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P Anisic Acid isn’t soluble.

June 26, 2019

I thought I’d write a little about this ingredient as it is a very good natural preservative, especially against mould but it is quite hard to use so we should explore that.

P Anisic Acid is supplied as a white powder that is stubbornly resistant to solubilising into your cosmetic formula.  I’ve had many a chat about this ingredient and mostly it is about how formulations containing it can look gritty.

Now the ingredient is mostly used at between 0.05-0.3% so not very much, but that little bit can cause people a lot of problems hence my blog post.

If we look at the solubility, we must first remember that the word is an applied term i.e. we must talk about both solvent and solute in order to establish solubility.  In this case the solvent is water and the solute is p anisic acid:

That graph comes from here and relates to an Experchem spec of p anisic acid.  Being an individual chemical other suppliers data is likely correlate well with this.

This graph shows me how solubility of p anisic acid rises as the temperature rises.  The temperature is plotted along the bottom (X Axis), the solubility along the side going up (Y Axis).   Between 30-40C we get what looks like a massive jump in water solubility when compared to temperatures below 30C but in reality we are only talking a big jump in what is a tiny amount of ingredient.  At 40C only 0.03g per 100g (or 0.03% of p anisic acid is soluble. That’s not really enough to be useful in the average cosmetic formula as we require at least 0.05% to be anti-fungal and typically use this up to 0.3% so 10 times more than this!

So what can we do?

First of all we have to think about how a preservative works and whether insolubility is a real problem or not.  Now fungus grows on the surface of a product so it makes sense that a good anti-fungal preservative will sit at the surface of a product.  There are many surfaces within a product that contains many phases. In an oil-in-water product there are surfaces between the dispersed oil drops and the continuous water phase.  It is helpful if anti-fungal and general anti-microbial preservatives sit there.  I like to think of my preservatives as border security guys.  They are no use to anyone if they are asleep inside the centre of the country, they need to be at the borders between countries, ready to check peoples passports and deny entry if needed.  Preservatives are the same, anti-fungal preservatives are most definitely needed on the front line!

So not being soluble may not be such a big deal?

The main problem with this lack of solubility is the ability of the user to detect tiny undissolved particles in their product. This is an aesthetic and maybe a safety problem (blobs of p anisic acid may be more irritating than if it was solubilised throughout the product).  It may also be a preservative problem if the preservative is poorly wetted and therefore has less surface area.  Again if we think of this as like a border patrol person, think of how less effective border security would be if all the guards were huddled into one spot – that’s what not wetting your p Anisic acid would look like, plenty or room to sneak around them!

How soluble is enough?

One could argue that the best outcome for p anisic acid is for it to be adequately wetted and partially dissolved without being fully dissolved.  That way it stays at the border, nicely spread out and ready for action while being harder to detect (on account of it being finely dispersed) and less likely to irritate (for the same reason).

So how do you achieve that?

Heat is an imperfect tool.  Sure if you heat the formula, the p anisic acid becomes more soluble but cosmetics are not stored hot, they are typically stored at room temperature and we’ve already seen that it doesn’t help us.

Changing the solvent helps.  Adding some alcohol will help boost p anisic acid solubility because this chemical is soluble in alcohol.  We could play around with the ratio of alcohol to p anisic acid in the formula to get it to just dissolve rather than become fully incorporated in the water phase.  Doing this can lead to crystallisation later down the track for the formula as crystals of the partially solubilised p anisic acid find their friends and start to migrate together but this may not be a problem for a long time – i.e. after your product shelf life has run out.  You’d need to check.

Another thing that can help is to alter the chemistry of the active.  P Anisic acid is not water soluble but sodium anisate is.  You can make this chemical change by altering your product pH with Sodium Hydroxide or equivalent.  Again you don’t want to make the p anisic acid folly comfortable in your water phase but you can encourage it in a little, lubricate its passage somewhat.  You may want to experiment with this.  Both p anisic acid and sodium anisate are antimicrobial, the p anisic works best as anti-fungal because it stays at the interface (that’s physics) rather than because of its chemistry so keep that in mind and you should still be ok.

Hopefully that gives you some tips of how to work with P Anisic Acid, a great naturally derived preservative that is good at preventing fungal growth in water based cosmetics.

Amanda x

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