While looking at the comments section looking for questions I could answer, I found a link to the Bancroft rule, and I thought I should post it here. (I can't remember who suggested this, sorry, but you are awesome!)
The Bancroft rule states: "The phase in which an emulsifier is more soluble constitutes the continuous phase."
Check out this post for more in depth information.)
Oil phase = internal, discontinuous, or discrete phase
Water phase = external or continuous phase
Looking at this picture, the oil is the orange part.
More quoting from Wikipedia: "What the Bancroft rule states is that contrary to common sense, what makes an emulsion oil-in-water or water-in-oil is not the relative percentages of oil or water, but which phase the emulsifier is more soluble in. So even though there may be a formula that's 60% oil and 40% water, if the emulsifier chosen is more soluble in water, it will create an oil-in-water system.
There are some exceptions to Bancroft's rule, but it's a very useful rule of thumb for most systems."
Wow! So what this says is that the emulsifier determines if it's an oil-in-water lotion (what we generally make) or a water-in-oil lotion, not the ratio of oils to water! Wow!
When we take a look at the hydrophilic-lipophilic balance system (HLB system), we can see that some emulsifiers are more soluble in oil and others in water. The lower the number, the more oil soluble. So glycol distearate (HLB 1) will more soluble in oil than polysorbate 20 (HLB 15). Which makes sense as most of those low HLB emulsifiers are waxy or oily things like glycol distearate or lecithin.
Yet more quoting: "To have a stable emulsion, the surfactant must be soluble in the continuous phase." So anything we use must be water soluble to create an oil-in-water product. Interesting...
This isn't a hard and fast rule, and it seems to be have been surplanted by the HLB system, but it gives us something to think about when creating lotions. As with so much of what we learn about making bath & body products, common sense isn't always the correct answer!
If we're using all-in-one emulsifiers like Polawax, emulsifying wax, Ritamulse SCG, Incroquat BTMS-50 we don't have to worry about solubility. And, to be honest, we really don't need to worry so much about it if we're using HLB system emulsifiers. I just thought this was really interesting!
More on phase inversion- Creating products: Mixing the two phases
Colloid chemistry power point (do a search for Bancroft, although the whole thing is fascinating!)
Colloid chemistry power point (page 16, or do a search)