Why Are Some Materials Hydrophobic?

[Gerenuk]

Explanation for "hydrophobic"?

Hi,

in chemistry class I was always wondering how materials can be hydrophobic. I understand that dipolar molecules such as water are hydrophil because they attract due to the dipole interaction, but why do they repell neutral molecules?
What is the interaction of hydrophobic molecules amongst each other?

 

[Gokul43201]

There isn't really any kind of "repulsion" of the water molecules from a hydrophobic material. The term, in that sense, is something of a misnomer. A hydrophobic molecule feels a very weak attraction to a polar molecule like water, compared to the attraction between the polar molecules. As a result the hydrophobic molecules tend to bunch up among themselves and ignore the water.

Perhaps, "unhydrophilic", would be more descriptive (but less fun to say).

 

[Gerenuk]

Hmm, hydrophobic molecule ignore water, but I didn't get why they prefer themselves and don't include water molecules as neutral participants.
Wasn't that the reason why fat dissolves only in non-polar solvents? Do you mean if you put water into some non-polar liquid, the water molecules will attract each other, not objecting to going next to non-polar molecule, but still prefering the polar ones?!

 

[Gokul43201]

Hmm, hydrophobic molecule ignore water, but I didn't get why they prefer themselves and don't include water molecules as neutral participants.

They are indifferent to the water molecules, but the water molecules are not indifferent to each other.

Wasn't that the reason why fat dissolves only in non-polar solvents?

Yes, the fat molecules will be unable to disperse themselves within a polar solvent. To do that they will have to first break the dipolar interactions between the polar molecules. That would only pay off if they had a stronger interaction to offer in exchange, but they do not. As a result, all the fat molecules tend to bunch together into globs or micelles.

Do you mean if you put water into some non-polar liquid, the water molecules will attract each other, not objecting to going next to non-polar molecule, but still prefering the polar ones?!

Exactly.

 

[Gerenuk]

I now started to wonder how soaps work. Don't they have molecules with one hydrophobic and one hydrophil end? How do they surround non-polar dirt? Don't they still prefer to exclude as much indifferent molecules as possible?

 

[rohanprabhu]

I now started to wonder how soaps work. Don't they have molecules with one hydrophobic and one hydrophil end? How do they surround non-polar dirt? Don't they still prefer to exclude as much indifferent molecules as possible?

They surround non-polar dirt by Van der Waals force. These forces are dependent on the molecular mass. The molecular mass of the dirt and the soap 'misclle' is quite high as compared to that of water. As Goku said, it is not that hydrophobic end 'hates' water, it's just that it likes the 'dirt' or itself more due to higher molecular mass. At the same time, since the other end is charged, it is attracted to the water molecule by electrostatic forces.

 

[Gerenuk]

OK. That seems to make sense. The heavy non-polar molecules attract each other stronger, than water. Is there a reference where I can find how the Van der Waals force depends on mass? I couldn't find it on online encyclopedias.
There is a post
https://www.physicsforums.com/showthread.php?p=836972
but I don't get what's the atom number density for two isolated atoms.

 

[rohanprabhu]

OK. That seems to make sense. The heavy non-polar molecules attract each other stronger, than water. Is there a reference where I can find how the Van der Waals force depends on mass? I couldn't find it on online encyclopedias.
There is a post
https://www.physicsforums.com/showthread.php?p=836972
but I don't get what's the atom number density for two isolated atoms.

When i said that the Van der Waals forces depend purely on the molecular mass, I was actually indicating the 3rd type of forces, existent in all molecules, which you got correctly i.e. London forces [or dispersion forces]. The other two forces are experienced by permanent/temporary or induced dipoles.

For the London forces, this is how it is caused:

1. The movement of electrons within the electron cloud cause temporary electron imbalances (self-polarization)
2. The resultant instantaneous dipole will induce polarization in neighboring molecules
3. The magnitude of self-polarization increases with increasing numbers of electrons

Thereby, to have a "high" number of electrons, we will most probably have a higher number of "protons" and "neutrons", since here we are dealing with uncharged species, so we can assume that the number of electrons will give us approx. half the molecular mass [assuming No. of protons = No. of neutrons].

Thereby, I used the word 'molecular mass', as my teacher taught me this as a commonly used tool for inspecting intermolecular forces.

The same forces act even among polar molecules, but the London forces are far weaker than other Van der Waals forces among charged species [or species carrying an apparent charge].

the list taken from: http://www.wou.edu/las/physci/ch334/lecture/intermol/london.htm