About the Like dissolves like principle

[cantgetaname]

About the 'Like dissolves like' concept...

I can understand why polar solutes would be better soluble in polar solvents.
But why are they not soluble in non-polar solvents?
Moreover, why are non-polar solutes only soluble in non-polar solvents? What stops them from being soluble in non-polar solvents?

I would expect the following order solubility...

Polar-polar> Nonpolar-polar (or vice versa) > Nonpolar-Nonpolar

(solute and solvent respectively)

 

[espen180]

Think about that it means for something to be in solution and for something to be dissolved. Polar solutes have relatively strong interactions with the water molecules, so they are able to separate the water molecules and be dissolved in them. In other words, the interaction between a polar solute and water is so stong that it is more energetically favorable for them to be close to the water molecules and the water molecules being close together.

Nonpolar solutes simply don't have a strong enough interaction with water to separate the molecules, and so we see layers forming when we try to dissolve for example hexane in water.

The same is true in the opposite case. Polar solutes are so strongly bonded to each other and don't really interact with nonpolar solvents, and the solvent can't separate the polar molecules from one another or break apart ionic salts.

 

[cantgetaname]

Thanks
However, the main confusion here was that in case where both solute and solvent are non-polar, then the interactions should be even less.
How do non-polar solute and solvent dissolve into each other then?

 

[Ygggdrasil]

In the case of a non-polar solute and non-polar solvent, it is true that the interactions between solute and solvent will be relatively weak. However, the interactions between solute molecules will also be relatively weak and easier to break up than interactions between polar solute molecules.

It is also worth noting that entropy plays a role here. When non-polar solutes are dissolved in a highly polar solvent with lots of hydrogen bonds (such as water), the solute molecules cannot participate in the hydrogen bonding. Therefore, the water molecules must arrange themselves in a ordered solvent shell around the solute particles in order to interact with the solute while still maintaining their hydrogen bonds to the other water molecules. This ordering of water comes with an entropic penalty.

In order to minimize the entropic penalty, the surface area of the solute molecules must be minimized and this is achieved by aggregating smaller groups of solute molecules into larger groups. This results in the separation of the non-polar molecules from the aqueous phase. Such an exclusion of non-polar molecules from aqueous solution is known as the hydrophobic effect.

 

[cantgetaname]

Thanks for that explanation!