Compatibility Investigation of Cationic Surfactants with Anionic Species

The objective of this study was to reexamine the general statement declaring that cationic and anionic species cannot be compatible in cosmetic products. This research demonstrated that there are considerable differences between the binding of cationic preservatives and various anionic compounds used in cosmetics, depending on the anionic functional group. Sulfate- and sulfonate-based molecules showed significantly stronger interactions with cationic surfactants than carboxylate-based anionic surfactants: This difference of affinity could reach a ratio of 1 to 10. We validated that conductimetry and isothermal titration calorimetry (ITC) can be used as predictive tools to determine the molecular interactions between any cationic and anionic species. Consequently, the correlation between compatible and incompatible cationic/anionic mixtures were verified and their corresponding anti-microbial activity using the challenge test was assessed.