Phase equilibria of some ionic surfactant systems with divalent counter-ions

Abstract

The phase diagrams of binary and ternary surfactant systems with divalent counter-ions have been studied by 2 H-NMR. In the sodium di-2-ethylhexylsulphosuccinate—deuterium oxide system at 300 K, the sequence of liquid crystalline phases remains unchanged when Na + is replaced by Ca 2+ or Mg 2+ . The stability ranges of reverse-hexagonal and cubic liquid crystalline phases are approximately the same in these three surfactant systems. On the other hand, the stability of the lamellar liquid crystalline phase is drastically reduced on the high water-content side, in the case of the divalent counter-ions. The thermal stability of liquid crystalline phases follows the order Na &>; Mg &>; Ca. The existence and stability ranges of the normal-hexagonal liquid crystalline phase, the isotropic aqueous phase and the decanolic solution phase change very little for the isothermal ternary system sodium octylsulphate—decanol—deuterium oxide on replacing Na + with Ca 2+ or Mg 2+ . In contrast, the capability of the lamellar liquid crystalline phase to swell and take up water is reduced very considerably in the presence of divalent ions. Moreover, the magnesium system has been found to form two more liquid crystalline phases — one reverse-hexagonal liquid crystalline phase and one cubic liquid crystalline phase. The cubic liquid crystalline phase is unable to solubilize decanol to any significant extent. The difference in phase diagrams between mono- and divalent counter-ion systems is discussed qualitatively in relation to Poisson—Boltzmann calculations and Monte Carlo simulations.