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What does the Hydrophobic Effect have to do with entropy?

  1. Oct 3, 2012 #1
    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.
  2. jcsd
  3. Oct 3, 2012 #2
    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.
    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.
  4. Oct 3, 2012 #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.
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