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.