From Wikipedia) Esters are chemical compounds derived by reacting an oxoacid (one containing an oxo group, X=O) with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group, and most commonly from carboxylic acids and alcohols.
If you'd like to know more about esters in organic chemistry, check out this great page!
So what does this mean to us? What is means is that esters can be derived from a carboxylic acid (which is an acid that contains oxygen) with a hydroxyl compound (one that contains an oxygen atom bonded to a hydrogen group). The carboxylic acid can be a fatty acid like oleic acid (C18:1) or stearic acid (C18) or any of the fatty acids we find in our oils. The hydroxyl group differs depending upon how the ester is processed. (If you want to learn more about how esters are processed, click here.) It can be an alcohol like ethanol or a fatty alcohol like cetyl alcohol or sodium hydroxide (lye, NaOH).
You can figure out what kind of ester you have by analyzing the name. The first part - the ethyl in the picture above - comes from the carboxylic acid. The second part - the ethanoate - comes from the adding -oate to the name of the hydroxyl chain. (The hydroxyl chain here could be ethanol, hence the eth- part.) So you can determine that you have an ester of the carboxylic acid ethanoic acid (also called acetic acid, what you find in vinegar) and an ethyl group. (As a note, eth- in organic chemistry means 2, meaning two carbon atoms in the chain. If you see something like methane, the meth- part means 1, so there's one carbon atom in the chain.)
To relate this to the esters we'll be seeing in this series of posts, if you have cetearyl ethylhexanoate, then you can determine this was derived from the cetearyl fatty acid. Cetyl is also known as palmityl alcohol and palm- means 16 carbon atoms. The prefix stear- means 18 carbon atoms, like in stearic acid. The ethylhex- part means it has 8 carbon atoms in the hydroxyl compound and the -oate part is how you know it's an ester.
As a note, cetearyl alcohol is a combination of stearic and cetyl alcohol - C18 and 16 fatty alcohol chains - so it's two great things in one! I probably should have picked a better example here.
IPM, which doesn't follow this naming convention (more about this below). The myristate part indicates comes from myristic acid (C14). The propyl part means the hydroxyl compound had three carbon atoms (prop- means 3) and the iso- part means the propyl part was an isomer of the propyl (an isomer means it has the same chemical formula - for instance, -C3H7 - but it has a different structural formula, like being attached to a different carbon in the chain).
Did I mention there'd be a test? Well, there is. From which fatty acid is isopropyl palmitate derived? (Answer at the bottom of this post.)
It gets confusing naming things when we're working with cosmetic ingredients because there's one official naming convention for chemistry (IUPAC), but a lot of the names we see are what are called trivial names. For instance, we call it oleic acid but the proper name is (9Z)-Octadec-9-enoic acid. We might call it cetyl alcohol, but the proper name is really 1-Hexadecanol.
A lot of misinformation comes out of the difference between trivial and official names. For instance, you'll see people saying that something like isopropyl alcohol comes from propane because of the prop- part of the word. Or associating things with other things because of the trivial name, like people avoiding cetyl alcohol because they don't want to use alcohol (ethanol) in their products.
So what's the big deal about esters? Well, everything! Esters show up a lot in our bath & body products - they're fragrance and flavour oils when the carboxylic acid is combined with an aromatic ring (the one to the left smells like strawberries), they're thickeners like Crothix or glycol distearate, they're water soluble oils like PEG-7 olivate, they're emulsifiers like polysorbate 20 or 80, and they're emollients like my new favourites cetearyl ethylhexanoate and C12-15 alkyl benzoate, and my old favourite isopropyl myristate (or IPM).
As a point of interest, yes, polyester is an ester and esters are found in our bodies in fat as well in bee pheromones. In fact, the name ester refers to their odour, and we find it in anything in nature with a fragrance like fruit and flowers. So they can range from the incredibly natural to the incredibly synthetic.
I've already written about the basics of esters - as evidenced by the links above - but I will be going into more detail in this series. If you simply can't wait, click away!
If esters are derived from fatty acids found in our oils and butters, then why bother using esters?
- Esters tend to have longer shelf lives - 24 months or more - because they don't have the free fatty acids or unsaturated acids that can go rancid in something like olive or grapeseed oil.
- They tend to feel drier than oils or butters, and they can be added to surfactant mixes without having a huge impact on the foam and lather. If you've used an ester as the basis of a product you'll know they feel quite emollient and spread easily, but have a much less greasy feel. (More about this tomorrow...)
- They can also be water soluble (like the PEG-7 olivate), but the solubility goes down as the chain length increases (something like isopropyl myristate or IPM with 14 carbons in the chain will not be water soluble).
- They can solubilize other oil based ingredients in water based products (polysorbate 20 or 80) or they can mix with water based products without separating. This is because they are polar, unlike oils which are non-polar. (More about this in the post on solubility). This means that some esters can be added to water soluble products like body washes or shampoos without the need for an emulsifier.
- Esters can solubilize other oils in oil based products. The reason the complicated balm feels very dry is thanks to the esters: Adding esters to other oils makes them feel less greasy and easier to spread on your skin. (More on this tomorrow and throughout this series!)
- Esters are consistent. If you've ever wondered why commercial cosmetic products tend to have things like mineral oil or C12-15 alkyl benzoate (an ester) instead of olive oil or shea butter, this is the answer. Because there's a specific way for each ester to be processed and a standard of what each ester should be, you're guaranteed every time you buy an ester, it's the same as the last bottle. Natural oils see more variation based on crop, season, weather changes, soil, fatty acid composition, free fatty acids, and so on, which makes it harder to create a standardized product every time. I'm sure you've noticed the same lotion might be thinner or thicker or greasier than the last batch. By using esters, you are ensuring the product is the same every time. Not a huge issue with most homecrafters, but if you're selling your products, standardization is a bonus!
Oh, I almost forget - isopropyl palmitate is derived from palmitic acid (C16). Give yourself a cookie if you got it right. And if you got it wrong - go get a cookie anyway! (Never deny yourself a cookie!)