Hydrophobic interactions

How/Why do hydrophobic substances which are neither polar nor charged, dissolve in hydrophobic/organic solvents?

And if it is neither charged nor polar, how does oil attract water?
As said here:(http://www.princeton.edu/~lehmann/BadChemistry.html#hydrophobic)

I did a search on the net and most of the stuff says that hydrophobic substances repels water, which i’ve come to learn otherwise. And nothing explains why it would repel or attract anyway.

So help, anyone?

The attraction between hydrophilic substances is rather easier to explain. Opposite charges attract, right? So any compound with a dipole moment (a separation of electron into partially-positive and partially-negative charge by the different electronegativities [electron-attracting abilities] of two atoms*) has areas where the charge is consistently more negative or more positive. These areas are attracted to one another. (Water is more negative at the oxygen and more positive on the hydrogen, so water molecules are attracted to each other by the attraction of hydrogen to oxygen.)

*: Really, it has to be a net dipole moment; in a symmetric, linear molecule like carbon dioxide, the separation of charges to both ends cancels out.

If you understand this, it’s easier to understand how non-polar molecules can be attracted to each other, and thus how non-polar solutes can dissolve in non-polar solvents. Non-polar substances don’t have consistent areas of negative or positive charge, but that doesn’t mean they have the same charge everywhere. Since electrons are constantly moving around in molecules, sometimes they’ll happen to congregate at one part of a molecule, giving it a slight negative charge in that area and a slight positive charge in the area they don’t happen to be accumulated in (a dipole).

This really isn’t enough to attract molecules to each other by itself. But one spontaneously-formed dipole, however transient, has effects in other molecules. The areas of slight positive charge attract electrons on adjacent molecules, and the areas of slight positive charge repel them. This is called an “induced dipole”, a dipole formed by the influence of another nearby dipole. The usual Coulombic or electrostatic attraction between bodies having opposite charges occurs. The molecules between which the dipoles form need not be the same; this is how a nonpolar solute can dissolve in a nonpolar solvent.

Most “non-polar” substances are not entirely non-polar, though some (like straight-chain hydrocarbons) are very close. Many substances which are quite non-polar have some area with slight positive or negative charge (if they contain an oxygen atom, for example) – this usually helps induce dipoles, since there’s a strong, consistent charge to start the chain reaction.

There are several names for this phenomenon: dipole-induced-dipole bonding or dipole-dipole bonding, van der Waals forces and London dispersion forces. Generally, the attractive forces between non-polar molecules are quite weak and diminish rapidly with distance (to the sixth power, I think).

About the article you referred to: Remember how I just explained how non-polar substances can become momentarily slightly polar, and that this is how they are attracted to one another in solutions (and in pure samples, too). As I said, the attraction between, for example, oil molecules is very weak, compared to the strong attractive forces between water molecules. The attraction between water molecules is based on consistently negative and positive areas of the molecule, as well as by the ability of each water molecule to partially “share” hydrogen atoms with other water molecules (hydrogen bonding). The attraction between oil molecules is based on induced dipoles. Hydrophobic substances such as oil seem to ‘repel’ water because the water-water bonds are much stronger than any potential water-oil bonds. There is an energy cost associated with a water molecule giving up its strong bonds with other water molecules to form a weak bond with an oil molecule.

The article does not discuss the entropy cost of dissolving oil in water. In order to maximize the water-water interactions and the strength of the water-oil interaction, the water molecules need to form a sort of ‘cage’ around a droplet of oil molecules. This increases the order of the water molecules, which is not the way the Universe wants things to be – they have to expend energy to assume that ordered form, which further reduces the amount of oil that can dissolve in water this way. Detergents are much better at forming these ‘cages’ than water is – they have a hydrophilic end that forms strong detergent-water bonds and a hydrophobic end that forms strong detergent-oil bonds, so they allow oil to ‘dissolve’ in water.

The fifteen second answer: basically because there’s no reason for them not to. Put two things together, and they tend to mix unless there’s something preventing them (welcome to the wide world of entropy). Non-polar molecules have minimal interactions with each other (relative to polar ones of a similar size–hence the volatility), so there’s no real preference for sticking together rather than mixing up into solution.