You can read more about these concepts and terms in the chemistry section of the blog.
Amphoteric: This refers to surfactants that have different electrical charges at different pH levels. Cocamidopropyl hydroxysultaine has a positive or cationic charge at pH 6 or lower, and an anionic or negative charge at pH 6 and higher.
Anionic: (Link) This means something is negatively charged. Most of the surfactants that have “sodium” or “ammonium” in the name will be negatively charged.
Cationic: (Link) This means something is positively charged. Hair conditioners are positively charged because our hair and skin are negatively charged, so the cationic ingredient adsorbs to the hair strand in a process called substantivity. We use cationic polymers or positively charged ingredients like Honeyquat or various polyquaterniums in hair and skin care products to condition our skin.
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.
Interface: (Link) Where something meets something else. For instance, where a liquid meets air, like a shampoo in a bottle or a glass of water. Or where a solid meets the air. Or where oil and water might meet in a lotion. An emulsifier gets into that at that interface to create an emulsion. (Click here for a longer post.)
Interfacial tension: (Link) The tension that’s happening between two things – air and water, water and oil, water and silicone – and so on. Reducing that tension means we need less energy to mix the two things and, if we’ve done it right, reducing the amount of energy required to remain in that mixed states.
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…)
Newtonian fluid (Wikipedia): This is a fluid in which the “viscosity remains constant, no matter the amount of shear applied for a constant temperature. These fluids have a linear relationship between viscosity and shear stress.” (Reference)
Non-ionic: (Link) This means something has a neutral electrical charge. Most of our emulsifiers – Polawax, e-wax, Montanov 68, Simulgreen 18-2, Olivem 1000 – and solubilizers – polysorbate 20 or 80, caprylyl/capryl glucoside, PEG-40 hydrogenated castor oil – are non-ionic, as are all our oils, butters, fatty alcohols, and fatty acids.
Non-Newtonian fluid: This is one in which the viscosity does not remain constant when shear stress is applied, meaning the viscosity is dependent on the shear stress. These will have a yield stress point or a measurable amount of stress the system needs before it can start flowing.Shear stress is the force required to make the layers of fluids move. The yield stress point is the point we need to reach so the fluids start moving. So a non-Newtonian fluid is one in which the viscosity changes when the force required to make the fluids move is reached. What happens at that point is what defines a few different types of these fluids.
Refractive index: (Link) All materials reflect and refract light, which is to say all materials alter the angle of light. The higher the refractive index, the higher the light intensity reflected from the surface. Titanium dioxide has a refractive index of 2.7, one of the highest of all materials, and diamonds are about 2.419 – and we know how sparkly they can be! – whereas water is around 1.33. This is super relevant in hair care products. If you want shine to your hair, you’ll want to choose ingredients with higher refractive index levels. Dimethicone has a higher level than cyclomethicone, so adding more dimethicone to an anti-frizz spray will increase the shine on your hair.
Rheology: (Link) “The science of how things flow.” (RM). So rheology modifiers are ingredients that can change how fluids flow, altering the viscosity of a surfactant based product, like bubble bath, body wash, shampoo, or cleanser, or an emulsified product, like a lotion, cream, or moisturizer. You can also alter the viscosity of an anhydrous product, like those made with silicones, like a primer, anti-frizz spray, or facial serum, or oils, like a body oil, facial serum, or balm.
Shear: (Link) It’s defined as the “relative motion between adjacent layers of moving liquid”. As above, “Shearing occurs whenever the fluid is physically moved or distributed, as in pouring, spreading, spraying, mixing, etc.” A high shear mixer – like an immersion or stick blender – will brings those layers together quicker than something that’s low shear, like a hand mixer. High shear mixers spin quicker in the middle than the outsides.
Shear stress: “Shearing stress is a force that causes layers or parts to slide upon each other in opposite directions.” (Reference) If shear is the “relative motion between adjacent layers of moving liquid” and “shearing occurs when the fluid is physically moved”, then shear stress is the force that causes those layers of liquid to move. When we use a high shear mixer, we are using that mixer to apply shear stress to the product.
Shear thinning fluid: (Link) If the viscosity decreases when shear stress is applied, this is a shear thinning fluid. These fluids are also called pseudoplastic. These fluids return to their original viscosity pretty quickly after being annoyed in some way.
Specific gravity: This is a measurement of how many grams per millilitre something measures. 1 ml of water is one gram, which is our baseline. You may also see this as 1 cc or cubic centimetre. So 1 ml or 1 cc of water weighs 1 gram. This means that 5 ml or 1 teaspoon of water = 5 gram. (For a longer post, click here.)
If something is 1.03, it means 1 ml weighs 1.03 grams. If something is 1.35 it means 1 ml weighs 1.35 grams. If something is 0.90, it means 1 ml weighs 0.90 grams. Our oils are lighter than water, so if you have 10 ml of oils, you have 9 grams of oil. This means if you want 60 ml anhydrous facial serum, you need to use 54 grams of oil as 60 grams will be too much! This can mess you up when you’re teaching a class in which you’re making a 60 gram anhydrous facial serum that goes into a 60 ml bottle. 60 grams of oil.
Solubility: (Link or link) This just means how one thing mixes with something else. Something can be soluble, so it mixes completely, like salt dissolving in water, or it can be insoluble, meaning the two things won’t mix, like oil and water. There are limits on solubility. Sodium chloride or table salt dissolves well in water, but you’ll reach a point where you have too much salt and no more can dissolve. The stuff that didn’t dissolve is called the precipitate.
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)
Terpenes: (Link) These are organic compounds, the major building blocks within nearly every living creature. For instance, steroids are derivatives of the triterpene squalene. (Quote from Wikipedia .) They are derived from units of isoprene (that’s the picture up there, with a formula of C5H8) called the isoprene rule or C5 rule. Terpenes are created out of multiples of those isoprene units. When they are modified chemically through oxidation or rearrangement of the carbon skeleton, they becometerpenoids or isoprenoids (terpenoids are not the same as terpenes as they have been modified in some way, but some people group them together.) These can be cyclical or linear. Terpenes are everywhere in our products, especially in oils and essential oils.
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.
Wetting: (Link) This is about how a material coats or covers a surface.
Yield stress: (Link) This is the “applied stress we must exceed in order to make a structured fluid flow.” In other words, this is what we have to do to get the fluid moving, which could be squeezing, pounding, tapping, or shaking a bottle; pressing a pump; scooping it from a jar; and so on.
Yield stress point: (Link) “Minimum stress value applied to inital flow.” When the fluid is below this point, it displays the properties of being a solid or in that it isn’t flowing the way water might flow. When it’s above it, it’ll flow like a liquid. (Reference) In other words, this is the point we need to reach to get the fluid moving. We could measure this by saying something like, “It takes four taps on the table to get the ketchup out of that glass bottle.”