Phase transition of sodium bis(2-ethyl-1-hexyl) sulfosuccinate and sodium deoxycholate mixtures in aqueous solutions

The aggregation behavior of two anionic surfactant mixtures, sodium bis(2-ethyl-1-hexyl) sulfosuccinate (AOT) and sodium deoxycholate (SDC), in aqueous solution was investigated at room temperature. The mixture aqueous solution goes through a series of phase transition from a planar lamellar L_α phase to an L₁/L_α two-phase, and finally to the transparent isotropic solution with the increase in SDC molar fraction. The surface tension measurements show that the mixtures have synergistic surface activity. The small-angle X-ray scattering (SAXS) and ²H nuclear magnetic resonance (²H NMR), and freeze-fracture transmission electron microscopy (FF-TEM) clearly demonstrate a phase transition from lamellar structure to hexagonal structures with the addition of SDC to the AOT L_α phase. The rheological measurements show the highly viscoelastic property of the L_α phase formed by AOT, for which the viscosity decreases with the increase in the proportion of SDC because of the re-arrangement of the planar bilayers. The structure transition of AOT and SDC mixtures might be a result of the balance of the repulsive force between the negative headgroups of AOT and SDC, the hydrophobic interaction between the double chains of AOT and the steroid skeleton of DS^-, and steric effect of DS^-.