Many cosmetic products require the addition of a preservative - anti-microbial ingredient(s) designed to prevent spoilage and maintain product safety. It's important these ingredients are active within the conditions found in your formula and that they remain situated at the vulnerable interface. This isn't always the case as we will see here.
Preservatives are surface-active.
In the case below, the preservative chose to sit at the product: packaging interface instead of the oil: water interface where it was needed.
A while ago I was sent an enquiry from a client who was experiencing problems in their formula. They felt sure it was something to do with the preservative but couldn't work out what was going on.
I took a look and could see the problem was one of solubility as the preservative that had been chosen was just not mixing into the main formula.
I confirmed my suspicions with a few lab tests and also tested a couple of formula adaptations in the hope of providing the preservative with a more accommodating environment.
I kept the original samples to evaluate over time.
On returning to the samples after a couple of weeks, I noticed some of the samples had developed etchings on the side of the bottle while the rest of the formula was crystal clear with no sediment.
Clearly, in these samples, the preservative was preferring the packaging to the product and had found a way to precipitate out onto the side of the bottle.
I do see this behaviour from an ingredient from time to time and it can happen with other ingredient types too such as essential oils, fragrances and colours.
This situation takes a bit of time to develop but in this particular case, I could immediately see the preservative was not in solution so the etching effect developed quickly. However, this situation can and does happen more slowly and quietly, showing up as stability issues down the track either in terms of microbial failure (when the offender is a preservative) or a premature loss of colour or fragrance of a product (for dyes or aromatics).
I often hear people talking about solubility as if it is an absolute thing but it is not.
Solubility is always relative.
Cosmetic formulators have to establish the solubility of the ingredients in their formula. Solubility data from the internet will only guide you as to the solubility of the ingredients in binary mixtures (one solvent plus one solute). That's rarely what we are playing with in a formula.
Next, we also have to evaluate the product: packaging relationship, something that's typically carried out during stability testing. Solubility under optimal storage conditions should also be compared with solubility when the formula is under stress (heat, cold, cycling temps, UV exposure possibly) as these are also conditions a commercial product might be exposed to.
R&D takes time and is often very repetitive.
These are some of the reasons we take our time when developing new formulations. This is why stability testing is important and why on-shelf vigilance, especially of the first batch of new products comes in (as not all cases of packaging interaction happen during a standard shelf life test or in standard test conditions).
Sometimes, in order to solve a problem in a complex system like a typical formula, the formula has to be broken down completely and analysed as each ingredient is added to better understand the relationships that are existing inside.
It can be daunting to know of all the ways your formulations can fail, especially when it’s to do with microbial stability. However, it’s also interesting and empowering when you can spot and fix a problem.
The key is to give yourself enough time and develop enough curiosity and knowledge to turn these unfortunate events into opportunities. Another option of course is to seek professional help and input, especially if all of this is new to you.
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