Some observations on liquid crystallinity in barium surfactant systems

Abstract

The phase diagrams of barium surfactants in two and three component systems are investigated by water deuteron (²H) NMR and polarizing microscopic studies. Water-free barium di-2-ethylhexylsulphosuccinate (BaA₂) has two-dimensional hexagonal structure and forms a reverse hexagonal liquid crystalline phase (F) with water at low water contents (≼16 wt%). At the high water content limit the F phase is in equilibrium with a very dilute aqueous solution (L ₁). The solubility of BaA₂ does not increase significantly with increasing temperature. The two-phase region, L ₁+F, transforms, on heating, at 316 K (three-phase line) to two immiscible liquid phases, L ₁+L ₂, which were found not to mix up to the range where decomposition starts.

Barium octylsulphate (BaOS) is virtually insoluble in water and does not form any liquid crystalline phase with water between 298 and 323 K. Addition of decanol to the aqueous system leads to the formation of the lamellar liquid crystalline phase, yet incompletely characterized. However, it appears that the lamellar phase exists at low water contents and probably occupies a small area in the pared with those of the corresponding systems with Mg²⁺ and Ca²⁺ counterions and the results are discussed in the light of electrostatic theory.