What % ethanol is required for use as the sole preservative?

suswang8suswang8 Member
in Formulating
Hi, all.


Does anyone know with reasonable certainty the minimum concentration of ethanol that is required in a formulation in order to have it function as the sole preservative?  I have seen estimates online ranging from as low as 10% to a high of 20%.  

I am hoping to create a low pH (3.7-ish) toner, and I would be happy to use just 10% ethanol, but at 15%+ it's probably going to be too drying.

Thank you!


Comments

  • PhilGeisPhilGeis Member, Professional formulator
    20% is the recognized target.
  • RedCoastRedCoast Member
    You could get away with 10% ethanol if you enhance it with glycols and other ingredients that reduce the water activity and minimize botanical extracts.
  • PhilGeisPhilGeis Member, Professional formulator
    Please pursue preservation as an affirmative effort.  A lesser level of ethanol  may be useful as a supportive element but don't design your system around it.
  • FormulatorFormulator Member
    suswang8 said:
    I am hoping to create a low pH (3.7-ish) toner, and I would be happy to use just 10% ethanol, but at 15%+ it's probably going to be too drying.


    In my experience, 20% ethanol is not really drying, especially if you incorporate some glycerin.
  • PharmaPharma Member, Pharmacist
    RedCoast said:
    You could get away with 10% ethanol if you enhance it with glycols and other ingredients that reduce the water activity and minimize botanical extracts.
    Antimicrobial glycols don't usually act by lowering water activity. You'd need something in the range of ~40%, if memory serves me right, for most to reduce water activity below the 'growth possible' threshold. Some glycols act as antimicrobials and/or preservative boosters at levels in the low % range and that doesn't noticeably affect water activity.
  • RedCoastRedCoast Member
    Pharma said:
    RedCoast said:
    You could get away with 10% ethanol if you enhance it with glycols and other ingredients that reduce the water activity and minimize botanical extracts.
    Antimicrobial glycols don't usually act by lowering water activity. You'd need something in the range of ~40%, if memory serves me right, for most to reduce water activity below the 'growth possible' threshold. Some glycols act as antimicrobials and/or preservative boosters at levels in the low % range and that doesn't noticeably affect water activity.

     Ah, I assumed @suswang8 was going to go with the higher glycol route for solubility purposes and to potentially dupe the other popular low ethanol or ethanol-free SA toners on the market... that's why I was a bit vague on my comment. I should've worded it better.
    But yeah, 40% sounds familiar, and I've seen and purchased some 2% SA toners on the market with extremely high concentrations of glycols. Unsurprisingly, the glycol-based/ethanol-free ones feel sticky and tacky, unlike the ethanol-dominant ones.
  • suswang8suswang8 Member
    Quick question about the above % figures:

    I see in old posts on this forum that certain members indicated that the % of ethanol they recommended is the % of the water phase, not the % of the total formulation.  So when the above members say 20%, for example, are the above members speaking about 20% ethanol for the entire formula, or just that ethanol needs to comprise 20% of the water phase?

    @RedCoast:  Actually, I plan to use only small amounts of glycols (and specifically butylene), as I don't really love the way they feel on the skin.

    Thank you.



  • PhilGeisPhilGeis Member, Professional formulator
    formulation - and that is typically considred in a highly aqueous product context.
  • RedCoastRedCoast Member
    edited July 2021
    @suswang8 That makes sense. Keep in mind, though, you may have to add fragrance. 20% ethanol doesn't smell very nice to a lot of women.
    In my opinion, the best scents to cover up the sharp odor of ethanol are green, fresh, watery ones... think aloe or agave-like fragrances. Using heavier floral fragrances can give some people headaches.
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