Anionic and Cationic Surfactant Synergism: Minimizing Precipitation, Microemulsion Formation, and Enhanced Solubilization and Surface Modification

Anionic–cationic surfactant mixtures are known to exhibit synergistic effects ( e.g ., low critical micelle concentration, ultralow interfacial tension, middle phase microemulsion formulation, and increased solubilization and adsolubilization). However, the anionic–cationic surfactant mixtures are also prone to form other unique phases such as precipitates, gels, and coacervates in place of middle‐phase microemulsions. Research summarized in this article demonstrates that asymmetric anionic–cationic surfactant mixtures have been shown to promote middle‐phase microemulsions instead of these other phases, albeit with a slight decrease in synergism when using these asymmetric mixtures. The use of anionic–cationic surfactant mixtures also is shown to enhance or decrease surfactant adsorption depending on anionic–cationic surfactant ratios. Middle‐phase microemulsion formation is demonstrated using anionic‐rich or cationic‐rich alcohol‐free microemulsions by anionic–cationic ratio scans while also reducing or eliminating electrolyte requirement. Solubilization and adsolubilization are shown to increase for mixed anionic–cationic surfactant systems, especially for hydrophobic solutes. Thus, by exploiting these synergisms while avoiding phase separation, properly designed anionic–cationic surfactant mixtures can be advantageous for a wide range of applications.