What does the Hydrophobic Effect have to do with entropy?

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In summary, the hydrophobic effect is the exclusion of hydrophobic molecules by water, which leads to an increase in entropy of the solvent water molecules. This is because when water molecules stick to hydrophobic molecules, they are restricted in motion and have less available energy. However, when the water molecules are excluded, they have more degrees of freedom and therefore, an increase in entropy is created. This is also why hydrophobic molecules tend to clump together in water, as this reduces the surface area and overall energy of the system. This phenomenon is studied in branches of physical chemistry and chemical engineering, and is caused by the higher surface energy of water at the interface with hydrophobic materials.
  • #1
phoebz
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Hello,
I am taking a biochemistry course right now, and I am so confused by this 'hydrophobic effect' and how it relates to entropy.

Hydrophobic effect: THe exclusion of hydrophobic groups or molecules by water. (I get this part!) This appears to depend of the increase in entropy of solvent water molecules that are released from an ordered arrangement around the hydrophobic group. (I don't get this part)

From what I have gathered, entropy is the amount of unavailable energy in a system. If hydrophobic molecules clump together because it is more energetically favourable (the solvent doesn't have to cover as much surface area)- why would entropy increase?

And if you say entropy is the amount of disorder in a system- again, why would there be more disorder after the hydrophobic molecules clump together? I though this was more ordered.
 
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  • #2
phoebz said:
Hello,
I am taking a biochemistry course right now, and I am so confused by this 'hydrophobic effect' and how it relates to entropy.

Hydrophobic effect: THe exclusion of hydrophobic groups or molecules by water. (I get this part!) This appears to depend of the increase in entropy of solvent water molecules that are released from an ordered arrangement around the hydrophobic group. (I don't get this part)

From what I have gathered, entropy is the amount of unavailable energy in a system. If hydrophobic molecules clump together because it is more energetically favourable (the solvent doesn't have to cover as much surface area)- why would entropy increase?
The water molecules are restricted in motion when they stick to the hydrophobic molecules. Therefore, the specific heat of the water molecules is smaller. The specific heat of the water molecules will increase once they are excluded. So the increase in specific heat means that less of the internal energy is available for work.
Basically, exclusion of water molecules means the water molecules have more degrees of freedom. They have more directions they can move. This means that entropy has been created. Entropy, once created, can not be destroyed. So there is less free energy in the water molecules.
phoebz said:
And if you say entropy is the amount of disorder in a system- again, why would there be more disorder after the hydrophobic molecules clump together? I though this was more ordered.
What you quoted said the opposite. The water molecules are "released from an ordered arrangement around the hydrophobic group". Therefore, the water molecules are in a less ordered arrangement. Therefore, entropy of the water molecules have increased.
You didn't quote anything about the hydrophobic molecules clumping together after the water molecules are excluded. The clumping, if this exists, would decrease the entropy of the hydrophobic molecules.
However, usually the hydrophobic molecules are bigger than the water molecules. There are a lot more water molecules excluded then hydrophobic molecules clumping together. So the total entropy will increase.
Note, the clumping of hydrophobic molecules does the opposite of the exclusion of water molecules. So you have to calculate which does more. I think the water molecules usually win because there are more of them.
 
  • #3
solvation phenomena, and associated things like colloids and aerosols, are nontrivial questions even for research purposes today. major branches of physical chemistry and chemical engineering are dedicated to their research.

in general, your textbook meant that when hydrophobic molecules are in water, the water forms stronger associations with each other than with the hydrophobic molecules, and thus surrounds the hydrophobic molecules. This forms a surface; a water-hydrophobic material interface. In general, surfaces have dangling bonds, in this case, hydrogen bonds for the water are incomplete at the interface, and that gives rise to the phenomena of surface energy - the surface is "higher in energy" than the bulk. This causes the water molecules to line up on the interface in an ordered way.

For the same volume, dispersed hydrophobic material has a much larger surface area than the hydrophobic material clumped up (1000 cubes of 1 cubic meter each have an area of 6000 square meters, but a 10x10x10 cube only has 600 square meters of surface area) and thus has a higher energy since there's more surface; it is unfavorable, so the hydrophobic material clumps up.
 

1. What is the Hydrophobic Effect?

The Hydrophobic Effect is a phenomenon in which nonpolar molecules or substances tend to cluster together in water due to the energetically unfavorable mixing of water with nonpolar substances. This effect is caused by the tendency of water molecules to form hydrogen bonds with each other, leaving the nonpolar substances with less favorable interactions.

2. How does the Hydrophobic Effect relate to entropy?

The Hydrophobic Effect is closely related to entropy, which is a measure of the disorder or randomness in a system. In the case of the Hydrophobic Effect, when nonpolar molecules cluster together, they decrease the disorder of the surrounding water molecules, resulting in an increase in entropy.

3. Why is the Hydrophobic Effect important in biological systems?

In biological systems, the Hydrophobic Effect plays a crucial role in the folding of proteins, as well as in the formation of cell membranes. The clustering of nonpolar amino acids in the interior of a protein helps to stabilize its structure, while the formation of lipid bilayers in cell membranes is also driven by the Hydrophobic Effect.

4. Can the Hydrophobic Effect be observed in other solvents besides water?

Yes, the Hydrophobic Effect can be observed in other solvents as well, but it is most pronounced in water due to its unique properties. Other solvents that have a strong hydrogen bonding network, such as alcohols, also exhibit similar effects, but to a lesser extent.

5. How does temperature affect the Hydrophobic Effect?

Temperature has a significant impact on the Hydrophobic Effect. As the temperature increases, the disorder of the water molecules also increases, making it easier for nonpolar molecules to dissolve. This results in a decrease in the strength of the Hydrophobic Effect. Conversely, at lower temperatures, the Hydrophobic Effect is more pronounced.

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