Hydrophobic and Hydrophilic

The combining form -phobe means to fear while the word -phile means to love. The prefix hydro- refers to the presence of hydrogen or water. If a molecule is attracted to water it is called hydrophilic and if it does not like water it is called hydrophobic.

• If you built up a static electric charge on a glass rod, what do you think will happen when that rod is placed near a very small stream of water? Defend your answer.

Hydrophobic molecules are nonpolar molecules. A good example of a nonpolar molecule is oil. If you ever rinsed off your plate of spaghetti without soap you probably noticed the oil floating on top. This is because the oil is nonpolar. When water contains hydrophobic molecules like the large molecules to the left, the water molecules can only interact with each other.

On the other hand, hydrophilic molecules are polar molecules which means that they have an electrical charge. Because of this charge, polar molecules can form hydrogen bonds with water molecules. This would explain why sugar dissolves in water.

• Would you consider the cellulose in your cotton bath towel to be hydrophilic or hydrophobic?
• Why does clothing containing nylon or polypropylene dry so quickly?

In the presence of water, nonpolar molecules are excluded from water which wants to form hydrogen bonds. As a result, hydrophobic molecules are forced together, causing these molecules to form a variety of shapes. If you add oil to water and stir it vigorously, you will notice that the small droplets of oil eventually combine into larger droplets. This because of the hydrophobic exclusion of oil by the water molecules.

• How do you think that the hydrophobic exclusion principle applies to the theories explaining the formation of the first cells?
• How do you think that the hydrophobic exclusion principle applies to the maintenance of the integrity of your body cells?
• Why does plaque form in some people's arteries?

When you are doing dishes, you use soap to make sure that the dishware is clean. Soap helps remove the oil films that are created during cooking. You probably noticed that there is no oil floating on top of dishwater.

• How does soap change the hydrophobic nature of oil?
• How are soaps, or surfactants, useful in society?

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