7. The Hydrophobic Interaction

Question 1

Describe the entropy of water (liquid) in the absence of non-polar molecules

Answer 1

It is large, making it a favourable state for water.

Question 2

What happens to water when a non-polar molecule is added to it

Answer 2

The water around the non-polar molecule becomes more ordered (low entropy) around the molecule because water cannot hydrogen bond with a non-polar molecule.

Question 3

What happens if multiple non-polar molecules are placed in water?

Answer 3

They will merge to maximise entropy because it reduces the surface area in contact with the water.

Question 4

What is the hydrophobic effect?

Answer 4

An effective driving force that pushes non-polar molecules together in water.

Question 5

What causes the hydrophobic effect?

Answer 5

A system is required to be in the highest entropy state. This causes the liquid water molecules to reorganise themselves.

It is not the result of a direct interaction between non-polar molecules.

Question 6

Give the equation that describes the interaction energy between two non-polar surfaces in water

Answer 6

U = interaction energy
γ = interfacial energy with water
S = interfacial area
λ = range of the interaction
D = separation distance

Question 7

Describe the change in the interaction energy between two non-polar surfaces in water with distance

Answer 7

The interaction energy decays exponentially with the separation between surfaces.

Question 8

Give 3 examples of hydrophobic interactions in nature

Answer 8

• Folding of protein molecules
• Cell membranes (phospholipid bilayer)
• Self-assembly of micelles or liposomes

Question 9

Are hydrophobic interactions attractive or repulsive?

Answer 9

Attractive