parabens - can be inactivated by non-ionic ingredients, including some of the fatty alcohols (like cetearyl alcohol), fatty acids, esters, and polysorbates, when they're used as emulsifiers (using the HLB system). The paraben based preservatives include Phenonip, Liquipar, and Germaben II, and the study to which I'm referring tested Phenonip at 0.3% in various non-ionic ingredients. (Liquipar PE and Germaben II aren't affected by non-ionic ingredients.)
Polysorbate 80 is the worst culprit, and at 2.5% in an emulsion it completely inactivated Phenonip! Polysorbate 20 partially deactivated it at 2.5% and 5.0%. Cetearyl alcohol and glyceryl stearate had no effect at 2.5% and 5.0%. Ceteareth-20 caused no problems at 2.5% but completely deactivated Phenonip at 5.0%.
So what do we do here? Obviously we can use other preservatives for products containing those ingredients or we could add another preservative to boost the efficacy of our parabens. And we can make sure we're using our ingredients at the right levels in the right products.
Join me in about an hour for inactivation of preservatives by pigments!