When I see drops of oil floating on water, at times, oil comes across as a stubborn liquid which refuses to mix with water. At other times, I wonder whether it is water to be blamed for not allowing oil to mix with it. Perhaps, both oil and water do not want to mix with each other or science does not let them gel! Let's look at what exactly stops them from mixing.
Both oil and water are liquids, but their chemical compositions are very different from each other. The sizes of oil and water molecules are different and so are their intermolecular forces of attraction.
The mixing of any two liquids is attributed to the similar nature of their molecules. The difference in the molecular structures of oil and water is one of the main reasons why they do not mix with each other. They are a well-known example of immiscible liquids.
One reason why water and oil do not mix, is surface tension. When water molecules come together, they form a network of hydrogen bonds which results in very high surface tension. Both oil and water have high surface tensions, due to which the adhesion between them is weak and they do not mix.
For two liquids to mix, the chemical bonds holding the molecules together, need to be broken and new ones need to be formed. The bonds between water molecules are very strong, whereas the bonds between oil molecules are weak. Hence, a lot of energy needs to be consumed in an effort to bring the water and oil molecules together. This explains why oil does not mix with water.
Molecules that form an ionic or a hydrogen bond with water are known as hydrophilic or water-loving, while non-polar molecules that repel water are called hydrophobic, which means they lack water affinity. As oil molecules (which are long chains of hydrocarbons) do not carry charge, they cannot bond with water. As a result, oil and water don't mix.
Why does oil float on water and not drown in it? It's because its density is less than that of water. However, oil and water can be forced to mix with the help of an emulsifier. It is any substance that can cause oil and water to mix. Add salt to oil and you'll see the oil drowning. Add a detergent, and the oil will readily mix with water. The detergent molecules bridge the 'rift' between the oil and water and allow them to mingle. Adding an emulsifier to oil and water gives a stable mixture.
One thing's for sure, although oil and water do not come to terms with each other under normal conditions, water is gracious enough to let the oil float atop!