What is phase inversion? We know we have two phases in our lotions - the oil phase and the water phase (if you want more technical terms or to learn more about emulsification, please click here). In an oil-in-water lotion you have the external, continuous phase (the water) and the internal dispersed lipid phase (the oil).
When we have heated and held our two phases, we add them together. In the past, I'd add the oil to the water phase because the container was usually bigger because I had more water based ingredients. But I was wrong!
If you add your oil to the water phase, you are creating an oil-in-water emulsion. This is what we want, right? Well, yes and no. The oil-in-water emulsion is our end goal, but if we add the water to the oil, we can get a more stable emulsion and, therefore, a more stable lotion.
When we add the water phase to the oil phase it produces a temporary water-in-oil emulsion (w/o). It will eventually "flip" to become an oil-in-water lotion at the point of phase inversion. This makes the emulsion more stable, which is, again, something we like.
How exactly does phase inversion work? Most of the emulsifiers we use are ethoxylated fatty materials. The hydrogen bonding between the ethoxylated molecules and the water is what helps with the emulsion. When they are heated two things happen: their HLB changes from about 12 to about 5, which means they are more suitable for water-in-oil emulsions, and their ability to form these hydrogen bonds with water decreases. So they're really acting as water-in-oil emulsifiers when heated.
As the lotion cools, the HLB of these emulsifiers starts to increase back to the proper HLB value and their ability to hydrogen bond with water increases. The lotion switches from being a water-in-oil emulsion to being an oil-in-water emulsion.
This is more stable because the oil droplets in a phase inverted lotion will be smaller than those found in a non-phase inverted lotion. We know that oil droplet clumping can be one of the biggest reasons for separation - the smaller the droplets, the easier it is to keep them apart, thus less likelihood of clumping.
Isn't chemistry fun?
For a fantastic PDF on phase inversion, click here for Dr. Z's explanation!
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