Physicochemical investigation of mixed surfactant reverse micelles: Water solubilization and conductometric studies

Solubilization of water in mixed reverse micelles (RMs) comprising sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and polyoxyethylene (20) sorbitan trioleate (Tween-85) or polyoxyethylene (20) sorbitan monooleate (Tween-80) or sorbitan trioleate (Span-85) has been studied at different compositions (Xnonionic = 0–1.0) at a total surfactant concentration, ST = 0.1 mol dm−3 in polar lipophilic oils of different chemical structures: viz., ethyl oleate (EO), isopropyl myristate (IPM) and isopropyl palmitate (IPP) at 303 K. The enhancement in water solubilization (i.e., synergism) has been evidenced by the addition of nonionic surfactant to AOT/oil(s)/water systems. The maximum water solubilization capacity (ω0,max) and Xnonionic,max (mole fraction at which synergism occurs) have been influenced by polar head group and hydrophobic moiety of nonionic surfactant. The standard free energy change of dissolution of water ( Δ G s 0 ) of these systems depends on water content, XTween-85 and oil. Solubilization efficiency parameter ( S P water * ) has been evaluated to underline the efficacy of oils in obtaining maximum water solubilization capacity in mixed RMs. Conductance behavior of these systems in absence and presence of additives (bile salts and hydrotrope) has also been investigated under varied water content (ω) at 303 K. Volume-induced percolation threshold (ωp) depends on XTween-85, oil type, and additives. An attempt has been made to give an insight to the mechanism of solubilization phenomena, standard free energy change of dissolution of water, percolation in conductance and microstructures of these systems by dynamic light scattering (DLS) measurements, wherein the chemical structures of both nonionic surfactants and polar lipophilic oils played significant role.