Why oil and water don't mix: the science of emulsions in cooking
Discover why water and oil do not mix, what an emulsion is and how ingredients such as egg yolk bring them together in recipes such as mayonnaise or milk.
Like oil and water
We have always heard the saying that they are like water and oil, to indicate that two things do not get along. The phrase has a scientific basis, since these substances do not mix naturally. The curious thing is that in the kitchen we have examples where they do coexist in a stable way, such as mayonnaise, a vinaigrette or milk, which are what in chemistry are called emulsions. An emulsion is a special mixture in which two liquids that do not normally combine remain together thanks to the help of a third component.
Why then do water and oil not mix?
A first explanation has to do with density. When you put vinegar (or water) and oil in the same container, at first they seem to come together, but if you wait a few minutes you will see that they separate into two distinct layers. This happens because oil is less dense than water or vinegar and therefore floats on the surface. Density, in simple terms, is a measure of how much something weighs in relation to its volume. However, density is not the only reason why they do not mix. The most important explanation lies in the chemistry of each liquid, in its polarity.
What does it mean for a substance to be polar or non-polar?
In nature there are polar, non-polar and also amphipathic (it even looks a bit like the word antipathetic, which means unfriendly) substances. Polar substances have a positive and a negative part in the same molecule, as if they were a small magnet; non-polar substances are electrically neutral, their bonds are offset and do not show poles; and amphipathic substances are a mixture of the two, with a polar and a non-polar part, which allows them to interact in both environments, so not so unfriendly after all.
The water molecule, a classic example of a polar substance, consists of two hydrogen atoms and one oxygen atom (H2O). Since oxygen attracts the shared electrons more strongly than hydrogen, it becomes partially negative, while the hydrogens are left with a slight positive charge. In addition, the bonds between the atoms are not in a straight line, but form an angle, which prevents their charges from canceling and causes the molecule to have both positive and negative areas. Vinegar is also a polar substance, not only because it is an aqueous solution but also because its active component, acetic acid, is also a polar molecule.
In contrast, oil is composed of long carbon and hydrogen molecules, which share electrons fairly evenly, and this makes it a non-polar substance. Water and oil have different polarities, or rather, of different polar nature, one is polar and the other is non-polar. When we try to mix them, the water molecules are held together by hydrogen bonds and the oil fails to integrate into that network. So, instead of combining, each liquid clumps together with its own kind. The result is the layers we are all familiar with. In general, like with like mixes well, but unlike does not mix so well together.
What is the role of emulsifiers?
A third actor comes into play here: the emulsifier. An emulsifier, also called a surfactant, is a molecule with a dual personality (an amphipathic). One of its parts is hydrophobic, meaning that it rejects water and is comfortable with fats, while the other part is hydrophilic, meaning that it is attracted to water. This combination allows it to cling on one side to oil and on the other to water, breaking the barrier that normally separates them. In mayonnaise, this role is played by egg yolk, which is rich in lecithin, an amphipathic molecule.
When we beat egg yolk, vinegar and oil, what happens is that the large drops of oil break into tiny particles that are surrounded by a layer of egg yolk and vinegar molecules. This forms an oil-in-water emulsion, which is stable and creamy. Light passing through this microscopic dispersion is scattered in all directions, a phenomenon called Mie scattering, which is why mayonnaise is not transparent, but white and opaque. Something similar happens with milk, which is also a natural emulsion of water and fat stabilized by proteins and phospholipids.
These emulsions are stable to a certain extent, as long as we take care of the conditions; these products should not be frozen or left too long without refrigeration, because sudden changes in temperature or the passage of time can break the emulsion. That is why in the industry (not only in the food industry) it is usual to add additives to stabilize the mixture and make it last longer.
In the meantime, we can enjoy this chemical phenomenon of cooking, which reminds us that behind every recipe there is science. So every time you see mayonnaise or milk, remember that the kitchen is full of scientific wonders.
If you want to know more about emulsions and explore this topic with spectacular photographs, I recommend you to visit this link I found: How mayonnaise is made: the chemistry behind it.
And if you want a more technical explanation from a scientific perspective, the American Oil Chemists’ Society (AOCS) offers a very clear article on how emulsions and emulsifiers work: Emulsions: making oil and water mix.
Here is a step-by-step activity for you to try at home.
Activity: Oil and Water Experiment to Understand Emulsions
Explore the science behind why oil and water do not mix and how surfactants, such as soap, change this interaction.
Materials:
- A clear glass or small jar.
- 3/4 cup water
- 1/4 cup vegetable oil
- Liquid soap or detergent (a few drops).
- Food coloring (optional, for greater visual effect).
Procedure:
Step 1. Fill the glass with water.
Pour the water into the clear glass, leaving room for the oil.
Step 2. Add the oil.
Add the oil slowly and let the children observe how it floats to the surface.
Step 3. Add colorant
If you use dye, add a few drops to the water to make the contrast more striking. Notice how the dye mixes with the water but not with the oil.
Step 4. Mix water and oil
Use a spoon or whisk to try to mix the oil and water. Let the children see how quickly they separate again.
Step 5. The power of soap
Add a few drops of liquid soap to the glass.
Step 5. Mix again
What’s going on? We have created an emulsion.
Questions for discussion
- Why do you think oil does not mix with water?
- Why can soap mix water and oil?
- What type of polarity does the soap have?
- Why do you think we wash greasy dishes with soap?
What do we learn from this experiment?
Water and oil, although they share the same vessel, do not get along well because they are very different. Oil always floats, since it is less dense than water.
In addition, their molecules are not of the same nature; water molecules are polar, like small magnets with a positive and a negative part. While those of oil are non-polar and do not show poles and therefore do not mix. Remember this simple rule: like with like they mix, unlike they repel.
Soap shows us the most surprising part, inside its molecule there is a “split personality” (amphipathic): one side is hydrophilic, which means it is attracted to water, and the other side is hydrophobic, which means it does not get along well with water but with fats. That combination makes it a bridge between two worlds, like an invisible force that allows water and oil to temporarily unite.
