Disconnected lamellar phases (Lalpha) in pseudobinary water-non-ionic surfactant systems: a general phenomenon

This study provides new experimental evidence for the disconnection of the lamellar phase (L(alpha)) in pseudobinary water-non-ionic surfactant systems. To prove that the disconnection is indeed a general phenomenon the phase behavior of the pseudobinary system water-pentaethylene glycol dodecyl ether/hexaethylene glycol dodecyl ether (H(2)O-C(12)E(5)/C(12)E(6)) was investigated as a function of the surfactant composition delta and the total surfactant concentration gamma. At a fixed gamma of 0.10 the extension of the highly diluted L(alpha) phase shrank continuously with increasing amount of C(12)E(6), i.e., increasing delta, until it disappeared at delta=0.60. The gamma-T phase diagram of this particular surfactant mixture was found to have a disconnected L(alpha) phase. For the first time, SAXS measurements were carried out to monitor structural changes related to the disconnection. For this purpose the interlayer spacing d and the effective cross-sectional area a(s) were determined from the SAXS data along characteristic paths through the L(alpha) phase.