Let's take a look at solubility for a moment before we take a look at the ingredients we call solubilizers. (Originally from this post. Click on it for more detail.)
Solubility is a pretty important part of making bath & body products, but we don't talk about it much. In essence, solubility is the ability of a solid, liquid, or gas (the solute) to dissolve in a liquid solvent to create a homogeneous solution.
Solute - the thing we're dissolving into the solvent.
Solvent - the thing into which we're dissolving things. It's usually a liquid for bath & body products.
Homogenous solution - a mixture that shows no variation in its properties, like Kool-Aid or salt dissolved into water. You won't see any large particles floating in the water. (I always remember this by thinking about homogenous milk. There aren't any large milk particles in milk!)
Precipitate - a solid that can happen when we add too much of something or have the wrong temperature.
Solublity is a measure of how well the thing we want to dissolve will dissolve in the appropriate liquid.
The solubility of the solute depends upon the type of solvent, temperature, and pressure. For instance, we can easily dissolve salt into water, but it's hard to dissolve in oil. Allantoin has a solubility of 0.5% at 25˚C, which means we can dissolve 0.5% allantoin into 100% water when the temperature of the water is at 25˚C or higher. If we try it at 5˚C (cold water), the solubility is reduced, meaning we won't be able to dissolve 0.5% in the water. (This is one of the reasons we heat and hold. A warm solvent will allow for more stuff to be dissolved in it than a cold solvent.)
As an aside, almost everything dissolves better when we increase the temperature of the solvent. Except for carbon dioxide into water, which is one of the reasons we keep pop in the fridge to remain fizzy!
There's an idea call the SATP or standard ambient temperature, and pressure. It can differ slightly from textbook to textbook, instructor to instructor, but the one you'll probably see most is that solubility is determined when at 25˚C and 0.986 atmospheric pressure (sea level).
If something like allantoin is judged to be 0.5% at 25˚C, will it precipitate when the temperature drops below 25˚C? Yes, but only a little. 20˚C is room temperature, so you won't notice a huge difference in the product. If you put it into the freezer, you might see shards of allantoin - the precipitate or solid - in the product. Not a good thing! Check out this post on slight changes in temperature and how it affects our products.
If two things are immiscible, it means they will not mix or dissolve well (like oil and water without an emulsifier). Some things are very soluble - salts, for instance - and some things are kinda soluble, meaning they aren't completely soluble or need some help to be soluble, like an increase in heat or some serious stirring. If we add too much of a very soluble thing or take something that isn't completely soluble and add more of it or fail to raise the temperature, we get a precipitate, which is a solid of the solute that can fall to the bottom of our creation. We see this kind of thing when adding too much salt to water (the stuff on the bottom is the precipitate).
This is one of the reasons we can make sugar or salt scrub in oil, but wouldn't be able to do that well with a water based thing - say a gel or surfactant mix. Sugar and salt dissolve very very well in water based things, but not well in oil based things. If you wanted to make a surfactant based scrub, you might consider using something like jojoba beads, which are oil soluble. They won't dissolve in the water based surfactants!
A rule of thumb is "like dissolves like". Water dissolves water soluble things; oil dissolves oil soluble things. Water is polar; oil is non-polar. Alcohol is also a great solvent, as is propylene glycol, glycerin, and the other polyalcohol humectants. Check the solubility of your ingredient and you'll be able to figure out whether it goes into the water or oil phases of the product.
When something is dispersible, it means it will stay suspended in the mixture, but it will always be separate and won't form a homogeneous solution. When we use fragrance or essential oils in something like a body wash, the oils aren't dissolved but suspended. We don't notice a huge difference unless those oils separate out and form an oily mass on top of the body wash, which can happen if you use too much! So if something is a disperser, it means it keeps the ingredients suspended in the product and you won't get a homogeneous solution. You might not notice this - you don't notice a water soluble ester in a body wash because it looks like a homogeneous solution, but there might be tiny little fat molecules that are separate from the actual solution.
For our purposes, we'll categorize our ingredients as solubilizers and emulsifiers. Solubilizers are generally used to incorporate oil based ingredient into a water based product. 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.
Join me tomorrow as we take a look at caprylyl/capryl glucoside, an Ecocert non-ionic solubilizer!