I am writing a book on advanced formulation techniques. It is a long way from being completed, but a question about order of addition for emulsions came up on a Facebook forum and as there was a section that answered the question, I thought I would use an extract from it as a blog post. If you are interested in the book itself, you might have a long wait but you’ll hear about it first if you sign up for my newsletter.
So if your aim is a water in oil emulsion, you start with a continuous oil phase and slowly add the water phase to it while agitating the batch. At first the water will be dispersed into the oil, but at some point the phases will invert leaving the erstwhile external phase as the new internal phase. The droplet size you get will be determined by the surfactants you use, the temperature, the mixing speed and the proportion of oil to water. If you get the mixing conditions and rate of addition of the water just right, then the point at which the phases invert will give you exactly the nature of oil phase that you desire. In theory you can produce an emulsion without any high shear mixing at all. It doesn’t in general give very good results, but as Doctor Johnson said about a dog playing chess the wonder isn’t that it works well but that it works at all.
Modern plant and modern energy costs all push in the direction of adding the smaller phase to the larger one. This is usually the most convenient way to work and my guess is that this is how the vast bulk of cosmetic emulsions are produced at the moment. But there are always exceptions and it is worth always bearing this feature of emulsion formation in mind. In particular, don’t treat which phase is added to which as a minor detail. If you are transferring from one plant to another and your plant manager wants to do it the other way around don’t assume that the product will come out the same. You will need to investigate whether or not this change is significant or not before recommending it.