Pka of benzoic acid in presence of surfactants

For those who want to use sodium benzoate as preservative for there cleansing product, here is a data that shows the pka of benzoic acid in presence of some surfactants. Hope it is useful for you.

According to this, in shampoo with 11%% ammonium lauryl sulfate+2% CAPB at pH 4, would sodium benzoate with 0.2% EDTA be enough as preservative? If yes, at what percentage? 


  • PharmaPharma Member, Pharmacist
    In theory, acidic preservatives would get more efficient at a given pH when their pKa increases ;) .
    However, a change in apparent pKa can also mean that water-to-oil/micelle distribution is shifted towards the interface or even into the micelles which corresponds to less available surfactant in the water phase and potentially lower activity (similar to the incompatibility of parabens with PEGs).
    It is impossible to judge the reality only based on the data provided... one would have to measure antimicrobial activity.
  • @Pharma thanks a lot
  • PhilGeisPhilGeis Member, Professional formulator
    Pharma - micro data certainly support this phenomenon.  Note preservation  of shampoos - Pantene, Heads & Shoulders, Aussie, Herbal Essences

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