Binding free water....explanation please.

I like to know how things work.... as I have researched a little tougher projects preservation system, and read many old forums, I have seen the concept that some things like propanediol are preservative enhancers, in that they 'bind free water'.

Could someone give an explanation of that term as it applies to cosmetic emulsions and preservation, and in a manner that someone with a business degree would understand.  :)


  • PharmaPharma Member, Pharmacist
    Free water is water not 'occupied' by dissolving stuff (especially electrolytes) such as salts or amino acids but also polyols, sugars etc.
    In order to bring something into solution, said substance needs to interact more with the solvent (water) than with its own kind. Like a flock of men 'dissolving' a flock of women. Both flocks hold hands with their own kind and dissolving means changing your hand-holding partner with the opposite sex. Once all the women have 'dissolved' in the male crowd, those men not holding hands with a woman are 'free' and may therefore cause trouble ;) . It's the same with water molecules not busy holding hands with solutes; these are accessible for example to nourish microbes.
    Knocking free water down to a self-preserved level isn't easy in an emulsion! Usually, free water is just reduces enough to put microbes on a diet, not starve them to death, so they will be more susceptible to preservatives and have less energy to deal with other hurdles set in their way.
    It's like preparing a field for a new crop. You have to get rid of the weeds when you can't (or shouldn't) use herbicides. Plucking them by hand helps but it's not enough. Not fertilising will slow their growth but only that. Not watering will also help (okay, YOU never water anyway) but due to rain, it's not enough by itself. You can also put up shades or plant trees to reduce sunlight or let chicken or goats graze the land. The list goes on and you might know better than I do what else there is. The trick, in gardening as well as cosmetics, is to use all possible options at once because together they will (hopefully) succeed. Else, you resolve to the use of a herbicide/preservative which in this case will work at a way lower amount than without all the additional hurdles.
  • GraillotionGraillotion Member
    edited December 2020
    Thank you @Pharma.  And so colorful!  :D

    So using propanediol as an example... will something like 2% have much of an effect as a hurdle....or does it need a higher inclusion rate, say 5-6%?

  • PharmaPharma Member, Pharmacist
    10% propanediol would keep many bacteria at bay, 15% would be the minimum against many yeast and moulds including common spoilage microbes, 20% is a good hurdle against all pathogenic bacteria and most moulds/yeast, and 40% is enough to keep also xerophilic moulds and osmophilic yeast at bay. The few microbes still able to grow in there are very rare and would require about 50% propanediol.
    Mind, this is only for propanediol and propylene glycol. Other substances have different % requirements.
  • PhilGeisPhilGeis Member, Professional formulator
    edited December 2020
    Please work off water activity of the product, not percentage of any ingredient.   As a stand-alone agent of preservation, water activity should be less than 0.7.  As shown in the attached, you'll need much greater levels for the polyols in aq. solution - e.g. >60% propanediol.  As pharma said, lesser levels may contribute but  please measure water activity of the product as the control element not their %. per se.   and be aware - lowering Aw will inhibit but not necessarilly kill - and may not contribute in  challenge test.

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