If Oil and Water Don’t Mix, How Does Soap Remove Grease?

When a greasy plate is rinsed with water alone, the water often slides over the oil. This is not because water lacks force. Water molecules attract one another, while oil molecules prefer the company of other oil molecules, so the two liquids do not readily form a stable mixture.

Soap and many detergents contain surfactant molecules. One end interacts readily with water; the other avoids water and enters the grease. During washing, these molecules gather at the boundary between oil and water and make it easier for the grease to detach from the plate. As the oil breaks into smaller droplets, the water-loving ends face outwards, allowing the droplets to remain dispersed rather than immediately joining into a layer again. Under suitable conditions, surfactant molecules also form micelles, with their water-avoiding parts tucked inside.

Soap therefore does not turn oil into water, nor does it make oil dissolve in the same way salt does. It creates a molecular interface that can engage with both oil and water; rubbing, flowing water and rinsing can then carry the dispersed droplets away. Foam merely forms when gas is enclosed by surfactant films. More bubbles may look impressive, but they do not by themselves prove that cleaning is more effective. What matters is whether the molecules reach the grease, help loosen it from the surface, keep it dispersed and allow enough water to rinse it away.

https://openstax.org/books/organic-chemistry/pages/27-2-soap
https://chem.libretexts.org/Ancillary_Materials/Laboratory_Experiments/Wet_Lab_Experiments/General_Chemistry_Labs/General_Chemistry_Labs_II/12.1%3A_Soaps_and_Detergents