this post on solubilizers, someone asked: I still don't get the difference between a solubilizer and a emulsifier. If I mix propylene glycol (in substitution for the surfactant), cetyl alcohol or stearic acid in water, will I be getting a lotion?
The quick answer is no because there's nothing in your question that is an emulsifier. The longer answer is still no, but there's an explanation.
Let's start with the basics. What is the difference between a solubilizer and an emulsifier? For our purposes, we'll categorize our ingredients as solubilizers and emulsifiers. Solubilizers are generally used to incorporate oil based ingredients into a water based products. Ingredients like polysorbate 20, polysorbate 80, Caprol Micro Express, Cromollient SCE, and caprylyl/capryl glucoside would be considered solubilizers. Ingredients like BTMS-50, Polawax, e-wax, Ritamulse SCG, Sucragel AOF, and HLB based emulsifiers are emulsifiers. And emulsifiers actually combine the water and oil together to create an oil in water or water in oil lotion to create an emulsion.
The big difference is this...Solubilizers allow you to create a water based solution by making something that wouldn't normally dissolve in water - like an oil - dissolve into water and create a mixture. If you mixed your oil with something that allowed that oil to dissolve better - like our solubilizers - you would have a toner with some oil based stuff in it that may or may not be clear but would have oil suspended in it, instead of floating on the top of the water.
Emulsifiers allow you to create a product that is water-in-oil or oil-in-water where it's a whole different product entirely. Adding oil and water together isn't something where you have a toner with globs of oil in it, but a product called a lotion. Solubilizers mix in a small amount of oil, whereas emulsifiers can mix in a lot of oil.
When you want to make a lotion, you need what's called an all-in-one emulsifier or you need to create one using the HLB system (more on this in a moment...)
Propylene glycol isn't a surfactant, so I'm not sure of a situation where you'd be using it in the place of a surfactant - and I'm guessing you mean a foaming and lathery one - will end up being a good substitution.
What's a surfactant? It's something that has a hydrophilic or water loving head and a lipophilic or fat loving tail. The hydrophilic head clings onto watery stuff - say the water phase of our lotion - and the lipophilic tail creates a ball around the oily stuff - the oil phase of our lotion. Our emulsifiers are surfactants. I know we generally only think of things that are lathery and foamy as surfactants, but things that bring oil and water together are surfactants.
Stearic acid is an emollient that has an HLB value, which means it needs to be considered as an oil when we're making a lotion. It isn't an emulsifier and can't be combined with anything to become an emulsifier. It's no more emulsifying than coconut oil or olive oil, which is to say it's a full blown oil with no emulsifying properties. Cetyl alcohol isn't an emulsifier either. It's an emollient that has an HLB value, which is to say it's treated like any other oil. (Both of these can help thicken and stabilize an emulsion, but they aren't emulsifiers or solubilizers in any way.)
So to mix propylene glycol - not a surfactant - with stearic acid or cetyl alcohol - neither of these are surfactants nor emulsifiers - you will end up with the water soluble stuff and oil soluble stuff just sitting there as individual things, not a mixed lotion or other product. If we were to add an emulsifier - let's say Ritamulse SCG - we would be able to bring these two things together!
We can also create our own emulsifiers, which is where I think things can get a bit confusing because things that aren't emulsifiers on their own - say, something like glycol distearate or ceteareth-20 - can be combined with something else to create an emulsifier!
hydrophilic-lipophilic balance system that helps us figure out the value of the oil phase of a lotion so we can figure out what emulsifier would work best with it. We don't need to do this with our all-in-one emulsifiers - like Polawax, Ritamulse, Incroquat BTMS-50, e-wax, and so on - because they've done all that work for us. But if you wanted to create your own emulsifier, this is what you'd use. When something has an HLB required value, that means it's part of the oil phase and not an emulsifier. Some things can be combined with other things to create emulsifiers, ingredients that aren't emulsifiers on their own, and those things can be found in the HLB table as potential emulsifiers. You'd combine one low HLB emulsifier with one high HLB emulsifier to create something awesome. For instance, you could combine glycol distearate with polysorbate 80 to create an emulsifier, but glycol distearate on its own isn't an emulsifier, nor is polysorbate 80.
So let's say you wanted to incorporate an oil soluble thing into a water based product. I like my toner, but I want some rice bran oil in there. I'd choose a suitable solubilizer - polysorbate 80 is a good choice - and I'd mix it with the oil based thing, then add it to the product. You'd still have a toner, but now it's a toner with oil dissolved into it.
If I wanted to make this product into a lotion, I'd use an emulsifier to incorporate a larger amount of oil into the water based product and end up with a completely different product, a lotion instead of a toner with rice bran oil dissolved in it.
Now here's a question - why are lotions white when the initial ingredients are generally clear? That's a question for another Chemistry Thursday!