The organic acids have moderate bacterial activity and great fungicidal activity. The acids have low water solubility, which is why we generally see them used as a salt or ester. They are best used in products with a pH of 2 to 6, and they can be incompatible with cationic and some non-ionic surfactants.
Sodium benzoate is bacteriostatic, which means it limits the growth of bacteria by messing with its metabolism, but doesn't kill it. It is also a recognized fungicidal ingredient.
The main problem in using sodium benzoate in our products is the pH level - sodium benzoate works best at pH 5 or less (possibly 6 or less), which means its use is limited to products more acidic products like toners or moisturizers with AHA or salicylic acid. You definitely want to own a pH meter if you're using this as your main preservative! Sodium benzoate is approved for us at up to 0.1% for food products and up to 1.0% for cosmetic and medicinal products. You don't want to use this with Vitamin C as studies have shown that together they can form benzene, which is carcinogenic. Geogard Ultra contains sodium benzoate as its preservative.
solubility (see potassium sorbate below). It's about three times more soluble in oil than in water. It's a good fungal, mould, and yeast inhibitor at pH 6.0 or less, and it's an okay bactericide. Its optimal range is at pH 2 to 6, and it's considered ineffective at pH 7. It can be slightly inactivated by non-ionic surfactants like polysorbate 20 and 80 (more about this in the near future). It's generally found in food stuffs at 0.01% to 0.1%. The maximum allowable for cosmetic use in the US is 0.89%. We find this in Optiphen Plus.
All of these organic acids work best when the pH of a product is 6 or less, and some are completely useless at above 7. Since most of the products we make are acidic (a shampoo should be at 6.5 or less, for instance), they can be appropriate for cosmetic usage. Having said this, most of these are only okay bactericidal preservatives and will, ideally, be used in combination with a preservative that excels with bacteria.
Join me tomorrow for more information on parabens!