[Jose M.]

Dear all:

Hi.

I have a question about Van der Waals forces. I know they are the following four (4).

1) dipole-dipole
2)induced dipole-induced dipole (dispersion) (London)
3)dipole-induced dipole
4)hydrophobic

I have no doubts about the first two. I know that dipole-dipole are attractive forces of polar molecules. I know that London are the only attractive forces available to non-polar molecules. I am clear in that. However, I need some explanation of dipole-induced dipole and of hydrophobic. An explanation of these last two shall be truly appreciated.

This is my very first posted message here. I shall now introduce myself. My name is José Miranda. I am a Puerto Rican man. I am here in Puerto Rico. My first language is Spanish, English is my second. I am new here and probably the only Puerto Rican here.

Once again, please somebody give me an explanation of the Van der Waals forces (the last two).

Best regards,

José M.

[FeLiXe]

"hydrophobic" is not a Van der Waals Force, just a property describing a substance that does not mix with water

dipole-induced dipole means that you have one permanent dipole (like water) and a non polar molecule (like an alkane). Then the permanent dipole induces a temporary dipole in the other molecule. And you get a little bit of attraction.

[Jose M.]

Thank you for your help ! It is appreciated.

Look, friend, in case you didnt know, hydrophobic forces can be considered a form of Van der Waals forces. Its a little known fact, but it is that way. We always learn something.

Once again, thank you very much. I appreciate your help.

[Donaldson Tan]

a molecule consist of a few nuclei surrounded by electron cloud.

if the molecule is non-polar, then the distribution of electron density is uniform.

when a dipolar molecule approach a non-polar molecule, the dipolar molecule polarises the non-polar molecule. The +ve terminal of the dipolar molecule attracts electron density of the non-polar molecule towards itself. The -ve terminal of the dipolar molecule repels electron density of the non-polar molecule away. This creates a +ve terminal and -ve terminal in the non-polar molecule, making the non-polar molecule polar. This interaction is known as dipole-induced-dipole.

in aqueous system, there are hydrophobic and hydrophillic interaction. This is due to the presence of polar groups bonded to the molecules in the system. These polar groups rather bond to water because such interaction forms stronger intermolecular bonds. It's energetically favourable. The preference for such interaction is described hydrophillic. The results in an apparent repulsion between between non-polar groups and water in the aqueous system. This apparent repulsion is described as hydrophobic.