Magnesium and calcium surfactants Ternary phase diagrams of magnesium and calcium dodecylsulphate with decanol and water

Abstract

The isothermal ternary phase diagrams for the systems magnesium dodecylsulphate-decanol-water at 40 °C and calcium dodecylsulphate-decanol-water at 50 °C are determined by water deuteron NMR and polarizing microscopic studies. In the magnesium system, three liquid crystalline phases (lamellar and normal and reverse hexagonal) and two isotropic (normal and reverse) solution phases are characterized and their ranges of existence are obtained. The calcium system yields the same liquid crystalline phases, but only the lamellar liquid crystalline phase is investigated in detail. The important observations made are: (i) The lamellar liquid crystalline phase for the magnesium and calcium systems can incorporate, respectively, a maximum of 22.5 and 14.3 mole water per mole surfactant ion against 139 mole water for the corresponding sodium system. (ii) The reverse hexagonal liquid crystalline phase is formed for both the magnesium and calcium systems while no such liquid crystalline phase exists for the corresponding sodium system. (iii) The2H NMR quadrupole splittings obtained in the liquid crystalline phases for C8SO 4 − and C12SO 4 − surfactant systems with different counterions (Ca2+,Mg2+,Be2+,Na+) reveal that surfactant hydration is almost independent of alkyl chain length and counterions.