Salt (Sodium Chloride. Chemical name NaCl) actually contains ions that can impact our formulas. You may be familiar with ions when it comes to the 4 different surfactants.
Anionic - which are most commonly cleansing surfactants, but Ritamulse SCG / EcoMulse is anionic and so is Xanthan Gum, so there are always exceptions, I can just give general statements. Anionic surfactants have a water loving head with a negatively charged ion and typically foaming anionic surfactants are the ones t
hat give the best foam.
Cationic - have a water loving head with a positive charge ion (I like to remember that cationics are positive because they start with the word cat and cats are a plus in my book! lol) typically cationic surfactants are conditioning agents and since they have a positive charge they are not compatible with anionic surfactants. But once again, there are exceptions to this rule. There are some cationic surfactants like honeyquat that are okay to use with anionic surfactants. So again, always read the formulating guidelines to all of your ingredients.
Amphoteric - They have both a positive and a negative charge in their water loving head depending on the pH. In acidic solutions the positive charge is dominant. In alkaline solutions the negative charge is dominant.
Non-Ionic - as the name suggests, have no ionic charge in their water loving head. (Contain no ions)
We are all probably familiar with working with these ionic ingredients and know that they play a big impact on your formulas when it comes to stability, performance, feel, viscosity, etc. But Salt (Sodium Chloride) also plays a big role in cosmetic formulation.
To truly understand electrolytes you need to understand the chemistry part of it and I’m not a chemist, so I will do my best to explain. If you have anything to add to this post or anything to correct, just let me know. I will be giving my understanding of electrolytes when it comes to formulating, as a none professional.
What are Electrolytes?
The reason I think electrolytes are so important to understand is because they can affect the stability, viscosity, and overall performance of a product. Often times we run across ingredients that say they are “sensitive to electrolytes.”
What does that actually mean when an ingredient is sensitive to electrolytes?
The most common electrolyte in cosmetics is sodium chloride. Sodium chloride is salt, but it’s not just actual salt that is an electrolyte. Other raw materials and ingredients actually contain salt as a chemical component. If you see ‘Sodium’ in the name of an ingredient or in the INCI name, that means this ingredient is an electrolyte. Sodium Lauryl Sulfate, Sodium C14-15 Olefin Sulfonate, Sodium Cocoyl Isethionate, Sodium Laurel Ether Sulfate, Sodium Lactate, Sodium PCA, Sodium Phytate, ETC. Not all surfactants are electrolytes though. For example, coco glucoside is not an electrolyte.
So when you are reading the formulating guidelines to an ingredient and it says “sensitive to electrolytes” this means that any ingredient with sodium in the name (or INCI name) will have a negative impact on the formula. Which means it can destabilize the formula. For example Aristoflex® AVC is sensitive to electrolytes and in the formulating guidelines it says the following “is sensitive to electrolytes and is not suitable for use in certain surfactant systems (such as cleansing products) which may contain high levels of salts.” This means we can not use Aristoflex® AVC to thicken a foaming surfactant system made with these “sodium” based surfactants. You also can’t use sodium lactate as a humectant as it will most likely thin out the product. Of course there may be some exceptions with some ingredients, so this is why experimenting is always important. Remember that formulating is a science and a part of science is experimenting. Don’t be scared to try knew things and don’t be scared to fail. Science is all about experimenting and learning from failures. You can learn something knew with each failure.
Sodium in Foaming Surfactant Systems
Another reason electrolytes are so important to understand when it comes to formulating is because they can have an effect towards surfactants and their micelles. Specifically I’m
talking about salt interacting with an anionic foaming surfactant. (Anionic surfactants have a water loving head with a negatively charged ion)
If you aren't familiar with micelles, you should go watch my video I just posted here on Patreon titled What are Surfactants & Micelles - Chemistry of Surfactants.
As mentioned in that video, when you pour a surfactant into water the oil loving tails try to escape the water. (See figure to the left)
Once you add so much surfactant that the oil loving tails have no more room to escape they form a micelle by sticking all their oil loving tails together to form a ball shape. This is called the critical micelle concentration (CMC). (See figure to the right)
If you keep adding surfactant to that water more and more micelles will form and they will also get larger.
But remember, like repels like. All of these micelles have their negatively charged water loving heads sticking out. So all of these micelles will want to push away from each other. Imagine when you try to put two magnets together, it doesn’t work. This is basically the same thing happening to these micelles. (see figure to left)
So in order to combat this we will need to increase the ionic strength of the water in the solution. We can do this by adding in just ordinary table salt (Sodium Chloride). Adding the salt actually allows the micelles to come closer by increasing in size, thus increasing the viscosity of your product. (see figure to right)
Keep in mind, this is in regards to those “Sodium” containing foaming surfactants. Not foaming surfactants like coco glucoside. It needs to have ‘Sodium’ in the INCI name for it to be impacted by salt.
It is also important to note that there is a limit to how much salt you can add. If you add too much, your formula will actually lose its viscosity. This is because micelles are constantly breaking down and reforming in our formulas. If you add too much salt then the micelles will breakdown way too quickly and not reform quick enough, thus causing the product to lose it’s viscosity. The large micelles are what’s creating the thickness in the product. So if they are breaking way to quickly and not reforming fast enough the viscosity will be lost. This is called the “salt curve”
You want to make sure you are on the left side of the salt curve, meaning, don’t use the highest amount of salt possible because it will be way to easy to accidentally add in too much. (see figure to right)
If you want to read about how to do your own salt curve test with your surfactant system read here. Also, let me know if you would like to see me do this “salt curve test” as a patreon exclusive. It sounds like it would be a fun experiment to share.
Sodium in Emulsions (Specifically Oil in water Emulsions)
Unlike the foaming anionic surfactant systems that thicken from sodium. Sodium can sometimes decrease the viscosity or destabilize oil in water emulsions. Oil in water emulsions are the type of emulsions most of us are making. This is when oil is the internal phase being dispersed into water. Recently I had a patron ask me about why their emulsion failed. They were using GelMaker NAT as their cold process emulsifier and they were using sodium lactate as a humectant. GelMaker NAT (INCI; sodium acrylate/sodium acryloyldimethyl taurate copolymer, C15-19 alkane, polyglyceryl-6 laurate, polyglycerin-6) As you can see has sodium in it. Usually when I see sodium in the name and there is no mention of being electrolyte sensitivity I always proceed with cation using electrolytes (or sodium containing ingredients). This is why you always need to do small test batches of your products before making a large batch. Also, I always recommend keeping out the expensive ingredients like the actives (retinoids, vitamin C, ceramides, ETC) because you might just end up with a failed product and it’s easier to figure out why it may have failed when you have less ingredients involved. On the other end, electrolytes can actually make water in oil emulsions more stable. I haven’t talked or experimented much with water in oil emulsions, as that is still something I want to learn more about before ‘teaching’ about them. So I will just leave it there. Water in oil emulsions have water as the internal phase being dispersed into oil.
Other Electrolyte Ingredients
There are some other ingredients that are technically electrolytes, but you might not realize it. Aloe Vera or even plant extracts, because they may contain other mineral ions like potassium, sodium, magnesium, or calcium. Which reminds me, potassium, magnesium, zinc, bicarbonates and calcium are all also considered electrolytes and might just throw you a curve ball in your formulating process. So if your formula fails and it contains any of these electrolytes then they could be the culprit.
Well that is all I have for now. Let me know if there is anything I discussed in this post that you would like me to expand on. Also, if you have any important info to add, comment it below.
Sources
Skinchakra cosmetic lab - video titled what are electrolytes? What do they do in cosmetic formulations?
https://www.sciencedirect.com/science/article/abs/pii/S0021979783713056#:~:text=The%20electrolytes%20appeared%20to%20enhance,the%20water%20droplets%20to%20coalescence.
Thank you for this information! I’ve been playing with adding salt to a foaming sugar scrub formula since I learned it can help with thickening. Also with body wash, I was trying to figure out why some fragrances thin out the formula and I heard salt will help bring back the thickness. So this post is super helpful. Yes please more on the salt curve. I really would like to learn how to find that sweet spot. I get confused, I use sci and capb; since both of these ingredients contain salt, will my salt % change depending on how much of those ingredients are used in the formula? Or can you say what a safe salt % range to…