Oil and water don’t mix. That’s just the way it’s been through the millennia and the way things will always be. In fact, “when oil and water mix” is now commonly being used by the hip kids to mean “when pigs fly”.**
But why don’t they mix? What if you really, really, really, really mix them? Let’s look at some fundamental properties of water and fats and see if we can’t get to the bottom of some of this.
Water is nothing but a compound made up of two hydrogen molecules and an oxygen. Of all of the chemical compounds out there, it’s one of the simplest in composition. However, our friend H2O has some amazing chemical properties. It reacts with some metal oxides to form bases, and some non-metal oxides to form acids. It becomes denser when heated from 0 – 4 Celsius, making liquid water more dense than ice.
Most importantly for us right now, though, is that water is highly polar, meaning that it has a positive charge at one end and a negative charge on the other.
Fats are one of a larger class of organic compounds called lipids (which also include steroids, waxes & phospholipids). Fats are big molecules that are made up of fatty acid (FA) molecules chemically bound to a glycerol molecule.
Fatty acids are generally long hydrocarbon chains with an even number of carbons with a carboxylic acid group on the end. If there is one FA bound to a glycerol, that makes a monoglyceride. Two FAs make a diglyceride, and three combine to form a triglyceride. The carbon-hydrogen bonds in fats are non-polar.
So the take-home message is that fats are nonpolar and water is polar. The water molecules are all bonded together like magnets (positive ends bound to negative ends, so the fats can’t get in. Water molecules don’t want to bond with something without a charge. So, like a middle school dance, the water molecules are all grouped together, and so are the fats. Nary the twain shall meet.
If you take oil and water and put them together, they form two separate layers. If you give that a shake, some oil bubbles get down into the water, but shortly afterwards the oil is all back togehter. It you shake it for a long time, you get tiny bubbles of oil in the water. Congratulations! You just made a suspension of oil in water! If you let it sit, they’ll still separate back out, but the oil droplets will still stay suspended in water for a while.
Emulsions – Suspensions with an agent
There’s a special kind of suspension called an emulsion. Emulsions are nothing more than stable suspensions. They are stabilized by some third agent (emulsifying agent) that will form a thin film at the droplets’ surface, allowing them to mix.
A common emulsion that can be found in many refrigerators (in addition to hollandaise, beurre blanc, and ice cream) is mayonnaise. I personally can’t stand the stuff, but we’re going to talk about it anyway. In this case the water is being replaced by vinegar, which is basically an aqueous solution (so it’s water-based). The emulsifying agent is egg yolk.
You can make mayonnaise at home with oil, vinegar, & egg yolk. Basically, you add a little oil to the egg, mix, slowly add more oil to the mixture, then add some vinegar, then alternate between oil and vinegar until it’s all thoroughly emulsified. Please don’t try to use that as a recipe for mayonnaise.
What happens is that the oil droplets become coated in the egg yolk, which keeps them from coming together to form a separate layer. If you add the oil too fast, or if you add too much oil at a time, the droplets will come together before they can be forced into the egg yolks; the mayonnaise will curdle, or separate. So the egg yolk allows the oil and vinegar to come together and stay that way.
So there’s your nutshell version of what happens with suspensions & emulsions. We’ll revisit some of the chemical properties of fats and water in some postings coming up!
* With apologies to the Bee Gees.
** I made that up.