Abstract
The phase behaviour of some surfactant systems with divalent counterions is examined using 2H nuclear magnetic resonance. Systems investigated are calcium octyl sulphate-decanol-water, calcium di-2-ethylhexyl sulpho-succinate-water, and magnesium di-2-ethylhexyl sulpho-succinate-water. The phase diagrams are compared with those of the corresponding systems with monovalent counterions. As regards the existence and stability ranges of isotropic solutions, normal and reversed hexagonal liquid crystalline phases, and cubic liquid crystalline phases there are only minor influences of counterion valency. On the other hand, the stability range of the lamellar liquid crystalline phase is dramatically reduced on replacing sodium with calcium or magnesium. The very much reduced capability of the lamellar phase to swell and take up water with divalent counterions is in agreement with recent theoretical Poisson-Boltzmann and Monte Carlo studies by Wennerstrom et al.
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