DEFINING SENSORY CHARACTERISTICS OF OUR INGREDIENTS
Why is the cosmetic formulator concerned with solubility, viscosity, surface tension, contact angles, wetting, refractive index, specific gravity, spreadability, play time, cushion, drag, and melting point?
ENTRIES FOR SOME RHEOLOGY MODIFIERS – see the complete list by clicking here!
Contact angle: “The contact angle is defined as the angle the water makes with the surface. In this illustration, a low contact angle (θ<90 ̊) means the surface is hydrophilic. Large contact angles (θ>90 ̊) lead to a surface that is hydrophobic.” (Reference, PEG/PPG PDF) Or, more simply, “The angle where a liquid or vapour interacts with a solid surface”. (Reference) This is important as it can determine if something is hydrophilic and water loving or hydrophobic and water hating.
Cushion: The thickness of the film on our skin. “Cushion is related to the viscosity of the liquid, the volatility of the liquid, the surface tension of the liquid, and the tendency of the liquid to be absorbed into the skin.” Volatile liquids are those that evaporate, like cyclomethicone or alcohol, which tend to be thinner ingredients and provide less cushion.
Lubricity or calling something “lubricious”: This means something feels smooth, moisturizing, emollient, and not waxy, draggy, thin, or too greasy. One textbook defined smooth as “slippy, slippery, velvet, flowing”, and moisturizing as “oily, wet, heavy, moist”. (Reference, Formulas, Ingredients, and Production of Cosmetics) In general, it means an ingredient that feels rich and emollient, so thinner ingredients, like fractionated coconut oil or isopropyl myristate, wouldn’t be considered lubricious. Our liquid oils are considered lubricious, while solid oils with shorter fatty acid chains and some butters are considered “coarse”. Waxes are considered draggy and “adhesive”.
Surfactants can be considered to be lubricious if they feel rich and creamy, like the elegant foam and lather we get from sodium cocoyl isethionate (SCI) when compared to foaming silk protein, which has a lacy glove lather.
Melt point or melting point: This is the temperature at which something melts (obvious, right?) This is an important consideration when formulating to make sure things like lotion bars or whipped butters won’t melt in a purse, steamy bathroom, or hot car. It’s also important when it comes to application on the skin. If the ingredient has a melting point at lower than skin temperature, like babassu oil, it will melt quickly when applied to the skin. If it has a melting point higher than skin temperature, like cetyl alcohol at 49˚C, it won’t. How do we figure out the melting point of a product? We have to determine that experimentally, by actually seeing when it melts. (More in this post…)
Play time: How long it takes an ingredient or product to spread on our skin then disappear.
Spreadability: How well an ingredient or product spreads on our skin when applied, then rubbed. In general, the heavier the emollient, the harder it is to spread.
Surface tension: (Link) “The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface…form a surface “film” which makes it more difficult to move an object through the surface than to move it when it is completely submersed.” (Reference)”In even simpler terms, it measures how much force it takes to keep a liquid together.” (Reference)
Viscosity: (Link) The easy way to think about viscosity is about the thickness of a fluid. Water is thin, so it’s low viscosity, while glycerin is thick and has higher viscosity. I liked this definition that “Viscosity is resistance to flow” (reference), that more viscous things require more force to make them flow. So something like glycerin takes more force to make it flow when compared to water. You may see some things measured in centistokes or cS. (1 cS = 1 mPa.s) Silicones are measured this way, especially dimethicone. I’ve worked with 5 cS, 350 cS, and 1000 cS, which you might be able to picture as being as almost as thin as water, thicker than olive oil, and less thick than glycerin respectively.