What exactly is an emulsion? For the purposes of this discussion, I'm going to work with the oil-in-water lotion, in which droplets of oil are suspended in water. This is generally the type of lotion you'll see, where generally the water phase is much larger than the oil phase.
If you want to know more about water in oil emulsions, please click here for a comparison of o/w and w/o.
An oil-in-water emulsion is defined as an internal lipid phase dispersed in the external or continuous aqueous phase which is stabilized by surfactants. The internal phase is lipophilic or oil loving and the external phase is hydrophilic or water loving. Adding a surfactant creates a film at the interface of the two phases (oil and water), which creates an emulsion.
In other words, the oil is dispersed through the water phase and stabilized by our emulsifiers.
You might see the oil phase called the internal phase or the discontinuous phase - I'm going to call it the "oil phase" to make life easier. Similarly, the water phase can be called the external or continuous phase - I'm going to call it the "water phase". I realize this isn't completely accurate if we're considering all lotions, but since I'll be writing mostly about oil in water emulsions, it works for me.
Oil and water normally don't want to mix, as you can see in salad dressing or failed lotions. Emulsifiers decrease the interfacial tension between these two phases that refuse to mix (they are considered immiscible, meaning in any proportion do not form a solution) and allow them to come together in a lotion-y goodness. Emulsifiers also act as stabilizers for this mixture.
There are three aspects of emulsification - chemical, heat, and mechanical. The chemical is the emulsifier, the heat is the heating and holding we do, and the mechanical is the mixing process. As our emulsifications are based on kinetic (or physical) stability, they will fail eventually. It may take minutes, it may take years, but eventually the emulsification will break down. We want that date to be well after we've finished the bottle! I'm going to be focusing mainly on chemical emulsification, but temperature and mechanical emulsification will be making appearances.
Why did I mention surfactants? Surfactants are SURFace ACTive agENTS (surf-act-ents) with a hydrophilic, or water loving, head and a hydrophobic, or water hating, tail. (Sometimes you'll see the hydrophobic tail listed as lipophilic or fat loving.) Surfactants can be foamy, lathery things like detergents or they can be emulsifiers. (And yes, theoretically foamy surfactants are emulsifiers as well, but let's not get into that right now!) For the purposes of these posts, emulsifiers are surfactants with at least 14 carbons on a chain that enable us to emulsify oil and water together.
How do emulsifiers emulsify? The hydrophilic head is in contact with the water phase while the hydrophobic tail is in contact with the oil phase. The hydrophilic tail connects to the oil and the head protudes into the water connecting the two. You can see why this is so unstable.
What can cause separation in an emulsification? If we don't have enough chemical emulsification or the right kind of chemical emulsification (meaning not enough or the wrong emulsifier), if we don't have adequate heat energy (heating both phases to the same temperature), and if we don't have enough mechanical energy (using a mixer or stick blender) we're going to see separation.
When you think of how much can go wrong, it's almost a miracle a lotion works sometimes!
So what can we do? We can use emulsification systems like Polawax, e-wax, or BTMS to help us create our emulsions or we can create our own emulsification systems using the HLB (hydrophilic-lipophilic balance) system.
We have a topic for tomorrow - the HLB system!