Effect of Polyoxyethylene Type Nonionic Surfactant and Polar Lipophilic Oil on Solubilization of Mixed Surfactant Microemulsion Systems

In the present report, the water solubilization capacity of water-in-oil microemulsion systems comprising mixed surfactants, that is, anionic sodium bis­(2-ethylhexyl) sulfosuccinate (AOT) and nonionic(s) [viz., polyoxyethylene (2) stearyl ether (Brij-72), polyoxyethylene (10) stearyl ether (Brij-76), polyoxyethylene (20) stearyl ether (Brij-78), polyoxyethylene (2) oleyl ether (Brij-92), polyoxyethylene (10) oleyl ether (Brij-97), and polyoxyethylene (20) oleyl ether (Brij-98)] was investigated in polar lipophilic oils [viz., ethyl myristate (EM), isopropyl myristate (IPM), and isopropyl palmitate (IPP)] with varying mole fractions of nonionic surfactant (X_nonionic) at a fixed surfactant concentration and 303 K. A maximum (i.e., synergism) in water solubilization (ω_0,max) was observed at an optimal value of mole fraction of nonionic surfactant (X_nonionic,max). The solubilization efficiency parameter (SP*_water) and the free energy of dissolution (ΔG⁰_s) were evaluated for these mixed systems. All of the physicochemical parameters for solubilization (viz., ω_0,max, X_nonionic,max, and SP*_water) were found to be dependent on both nonionic surfactants and oils. A minimum in ΔG⁰_s was observed at corresponding X_nonionic,max, where a maximum in SP*_water was evidenced. The IPP-based system was found to be most efficient, whereas the EM-based system was least efficient. An attempt was made to correlate the interdependence of physicochemical (ω_0,max or SP*_water) and thermodynamic (ΔG⁰_s) parameters.