dissolve in water. Lipids are hydrophobic because lipids are nonpolar molecules and water molecules are polar.

Background

• Lipids are a group of molecules that includes fats and oils, waxes, phospholipids, steroids (such as cholesterol) and other related compounds.

Polar Molecules

• A polar molecule has two bonded atoms that do not share electrons. Polar molecules are only attracted to other polar molecules. Polar molecules are hydrophilic because they dissolve in water by forming hydrogen bonds.

Nonpolar Molecules

• Nonpolar molecules are not attracted to polar molecules and are called hydrophobic because they do not dissolve in water.

Hydrocarbon Backbones

• According to Carnegie Mellon, lipid molecules are hydrophobic because they consist of long, 18-22 carbon-hydrocarbon backbones and contain only a small amount of oxygen.

Fun Fact

• One type of lipids, called phospholipids, are amphipatic, that is, both hydrophillic and hydrophobic.