Question

Thermodynamically speaking, why does water and oil separate when mixed?

Answer 1

there is a one thermodynamic fundamental rule that polar molecules only make a bond with another polar molecules and nonpolar molecules bonds with another non polar molecules.

we know that water is polar molecules which has positive and negative charge on it's element, while oil is nonpolar molecule.

so polar molecule is not making bond with nonpolar molecules and so water and oil is not able to make bond.

so they are easily separate.

if we want to mixed oil and water for longer period, we can used emulsifier, which having one end is positive charge and another is negative charge.

so water make a bond with positive charge of emulsifier and oil make a bond with negative charge of emulsifier.

in this way we make a oil and water mixer for longer period.