The reasons why water and oil are doomed to stay separated

We have all witnessed at some point the pronounced hostility between water and oil, which strive not to mix when placed together in a container. No matter how much we stir the mixture, they always end up forming two separate phases, with the oil floating on the water due to its lower density. They must have had quite a disagreement to end up in such a situation…

Sometimes it is said that the cause of this separation lies in the difference in density between the two substances, but the true culprits of this bad blood are the physicochemical properties of the molecules that make up both substances.

Entropy opposes union

In chemistry, there is a rule of thumb: “like dissolves like”, but water and oil could not be more different. Water, as indicated by its chemical formula, H2O, is composed of molecules with two hydrogen atoms that share their electrons with an oxygen atom to form covalent bonds. When these atoms bond, their electrons are distributed asymmetrically along the resulting water molecule. This happens because oxygen is more electronegative than hydrogen.

Electronegativity is the tendency of atoms to attract electrons from other atoms with which they bond. It is a property that increases as we move to the right and up in the periodic table of elements. For this reason, oxygen pulls electrons more strongly toward itself. Since electrons have a negative charge, oxygen acquires a slightly negative charge, and hydrogens a slightly positive charge. This results in a molecule with two poles, like magnets. That is why we say water is a polar substance.

This property allows water molecules to bond with other water molecules. The hydrogen atoms of each molecule can bond with the oxygen of nearby water molecules through hydrogen bonds. Eventually, all water molecules will form a three-dimensional network through hydrogen bonds to create the liquid element. However, as these bonds are relatively weak, at least compared to the intramolecular covalent bonds, they are constantly breaking and rebuilding, so water molecules have multiple options for organizing with adjacent water molecules.

Water molecules forming hydrogen bonds
Water is composed of a three-dimensional and dynamic network of polar H2O molecules (oxygen is represented by the red part and hydrogen by the gray ends) that bond with their nearest siblings through hydrogen bonds (blue lines), which are constantly breaking and rebuilding. If an apolar molecule incapable of forming hydrogen bonds were introduced in this network, this dynamic would be disrupted, water molecules would lose freedom of movement, and a decrease in entropy would occur. Splette – Wikimedia Commons

What happens with oil? In its case, the electron density is more evenly distributed among the atoms of its molecules, which are primarily triglycerides, so no poles are formed. In other words, it is an apolar substance. For this reason, the triglycerides in the oil cannot form hydrogen bonds with water and, therefore, cannot bond with it under normal circumstances. In fact, when triglycerides attempt to bond with water, they disrupt the network of hydrogen bonds and restrict the available options for water molecules to organize, resulting in a reduction of the system’s entropy. Or, as you may have read elsewhere, “the amount of disorder decreases.” And we know one of the implications of the second law of thermodynamics: natural systems tend to evolve towards states of higher entropy. In other words, entropy favors states with a greater number of available configuration options (“more disorder”). Because of this, triglycerides are “expelled” from the network of hydrogen bonds, which causes an increase in the entropy of the system. Triglycerides have no choice but to bind with each other and form their own layer separate from water.

A molecule to bind them all

Even so, it is possible to mix inherently immiscible substances, a process known as emulsion. For example, we can achieve a temporary emulsion by vigorously beating a mixture of water and oil. Initially, it may seem like oil droplets disperse in the water. However, after some time, oil droplets will rejoin to form a separate phase from the aqueous phase. If we want to obtain a stable emulsion, we will have to add an emulsifier to disperse one phase into the other. This is what happens when we make mayonnaise. In this case, we want the oily phase to disperse into the aqueous phase, and not the other way around, which is when it curdles.

Emulsifiers are characterized by having amphiphilic or amphipathic molecules. These terms simply define the structure of these molecules, which consist of two ends: one polar and one nonpolar. The polar end is also known as hydrophilic because it has an affinity for the polar molecules of water, while the nonpolar end is known as hydrophobic, as it avoids water. When we break down the oily phase and add the emulsifier, its molecules begin to surround the oil droplets. They do this by orienting their nonpolar end toward the oil droplet and the polar end toward the water. As a result, oil droplets cannot bond with each other, leaving the oily phase dispersed in the aqueous phase.

We can better understand this with the example of mayonnaise. The homemade recipe is very simple: it consists of egg, oil, and salt, although it is also recommended to add some acidic substance, such as vinegar or lemon; we will see the reason later. The egg is an archetypal case of a natural emulsion since it is composed of a mixture of water (80%), proteins, and fats, which are particularly abundant in the yolk. This part of the egg contains lecithins, a type of emulsifying fats responsible for mixing the egg with the oil to obtain mayonnaise.

Oil droplet surrounded by amphiphilic molecules
Molecules of an emulsifier surrounding an oil droplet by orienting their polar “heads” towards the aqueous phase and the apolar “tails” towards the droplet. In this way they will cause the droplet to repel other oil droplets also surrounded by amphiphilic molecules, since charges with identical sign repel each other. Anderl – Wikimedia Commons

First, you have to add the egg and then the oil. To ensure that the sauce is made correctly, it is essential to beat the egg very well to ensure the release of lecithins. It is advisable for the egg to be at the same temperature as the oil, as this requires less energy to release the lecithins. Once this is achieved, you can gradually start stirring the oil from bottom to top to disperse it into small drops that will be surrounded by the emulsifying fats of the egg. The addition of a little vinegar or lemon juice can help the emulsion to succeed. The acetic acid from the former or the citric acid from the latter will acidify the medium and help stabilize the emulsion, as increased acidity (not too much) promotes repulsion between the polar and non-polar ends of the lecithins and the separation of oil droplets.

Sometimes, when we don’t follow some steps correctly, the mayonnaise curdles, or in other words, an emulsion occurs in the opposite direction. In this case, what breaks into small droplets is the water from the egg. The lecithins will surround these drops, preventing them from coming together. In this way, the aqueous phase disperses in the oily phase, forming a liquid mixture that is not very appetizing.

Focused on preventing the mayonnaise from curdling, we overlook the intricate physical and chemical principles governing this culinary process. Surely, the next time you prepare the delicious sauce, you will see everything from a different perspective, one that will take you on a journey through the wonders of the microcosm that constitutes the essence of all things.

The fact

The cleaning action of soaps is based on the emulsion process, since soap molecules are amphiphilic. When soap and water are poured on a grease stain, their molecules are dispersed in the aqueous phase thanks to the intermediation of the emulsifier.

Soap molecule surrounding grease stain
In the image above, a representation of the chemical structure of sodium stearate, a molecule that is part of the composition of many soaps. It is an amphiphilic molecule, as it has a polar end (highlighted in yellow) and a nonpolar end (in blue). The polar head orients towards the aqueous environment, while the nonpolar part exhibits affinity for molecules of its same nature, such as those in fat. This property gives soaps the ability to remove grease stains. 20Minutos


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