Why does salt dissolve in water and not oil? Positive and negative charges on water molecules are attracted to sodium and chloride ions as seen in the image above. Water molecules "hydrate" sodium and chloride ions, keeping them separate in water, which, when you think about it, is quite extra-ordinary. And preventing salt's ions from re-combining is something that water is particularly good at because it forms this protective sheath around the ions. All the more interesting is that while water can keep salt ions separated, even at room temperature, oil cannot. What are the differences between water and oil? Water and oil have many differences; here, we will focus on their very different dielectric constants. Water has a dielectric constant of 80, while oil has a dielectric constant of only 2. This means that it is easier to polarize water than oil. It also means that the energy associated with 'how attracted' two ions of opposite charges are to each other in water is much weaker than in oil. The oppositely charged ions do not see each other in water because of this sheath that forms around them, and whether or not there is a sheath depends, in part, on the strength of the dielectric constant of the fluid they are immersed in.
It turns out that if the thermal energy, kT, is small enough, then salt will not dissolve because the attractive energy has to be less than kT for ions to remain dissociated. There is also a length scale to think about; this is called the Bjerrum length and it defines the distance between ions required for stable dissociation. For water, at room temperature, this distance is 0.7 nm which is the size of their hydrated sheaths, but for oil the distance is 28 nm. That means that salt ions cannot hide in oil unless there is a sheath of 28 nm in diameter to hide in. There are polymers or copolymers that are capable of sequestering ions into sheaths that are 28 nm; these sheaths with ions are called reverse micelles. (Image credit: L. L. A. Adams)
