Water in Oil Emulsions

by Belinda Carli B Nat Ther., Dip Cos Sci

Why go water in oil?

There has been a resurgence of interest in water in oil (w/o) emulsions because of their innate ability to provide better moisture protection than most standard oil in water (o/w) emulsions. In the past, w/o emulsions lost favour because they either needed to be created
using mostly silicones, silicone derivatives or mineral oils to feel nice; otherwise they would leave a heavy, greasy residue when created with natural materials. However, with the ever-increasing choice of naturally derived esters, w/o emulsifiers and w/o emulsion stabilisers, more elegant emulsions can now be created with a relatively high natural ingredient content.

How are w/o emulsions created?

A water in oil emulsion is one where the continuous phase is oil or lipid based and the internal (dispersed) phase is water. In order to create and stabilise a w/o emulsion:

  • Emulsifiers need to be selected that favour the ‘capture’ of water droplets and suspend them within the continuous oil/lipid phase. Suitable emulsifiers form chemical ‘stick-like’ barriers with the larger portion protruding into the oil/lipid phase to prevent water droplets from getting too close.
  • Polymers need to be compatible with the oil phase so that they create their stabilising network throughout the oil phase to prevent the water droplets from coming too close together. The compatible polymer forms a physical ‘net’ like barrier.

Here is a diagram to illustrate how this works – picture that the white space around the droplets is oil; the blue droplets are water; the ‘sticks’ are the correctly selected emulsifiers and the blue lines are the correctly selected polymer:

water-in-oil-emulsion

Not all oils are equal
To stabilise these types of emulsions, emulsifiers must have a low HLB value (hydrophile/lipophile balance), ideally around 3-5. For those of you who aren’t familiar with this terminology, look for emulsifiers and blends that specifically state they are good to create w/o emulsions – this takes the guess work out of selection for you.What emulsifiers and polymers are suited to w/o emulsions?

The right type of polymer will be ‘oil compatible’ or ‘oil soluble’. AVOID water soluble/compatible polymers like xanthan gum and carbomers – they are the worst possible choice and will pull water droplets together leading to rapid destabilisation!

When selecting materials to stabilise these emulsions, use:

  • A low HLB emulsifier blend; for example, Arlacell 1690, at 0.5 – 5.0%. This material is also approved by Ecocert, making it a very green choice.
  • A low HLB emulsifier ‘waxy’ material; for example, Glyceryl Stearate or Beeswax – it will help build viscosity to your product. Include this material in your formula from 2.0 – 5.0% depending on how viscous you need the end product to be. Product must be heated when using this material.
  • An oil compatible polymer, such as OleoCraft MP-32, at 1-5%. Product must be heated when using this material.

You may also like to consider lecithin, as it is also well suited to these types of formulations and imparts a beautifully cushiony skin feel. You can this in w/o emulsions between 1 – 5% for aesthetic purposes but it is not sufficient on its own to be the main emulsifiers.

To make your emulsion: combine your oil phase materials, emulsifiers and polymer together and heat until all melted; add equally hot water to this phase; and then homogenise. Allow to cool and then add suitable preservatives, antioxidants and other extras (fragrance, extracts etc) as required.

Not all oils are equal

Some final points to note about your oil/lipid selection when creating these types of formulas:

  • While silicones and silicone derivatives will provide a really light skin feel, you can also use a variety of light esters with similarly suitable aesthetic results – the fun is in experimenting with these materials until you get the skin feel just right!
  • Your formulas will typically contain 25% water or maybe a little less – so the selection of your lipids is crucial to get the right skin feel as they will make up the majority of your formulation.
  • Because the formula is mostly oil/lipid, it is very difficult to test and adjust pH as the low water content means the product won’t carry charge – in many cases, pH adjustment is not necessary even if you can test for it.

Finally, don’t be fooled into thinking that these products don’t contain enough water to need preservation – they most certainly do! Make sure you are selecting a broad spectrum preservative that is compatible over a broad pH range for the most effective protection.

Now, enjoy the fantastic moisture protection these products can provide while you play with the sensorial aspects for best results…

Happy formulating!

Belinda Carli is the Director of the Institute of Personal Care Science (IPCS). IPCS provides distance training from short course through to Certificate and Diploma levels in Cosmetic Science, Formulation, Brand Management and Regulatory Affairs. Contact Belinda and the team for assistance with your training requirements: belinda@personalcarescience.com.au or visit www.personalcarescience.com.au for more information.

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