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Solubility is relative, not absolute.

January 18, 2019

In spite of the dramatic rise in people making their own cosmetic products over the twenty years or so that I’ve been in business I haven’t really noted a comparable increase in science literacy. This is in spite of many people completing online or face-to-face courses.  Now I’m obviously part of the problem, being as though I’m an educator and I teach some short courses myself but I have to say that while I’m willing to try to push myself and my teaching harder, I am not prepared to take the whole blame for this and anyway, there’s no blame required as long as everyone stays open-minded, we’ll all learn as we go. Together!

So solubility…

Solubility is the art of disappearing one chemical into another.  It could be salt or sugar into water (where does it go?),  olive oil into avocado oil (do they combine into a new super-oil or do they stay separate?),  a food dye into water or some (but not all) essential oils into alcohol.

Is this magic or what?

To me chemistry IS magic,  a magic that only gets more magical on deeper understanding. There is no need for the slight-of-hand trickery, even when one knows exactly what is going on the reality is just magical and other-worldly. Solubility is one bit of the chemistry magic and this bit is relative.

Relative:  Considered in relation or proportion to something else.

We can only really have very meaningful conversations about solubility when we understand both the solvent and the solute.

Solvent: The medium that the ingredient is to disappear or dissolve into – it may not even disappear, colours don’t disappear do they? They merely integrate and spread out!

Solute: The thing that you are trying to solubilise or make disappear.

Us humans have a very large tendency to over-simplify everything in order to get things processed in a way that is as energy-efficient as possible – turns out our brains are super hungry and we don’t like churning through the calories just thinking.

So we tend to just stick to two categories of stuff in cosmetic science:

Oil or water. 

The question I get asked frequently is ‘is it soluble in oil or water’ and that’s a reasonable question but it is only a first step.

Finding out if something is generally oil or water-soluble helps us to understand the main nature of the ingredient, to visualise where it might sit in a formula and/or what type of issues we might face in using it.  But we shouldn’t stop there.

Once we get into the lab we might find that what we thought was going to be an easy job of disappearing our ingredient has turned into a nightmare.

This is what I mean about solubility being a bit more complex and with it being conditional.

Here are some of the conditions that can come into play with solubility:

  • Temperature
  • Concentration
  • pH
  • Molecular weight where a molecule has a variety of weights or sizes
  • Ionic strength of solute
  • Ionic strength of solvent solution.
  • Chemical interactions within the rest of the formula.

The above may mean nothing much to you until (or unless) you’ve been in the lab or your kitchen and things have’t gone to plan in the solubilisation realm.

The ionic strength part is actually quite a big thing in cosmetic chemistry, especially these days where people like putting all sorts of wonderful things in their water phase so people’s water phase is rarely just water.

If you have a formula that contains aloe powder and you next want to dissolve Sodium PCA, your water phase is no longer just water, it is aloe water.  This probably won’t stop the Sodium PCA from mixing in with your water but it might affect other things like, for example, how easy it is to solubilise essential oils into that ‘water’, how easy it is to form a stable emulsion with that ‘water phase’ or how readily the preservative dissolves through.

The other main thing is concentration.  Everything has its solubilisation limit and if you exceed that you may end up with sedimentation in your product, graininess, full product instability or crystallisation.   Published solubility data for water-soluble things is talking about pure water and not water that already has five different actives added so do keep that in mind too!

The bottom line is that things aren’t always infinitely soluble in each other so just because a chemical is soluble in water or oil doesn’t mean they will always be soluble in your water or oil phase or product, your product may have changed the solvent characteristics significantly enough to make the ingredient insoluble at the concentration you want to use.

Chemicals have their preferences and their limits.  If you are having issues with solubilising or stabilising something then do take a helicopter view of your formula and check that you are not over-loading the phases.  What I often do in that case is pair back and re-build one step at a time to see what kills it.

And don’t forget pH, pH can make the difference between something disappearing forever or staying stubbornly separate.


PS: When you combine oils, they generally just get tangled and co-exist rather than becoming a new super-oil. This is why some oil blends can soften in a non-even way so you get some liquid forming on the top while the bulk is still solid – well that has something to do with heat transfer too but wide and separate melting points also play a part.



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