Liquid crystallinity in systems of magnesium and calcium surfactants: Phase diagrams and phase structures in binary aqueous systems of magnesium and calcium di-2-ethylhexylsulfosuccinate

Abstract

The two binary systems of water and calcium or magnesium di-2-ethylhexylsulfosuccinate were investigated in the temperature range 298–423 K (25–150°C) concerning phase behavior, in particular, and also as regards to water binding and thermodynamics of phase transitions. The results are compared with those of the corresponding sodium system (Aerosol OT). In all three systems, the sequence of homogeneous phases with increasing surfactant concentration is: isotropic solution (L 1, lamellar (D), cubic (I 2), and reversed hexagonal (F) liquid crystalline phases. The single-phase regions are separated by two-phase regions. The Ca 2+ and Mg 2+ systems give closely the same phase diagrams which differ considerably in two respects from the sodium system phase diagram. First, the aqueous solubility is lower by orders of magnitude with the divalent counterions. Second, the lamellar phase can swell and incorporate water to a much larger extent with Na + than with Ca 2+ or Mg 2+ (difference in molar ratio of water to surfactant ion by more than a factor of 10). The gross features of the phase behavior are in qualitative agreement with electrostatic theory. Minor differences between Ca 2+ and Mg 2+ as regards water content of phases and thermal stability of phases are discussed in terms of ion specificity effects arising from differences in hydration, and the hydration aspects were also investigated directly from 2H nuclear magnetic resonance quadrupole splittings.