Back when we reviewed mixtures and compounds, we looked at some everyday liquids and I said that milk is a mixture of various molecules: butterfat, vitamin A, vitamin D, proteins, lactose, and calcium phosphate. What I failed to mention is that cow’s milk is also about 88% water by mass. So how do the nonpolar triglycerides in butterfat stay mixed in the polar water instead of separating out into a separate layer?

Milk is definitely not a solution, and the butterfat in milk will separate out if you leave milk sitting long enough. But the triglycerides and water molecules in milk are also not a heterogeneous mixture in the same way that they are in a pure oil and water mixture. If you could zoom in on a sample of milk, you would find fat globules filled with triglyceride molecules suspended evenly throughout the liquid water. These fat globules are, on average, about 4 µm (4 × 10-6 m) in diameter, which means that they each contain about 10,000,000,000 triglyceride molecules.

The fat globules in milk are what give milk its creamy, white color. The fat globules would remain suspended in the liquid water indefinitely if they were the same density as the liquid water. But because oils are generally less dense than water, the fat globules will, over time, slowly rise to the surface, forming a layer of butterfat above the milk. The homogenization process is designed to increase the density of the fat globules so that this separation does not occur (at least not as fast).

Fat globules are able to form because of the presence of phospholipids in milk. Phospholipids are a class of lipid that contains a phosphate group. Phospholipid molecules basically consist of a water-soluble “head” and a fat-soluble “tail.” One of the phospholipids in milk, shown below, is sphingomyelin (SPH). Polar covalent bonds are in red.

Phospholipids are able to form a barrier between water-soluble and fat-soluble substances, enabling the formation of fat globules, because of their dual polar and nonpolar nature. The polar end of a phospholipid molecule interacts with the polar molecules on one side of the barrier, while the nonpolar end interacts with the nonpolar molecules on the other side.

Fat globules form as phospholipids surround clusters of triglycerides, basically putting a water-soluble surface over a large collection of nonpolar molecules. This water-soluble surface is how fat globules are able to remain in a polar environment and to “mix” (at least to some extent) with polar molecules. In addition to triglycerides, other fat-soluble molecules, such as vitamin A and vitamin D, will also collect and dissolve inside of these fat globules.

Milk is an example of a colloid. Colloids are mixtures that are heterogeneous on a molecular level, but homogeneous on a microscopic level. In a typical oil and water mixture, the oil and water molecules do not mix at all. In an oil and water colloid, individual oil and water molecules still do not mix, but microscopic drops of oil and water molecules do. With the help of phospholipids, these microscopic drops of oil are evenly dispersed throughout the continuous liquid water phase. This means that, at least on a microscopic scale of about 10 µm (0.01 mm), the chemical composition of a colloid like milk is homogeneous.

In milk, the dispersed phase (triglycerides) and the continuous phase (water) are both liquids. However, there are other colloids where the substances in the dispersed phase and the continuous phase are in different states. Smoke consists of solid particulates dispersed in a gas. Each individual particulate contains trillions of molecules, and these molecules exist in a solid state. In general, solids are denser than gases and solid particulates will settle out of a gas. However, the solid particulates in smoke are so small (microscopic) that the natural currents in the smoke are enough to keep the particulates afloat. Think about dust in the air. Dust particles are solid particulates. They float in the air even though the dust is denser than air. But dust particles will settle out of air if the air stops circulating, such as in a sealed room.

Smoke is an example of a solid aerosol… a colloid where a solid is dispersed in a gas. When a liquid is dispersed in a gas, the colloid is called a liquid aerosol. Fog is a liquid aerosol. In fog, microscopic drops of water are suspended in air. Each microscopic drop contains trillions of water molecules, and those water molecules are in a liquid state. Fog is not the same as water vapor from evaporation. Water vapor is water molecules in a gas state. In water vapor, water molecules are not in drops; they are in a gas state and mixed on a molecular level with the nitrogen (N2) and oxygen (O2) molecules in the air.

The difference between carbonated water and whipped cream illustrates the difference between a solution and a colloid. Carbonated water is carbon dioxide (CO2) gas dissolved in water. This means that the CO2 molecules and H2O molecules are mixed on a molecular level, and the CO2 molecules are in a liquid state. On the other hand, whipped cream is air bubbles (not individual molecules) mixed in cream. The continuous phase in cream is liquid water. Dispersed evenly throughout the liquid water are microscopic drops of liquid butterfat and microscopic bubbles of air… and the N2 and O2 molecules in the air bubbles are in the gas state.

An emulsifier is a substance that stabilizes an emulsion… a colloid with a liquid dispersed phase in a liquid continuous phase. Phospholipids, such as sphingomyelin, act as emulsifiers in milk. Emulsifiers can be found in many common cooking ingredients, such as egg yolks and mustard. Egg yolks are used to stabilize mayonnaise and Hollandaise sauce while mustard is sometimes used to stabilize vinaigrettes.

Mayonnaise, Hollandaise sauce, and vinaigrettes are all made by whisking a fat (usually olive oil or butter) into an acid (usually lemon juice or vinegar). Vigorous whisking disperses the acid in the fat, creating a smooth, creamy sauce. But if an emulsifier is not used, these sauces will “break,” and the fats and acids will quickly separate. Because the fats make up approximately 75% of these recipes, the fats are the continuous phase and the acid and water are dispersed in microscopic drops. Using an emulsifier enables the emulsion to stay mixed by surrounding the drops of polar acid and water molecules with a nonpolar (fat-soluble) surface.

The primary emulsifying agent in egg yolks is lecithin… a yellow-brown fatty substance that contains phospholipids, such as phosphatidylcholine. Lecithin is sold commercially. It is added to chocolate to keep the cocoa and cocoa butter in candy bars from separating, cooking sprays to reduce sticking, and baked goods to make doughs smoother.

Soaps and detergents use emulsifiers called surfactants to remove oil and grease stains. Washing clothes or dishes in water will dissolve and remove dirt and stains that are water-soluble, but it will not remove dirt and stains that are fat-soluble. By adding surfactants to water, soaps and detergents enable fat-soluble molecules to dissolve in water and be rinsed away. A common surfactant in soap is sodium stearate.

When a surfactant is added to water, the polar head of the molecule is attracted to the surrounding water molecules, but the nonpolar tail is not. There are only two stable configurations for surfactant molecules in water. The surfactant molecules can form a single layer along the surface of the water… and once that space is filled, they can form clusters called micelles. Any fat-soluble molecules will then be absorbed and dissolved inside of the micelle.

You can see surfactants in action by adding dishwashing liquid to a mixture of oil and water. Whisk the mixture vigorously and you will see an emulsion form. If there is more water than oil, then micelles will form with oil inside. However, if there is more oil than water, then micelles will form with water inside instead.

Phospholipids and surfactants play an important role cell chemistry. You will learn more about phospholipid bilayers, cell membranes, and vesicles later in this unit.