Friday, August 27, 2010
Esters: PEG or polyethylene glycol esters
Point of interest: What we've been taking a look at for the last week are called alkyl esters, just one of the many types of esters! There are a ton of different types - I can't cover them all as this will become the "ester blog", but we'll go over the major categories!
PEG stands for polyethylene glycols or ethylene glycols, which are esters that have undergone a reaction with polyethylene glycol to create an ester that is water soluble and might behave as an emulsifier. This process is called ethoxylation and is an industrial process in which ethylene oxide is added to a fatty acid or fatty acid alcohol, and in the end non-ionic surfactants are produced. (Remember that surfactants are not necessarily foamy or lathery - there are many categories.) Surfactants have a hydrophilic (water loving) head and a lipophilic (oil loving) tail, which means they can help emulsify themselves or other oil based ingredients we might want to include in our water based products.
Generally, a PEG ester is named like this - PEG-# something-ate. The # stands for the number of polyethylene glycol molecules that have been reacted and the something-ate is the fatty acid with which it was reacted. So if we see something like PEG-12 laurate, it means 12 molecules of polyethylene glycol has been reacted with 1 molecule of lauric acid (C12 fatty acid, generally from coconut oil or palm kernel oil). The number is important - as the number increases, the water solubility increases. Something like PEG-8 oleate will be more water soluble than PEG-6 oleate, and PEG-10 would be more water soluble still.
Most of what we see are low ethoxylated esters like PEG-7 olivate, PEG-7 glyceryl cocoate, and PEG-12 laurate. (We will see higher chain lengths in things like PEG-150 glycol distearate when we get into the glycol and glyceryl esters, which tend to be used as thickeners, opacifiers, and emulsifiers.) These PEG esters behave as emollients, moisturizers, anti-irritants or mildness increasers, and can confer slipperiness to things like shampoo, body washes, and bubble baths! Some of them behave as very mild cleansers, which is why adding something like PEG-7 olivate to a make-up remover works so well!
The neat thing about using PEG type esters is the impact they have on our products. They don't hydrolyze in water and don't support mould growth, which makes them a great inclusion in any product where you're using water. (Hydrolysis is one of the mechanisms of rancidity, so having something that doesn't hydrolyze means you are less likely to see rancid oils and icky products!). They also form clear solutions with your surfactants - you won't get that when you emulsify that little bit of oil with something like polysorbate 80.
So when you see something called a "water soluble oil", it's probably a PEG ester (or possibly a PPG ester, which is a polypropylene glycol ester, which we'll be discussing soon) derived from that fatty acid or triglyceride.
There are PEG esters that are called an ethoxylated triglycerides and they're similar to the PEG esters, but they tend to be named things like PEG-20 almond triglycerides. You'll also see things like PEG-7 glyceryl cocoate, which are glycol or glycerol esters, which are created through a different process. And we have things like polysorbate 20 or 80, which are alkyl carbohydrate esters. All of these are considered water soluble surfactants and solubilizers or emulsifiers. We'll be taking a look at these shortly.
Let's take a look at one of favourite PEG esters tomorrow - PEG-7 olivate!
As a final thought, why is it everything I do comes down to surfactants, even when I don't plan on it? I guess I'm just a surfactant junkie!