Polawax, the general rule of thumb is to use 25% of the oil phase. So if you have 20% oil, you'd use 5% Polawax. If you have 30% oil, you'd use 7.5% Polawax. And so on. So why am I using 4% emulsifiers in yesterday's moisturizer recipe or more than 25% in other recipes?
A short aside on how to calculate your oil phase...Your oil phase is anything with an HLB value - any oils, butters, thickeners, "oil free" moisturizers, esters, and so on - or anything we'd consider not-water-soluble like silicones. Your fragrance or essential oil and Vitamin E also count as the oil phase. So if you have 10% oils, 5% butters, 3% cetyl alcohol, 1% Vitamin E, 1% fragrance oil, you have a 20% oil phase (so 25% would be 5%). Add 2% cyclomethicone, 2% dimethicone, and 1% C12-15 alkyl benzoate and you have 30%, so you'd increase the emulsifier to 6%.
In the case of the recipe I posted yesterday, I have 10% oils, so why did I use 4% emulsifier when it should be 2.5%? A few reasons...
I'm planning to tweak this recipe. As you can see from the dry skin recipe, I've added more oils (thus creating a larger oil phase of 16%), so I'll have enough in there when I make that tweaked recipe.
I might not use Polawax. The rule of thumb is specifically for Polawax. If you're using another emulsifier - Natramulse, BTMS, emulsifying wax NF, and so on - you have to find out what the suggested usage is for that product.
I might use BTMS. BTMS is not only a great conditioner but a great emulsifier (especially if you're making something silicone heavy). There is no real rule of thumb for BTMS. You can use it as low as 1% or as high as 10% as an emulsifier, and it depends on your desired skin feel. In the case of a facial lotion, I might be using it for the conditioning features and use it at 50% of the oil phase. Heck, there might not even be an oil phase (in the case of making an oily hair conditioner) or a really tiny oil phase (in the case of the upcoming moisturizer for oily skin), but I want that ingredient with its benefits in my moisturizer.
I've made this recipe, and I know if I use 2.5% it isn't stable enough. The goal of emulsification is to create a stable lotion that won't separate. All emulsions want to separate - it's not a natural state of being and one day it will fail, but we want to make sure it won't fail until the next century (you've seen lotions kept in the back of cupboards for years that are still emulsified - that's our goal!) Rather than go with the 25% rule here, I went with what I know worked for me, which was 4%. (Click here for my epic lotion fail. I used the suggested amount of emulsifier, but it was a new emulsifier to me, and it failed.)
As well, by using a little more, I eliminate the room for error. If I am to use 2.5% but I accidentally add 0.5% more oil, 0.4% more silicone, and 0.6% more Vitamin E oil, all of a sudden I have 1.5% more oil than I expected, which could mean lotion failure. If you're using 8 different oil ingredients and you're out by 0.5% on each one, suddenly we have 4% more oils than we thought. By adding a titch more emulsifier, I make sure my lotion has enough emulsifier to stabilize it. (If you're using a scale that goes to 1 gram, it is possible your 10% is actually as high as 10.9% so it's really easy to add more than you expected.)
When you increase the size of your lotion batch - say you make 2000 grams (20 x the recipe), and you add a little more here and a little more there, it's really easy to see a lotion fail. By increasing the amount of emulsifier slightly, it's easier to increase the size of your batches.
As well, sometimes I forget to include a fragrance oil in the recipe, and might add it at a later date. I might decide I need more Vitamin E. If I have a titch more emulsifier than I need, I can play around with a recipe without re-calculating it every single time.
And it's a personal thing. I remember LabRat suggesting we use 2% emulsifier when starting out with the HLB system. That amount scares me, so I choose to use 4% when I'm formulating my own emulsifiers with the HLB system. I don't get a lot of time to formulate, so when I do, I want to make sure it works. I'm happy to add a little bit more to ensure stability. He was an amazing cosmetic chemist with access to many interesting ingredients and a lab; I'm an aspiring cosmetic chemist with access to what's in the shop this week and workshop.
Remember, every recipe you see on any site reflects a number of things - what that person likes, his/her personal philosophy about formulating, what's in stock that week, what's on special at the supplier's store, or what's new and interesting! As you get more comfortable formulating, you'll learn what you can change and what to keep the same. That's why I take a look at tweaking suppliers' recipes and commercial products, and why I'm always tweaking my products for different skin or hair types - by doing this, I learn what I have to buy and what's just a suggestion, what feels nice on my skin or hair, and what I can do to make it more mine!
Where does the 25% of the oil phase rule come from, and can I provide a link? I'm not sure, and no. I remember reading the "25% of the oil phase" on the Dish and I remember looking for it and finding confirmation from a reliable source, but I can't find a link anywhere (I've been searching my computer and the 'net for over an hour - I'm done!) In their own literature, Croda comments about lotions made with Polawax being stable at "as low as 5%, although 10% might provide superior stability".
It's funny to me how we get something in our heads and we go with it as gospel. To me, this is more the reason I need to check and double check something before posting it - once it's on the 'net, it takes on a life of its own! And this is the reason that you, gentle reader, need to call me on something if you think I'm wrong. I encourage all feedback and criticism - it's the only way I learn! - but please do it in a nice way. There's no need for nasty names and swears!
In their sample recipe, Croda suggests you use as high as 10% Polawax in an 80% water, 10% oil lotion and 10% in a 40% water, 50% oil creation. (Click here for the data sheet.) But they suggest 15% for a 75% water, 15% paraffin wax creation. If you look through the Croda formulations for Polawax, at times they follow the 25% rule, other times they throw it out the window. One of the recipes calls for 12% Polawax! EEK! Most of our suppliers will state we should use 2% to 3% to 10% in our creations, but no one ever says "use 25% of the oil phase".
Having said all of this, it's a good rule. When following the 25% (or higher) rule, I've never had a lotion fail on me. (I have had lotions fail with other emulsifiers, and I had one lotion fail because I didn't get the water phase to the correct temperature, but otherwise no lotion fails in my entire lotion making career!) You can always go higher with emulsifiers, but never ever go lower (unless you're doing some experimenting and want to see what happens!).
I hope this answers the question about using Polawax at 25% of your oil phase in your lotions. As with every guideline, there are times to follow it and there are times to go nuts and try something different!