Click here to see the basic lotion making instructions post.
Whenever I make a lotion, I weigh out the heated water phase and the heated oil phase separately, and put each into my double boiler. I heat each until they reach 70˚C, then hold it there for 20 minutes. Why do we do this? The first is to eliminate any possible contamination by heating it, and the second is about the critical micelle concentration, which requires that both phases are the same temperature when we add them together. And finally, we need the phase inversion to create a stable emulsion.
Okay, so the contamination stuff is obvious. If we are using water with icky things in them, a prolonged period of heating and holding will help kill the nasties!
The critical micelle concentration is a little more complicated. When we reach the critical micelle temperature or Krafft temperature - suggested as 70˚C to 80˚C for Polawax - we will see our micelles form properly, which means the emulsification has worked and we will get a nice lotion that shouldn't separate. If our oil phase is at 70˚C and our water phase is at 100˚C (boiling point of water in Celsius), then they may not come together properly. If our oil phase is lower than 70˚C, we might see a very unstable emulsion.
We know that when we heat something we increase its solubility. Non-ionic emulsifiers (like Polawax and e-wax NF) are water soluble at low temperatures and oil soluble at higher temperatures. At the lower temperatures, the Polawax forms an oil-in-water emulsion. At the higher temperatures, the Polawax forms a water-in-oil emulsion. So shouldn't we do it at lower temperatures so the Polawax will create what we want - an oil-in-water emulsion?
Creating a water-in-oil emulsion that will eventually turn into an oil-in-water emulsion is a concept called phase inversion. We need to reach the phase inversion temperature to make this process work. (Click the link to read more about this...)
So if you are boiling your water and heating your oils in a microwave, odds are pretty good that you don't have both phases at the same temperature and you won't see the phase inversion. You will get an emulsion, but it won't be as stable as the one achieved with the phase inversion method.
Point of interest: Here's a post about all the different ways lotions can go wrong, most of which have to do with a failure to get the ingredients to the proper temperature.
To summarize: If we heat and hold our ingredients at 70˚C for 20 minutes, we are assured that our emulsifier will be very oil soluble, which means it will create a water-in-oil emulsion that will eventually become an oil-in-water emulsion, which is very stable. We are assured our micelles will be a nice size, and we have eliminated most of the nasty contaminants that could ruin our lotion. In short, by heating and holding, we are ensuring we have a stable lotion that will remain emulsified and bug-free for a very long time!