Article by: Perry Romanowski
Not everyone who formulates cosmetics received a degree in chemistry. In fact, most home formulators have very little chemistry training at all. While having a degree in chemistry is not required to be an excellent formulator, there are some chemistry concepts that can make your formulating efforts much easier.
In this post, we are going to talk about Solubility and how it impacts your formulating efforts.
Definition: The property of a raw material to dissolve in another substance.
Typically, when people talk about solubility they mean the tendency for a raw material to dissolve in water. In reality, all materials are soluble in water, they just vary in the amount that can be dissolved. But from a practical standpoint when only a tiny amount of a substance (less than 0.1 g in 100 mL of water) will dissolve we consider it “insoluble.” Most oils and silicones are insoluble in water which means that when you mix them, they separate into different layers.
Materials that are soluble in water will dissolve to a much greater extent. These include things like salts, sugars, and alcohol. The degree to which something is soluble depends on numerous factors related to the raw material and the characteristics of the system.
Why cosmetic formulators need to know
Solubility is an important concept to understand because it will allow you to more quickly know which materials can be used together as is and which will need emulsifiers to make them compatible. Basically, if you know the solubility characteristics of a material you will be a faster, better formulator.
Factors that affect solubility
As we said, there are a number of factors that affect solubility. These include the molecular structure of a raw material, the concentration, the temperature and the pressure.
Molecular structure – The molecular bonds within a substance has a big influence on how much of a substance will dissolve in another. The general rule is that “like dissolves like.” Therefore, since water is a polar molecule other molecules that are polar will readily dissolve. Salt crystals are composed of ionic bonds so they dissolve and sugar is made up of polar bonds so it dissolves. Oils are made up of nonpolar bonds so they don’t tend to dissolve in water.
Here are some simple rules for cosmetic formulators.
1. Raw materials with sodium, potassium, and ammonium will be soluble
2. Nitrates, chlorates, perchlorates, and acetates will be soluble
3. Chlorides and Bromides will be soluble
4. Most sulfates are soluble
5. Hydroxide materials are insoluble except those containing sodium, calcium & barium
6. Non-polar materials like oils and silicones are insoluble
Temperature – As you can imagine, temperature affects the degree to which a material will be soluble. Typically, if you heat a solid or liquid up, it will become more soluble. This is why most formulations are heated. It just makes things go faster.
Pressure – This has a big impact on solubility but only for gases. It has very little impact on solids and liquids so as a formulator, you don’t have to think about it much. Unless you are making aerosols of course.
For more on solubility see this lecture by the Khan Academy