Life at the interface – Preservative Tales
I don’t know why I feel like writing about preservatives tonight but I do so here’s another one.
One of the many reasons why a ‘one size fits all’ approach to preservative selection is foolish for a company is solubility. Most cosmetic brands have products that range from all oil through to all water with most products fitting in the middle (creams, lotions, spritzers, serums etc). The preservative is put into a product to keep microbes at bay thus keeping the product fresh and clean throughout its shelf life. In order for it to do that it has to be where the action is likely to occur so that it can POUNCE into action like a cat waiting around the corner of the door to grab your leg………….
This ‘pounce readiness’ is linked to solubility.
The place in your formula where you are most vulnerable to micro attack is at the interface.
An interface is the meeting of two surfaces:
1) Air and product.
2) Product and container.
3) Product and finger.
4) Oil phase and water phase.
Point 1 is often addressed by choosing good quality packaging that minimises the time the product is exposed to the air (if the product is high risk)
Point 2 is addressed by having clean containers and a clean product.
Point 3 is actually linked to the volatility (chemistry) of the preservative. This is where essential oils can help but more typically we use things like phenoxyethanol (or phenethylalcohol in organics) to do this job and protect our head space.
Leaving Point 4 to solubility.
The interface between the oil and water phases of a product are where microbes are served up everything they need to survive. Oxygen to breathe from the water, water to drink and food to eat in the way of tasty organic (as in carbon rich) vegetable oils. Many a formulation has failed a PET because the preservative was present but sleeping in the oil phase or too damn comfortable swimming in the water half of the formula.
The ideal preservative for a water and oil containing formula sits nicely at the interface there like a security guard ready to fight and defend.
And that is just one reason why preservatives can mess with your formula so much, crash viscosity and break emulsions – they want to sit with your emulsifiers……
Testing for emulsifier/ preservative compatibility is just one of the things us cosmetic chemists do early on in our new product development work. Testing for preservative efficacy is something that comes a little later, before a product launches but once we are relatively happy with the formula.
I drew you a picture so you didn’t have to just read my waffle. Sorry that I put one too many ‘e’s’ in interface – I got a bit carried away.
Anyway, hopefully you can now see a little more clearly that preservative solubility is actually quite important. I tend to go for preservatives that like oil and water a bit when I make a cream and go for preservatives that favour water when I make a spritz. However, as with all things preservative wise the only way you can tell if things are working is to test and while that does require a slight investment of dollars it is well worth it!
Yay for cosmetic science.
PS: You can be taught this and more exciting stuff about Cosmetic Chemistry by me via New Directions where I run the Cosmetic Chemistry workshops. Please enquire via the NDA website!