Preservative blend for pH above 5.0

Hey yall,

I have been given a set of ingredients from a customer and asked to turn it into a stable (and aesthetically pleasing) cream.

They have requested using EcoMulse or my own combination of Glyceryl stearate, Cetearyl Alcohol, and Sodium Steroyl Lacylate, and based on some research about EcoMulse, it seems my pH must stay at or above 5.0. I also have no experience using this emulsifier blend, and frankly, would love some feedback as to its strength, in reality, for an oil load like this (15-20%) and if I can add a co-emulsifier such as GSC to feel better about it. 

They have also requested use of Phenylpropanol-- I see a nice combo from MakingCosmetics with ethylhexylglycerin and my rep from Schulke has one, Sensiva PA 40, with caprylyl glycol. Anyone have much experience with these? I could not find much detailed information about phenylpropanol's effective microbial action.

Since I do not know much about phenylpropanol and I am dealing with a pH (5.0-6.0) above the truly effective levels for most organic acids (as far as i know), what would be a sufficient way to boost this product's preservation? I can request to use parabens, but I would not be surprised if that response is a big, fat "no". 

Here is the initial formula I have drawn up based on the set of ingredients I was given:


Water q.s.
Glycerin 1%
Propanediol 2%
Xanthan Gum 0.1%
EDTA 0.2%

Menthol 3%
Methyl Salicylate 3%
Sunflower oil 2%
Shea Butter 2%
Jojoba oil 5%
Hempseed oil 2%
EcoMulse 8%

Mixed Tocopherols 0.5%
Rosemary essential oil 0.1%
Lavender essential oil 0.1%
Devil's claw extract 0.1%
Comfrey extract 0.1%
Arnica oil 0.1%
Phenylpropanol EHG 0.5%
Citric Acid solution (50%) q.s.



Every response is appreciated! Thanks.

Comments

  • PhilGeisPhilGeis Member, Professional formulator
    Consider Phenoxyethanol - for that there is a lot of data for EHG boost.    I'd be cautious re a sales rep's suggestions for their own products.  Think you may have a fungal gap.  Perhaps add an organic acid.
  • Thanks @PhilGeis ! I appreciate the knowledge. It looks as though (from some quick, dirty, not long enough studies on google scholar) that the pKa of acetic acid is slightly higher than that of sorbic acid. In theory, under exactly identical conditions, would this mean acetic acid will have a higher level of activity (although still below 50%) at a pH of 5.0 in comparison to sorbic acid? 
  • For fungi you can use 0.05% Piroctone Olamine. 

    Can be solubilized in phenoxyethanol. 
  • PhilGeisPhilGeis Member, Professional formulator
    edited August 15
    Acetic acid is not a preservative - except as an acidulant. 
    Abdullah.  It's has chelator function like ZPT, not so cost effective as a preservative and has found little use in that context.  Don't think it's on the positive lists.
  • Ok i see. I have made an incorrect assumption about the connection between acetic acid and dehydroacetic acid.

    Just read through a full pdf of this article https://www.sciencedirect.com/science/article/abs/pii/S0740002009001798 , which gave me some insight comparing dehydroacetic acid and sorbic acids (and other weak acids) and their MIC at pH 5.0 and 6.0 with two different levels of innoculation. Seems promising (at least in this study) that dehydroacetic acid can perform as well as sorbic acid in the given pH range I am dealing with in my formula. 

    My lack of chemistry background is confusing me in terms of understanding a relationship between pKa and antimicrobial acitivity - if there is one. The study listed above hints to the fact that levels of both the dissociated and non-dissociated acid forms are relevant to its antimicrobial activity, but clearly, certain weak acids are highly pH dependent for their effectiveness. Is there some relevant reading you can point me in the direction of? Although, this level of study may be above my brain's pay grade...   

  • @PhilGeis do you mean that chelating agents will reduce the effectiveness of Piroctone Olamine? 
  • PhilGeisPhilGeis Member, Professional formulator
    edited August 16
    Piroctone is a chelating agent.  As a hydroxamic acid like Pyrithione Zinc, it has antifungal function.  

    This explains https://findanyanswer.com/what-is-the-pka-of-a-drug
    The protonated acid is the active antimicrobial.  DHA pKa~ 5 - so 50% dissociated at pH a little over 5 (haf proitinated, half ionized - salt).  Higher pH shifts to dissocated/ionic salt that is not active.  In product, other ingredients may effectively change pKa - e.g. benzoic acid and surfactants.  
  • thank you, @PhilGeis ! this article helped a lot. still need to learn my functional groups "like multiplication tables"... 
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