Monday, August 9, 2010

Spreadability, tackiness, and greasiness.

How many times have you heard a lotion, cream, or oil being described as "draggy" on our skin? This word is generally used to indicate there's some difficulty in spreading the lotion or cream on our skin. This has actually been studied and is considered a "sensory characteristic" of bath & body products, along with time to be absorbed, greasiness, and tackiness. Interestingly enough, there doesn't seem to be a general relationship between the physical characteristics of an oil and the sensory properties - at least not one we can say works with every single emollient ingredient.

As a general rule, the spreadability values of a lipid decreases with an increase in the viscosity of an oil and an increase in surface tension. (In this case, lipid means anything we'd think of as an oil, butter, or ester). And the spreadability of an oil correlates with our subjective experience of absorption into our epidermis. So the lower the viscosity of an oil and the lower the surface tension, the more we feel it absorbs into our skin.

Think about the difference in feeling between fractionated coconut oil and shea butter. Fractionated coconut oil is very thin, almost watery, and shea butter is a thick buttery substance. We always see descriptions of shea butter as occlusive or thick or rich, whereas fractionated coconut oil is considered non-greasy and light.

I realize I've just stated the obvious - shea butter has high viscosity and it feels thick, fractionated coconut oil has low viscosity and feels light but that's kind of the point of this post, so please excuse what might seem really elementary! 

Ever had a tacky feeling lotion? Tackiness increases with the surface tension of a lipid...

Definition of surface tension: Surface tension is a measurement of the cohesive energy present at an interface. The molecules of a liquid attract each other. The interactions of a molecule in the bulk of a liquid are balanced by an equal attractive force in all directions.

Viscosity is measured in milli-Newtons per metre (nM/m) and surface tension is measured in milli-Pascals (mPa). The lower the numbers, the lower the viscosity or surface tension. So something like avocado oil with a viscosity of 64.3 and a surface tension of 32.1 will feel thicker, tackier, and spread less easily than cyclomethicone with a viscosity of 3.8 and a surface tension of 18.3, which is considered to have "very good" spreadability.

To give you some perspective on just how awesome esters are at spreading over your skin, fractionated coconut oil with a viscosity of 23.7 and surface tension of 29.4 is considered "very poor"! (And this  is one of the oils I consider really really spreadable and light!) IPM with a viscosity of 4.6 and a surface tension of 28.4 is considered "very good" whereas C12-15 alkyl benzoate with a viscosity of 11.8 and a surface tension of 31.6 is considered to be a "very poor" spreader in the world of esters! Virtually every ester you see will have better spreading abilities than one of the lightest oils we can buy!

How do we decrease the viscosity of an oil? Well, we don't - at least not in our workshops. We can't change the viscosity of something like avocado oil but we can buy it in the form of avocado butter (which will increase the viscosity) or something like an ester of avocado oil (which decreases the viscosity).

We can increase and decrease the viscosity of a product like a lotion or body wash by adding more water, increasing or decreasing the thickeners, changing the butters and oils, and so on, but we can't change the viscosity of something like straight avocado oil! 

How do we decrease the surface tension? Surfactants - they're called surface active agents for a reason! By solubilizing or emulsifying the lipids, we decrease the surface tension, which decreases the feelings of tackiness and greasiness and increases the feelings of absorption and spreadability!

If you applied straight avocado oil to your skin, you'd find it more difficult to spread and far more greasy feeling than you would in a lotion or cream containing emulsifiers. Esters tend to be solubilizers for other oils, so when we add them to our creations - say, in the form of something like IPM - they will make the other oils feel less greasy (as demonstrated in this body spray).

To summarize: Esters tend to be lower viscosity, lower surface tension lipids compared to oils, which is why they feel less greasy, spread more easily, and tend to feel less tacky than natural oils and butters.

Join me tomorrow for more fun with esters!

1 comment:

Maria said...

Dear Susan,
I'm having an hard time to find spreading values of some oils I like to use in my formulas in order to get a sensorial cascading effect.
These oils are jojoba, rice bran, grapeseed, rose hip...
I've been cosidering to try Ziedler's procedure but it's quite impossible at home.
Can you be of any help?
Thank you from Rome.