Why does oil not mix with water?

Due to the electronegative oxygen atom's ability to remove charge from two hydrogen atoms, water molecules are highly polar, having partial positive and negative charges at one end of the molecule and partial negative charges at the other.

This characteristic is absent from oil molecules; while some oils may contain small amounts of polar groups, the bulk of the molecule will consist of carbon atom strings, indicating low polarity.

In other words, it is more advantageous from an energetic standpoint for water molecules to associate with one another than it is for them to associate with oil molecules. On the other hand, it is more advantageous from an energetic standpoint for oil molecules to associate with one another than with water.

Two layers is the end result.

Conversely, combining oil with a non-polar substance like hexane results in a much better mixture.

Oil and water don't mix due to differences in polarity; water is polar, while oil is nonpolar, causing them to repel each other.