Formation of intermediate micellar phase between hexagonal and discontinuous cubic liquid crystals in brine/N-acylamino acid surfactant/N-acylamino acid oil system

The phase behavior in the brine/sodium N-dodecanoyl sarcosinate (Sar)/isopropyl N-dodecanoyl sarcosinate (SLIP) system has been investigated by means of phase study, static light scattering, and small-angle X-ray scattering. The liquid crystal phases, hexagonal (H(1)) and discontinuous cubic (I(1)), melt upon the addition of NaCl, which shows the similar effect to the increasing of temperature. The addition of SLIP to the brine/Sar solution at high Sar concentration induces the phase transition from H(1) to I(1) via the isotropic micellar solution (W(m2)). The micellar structure in the W(m2) phase also changes from the wormlike to the globular micelle with SLIP concentration. Adding NaCl reduces the repulsive force between the Sar head groups and simultaneously the space of the solubilized SLIP in the palisade layer, leading SLIP to shift their location further into the micelle core. As a consequence, the hexagonal symmetry breaks into the micelle solution and the liquid crystal order is destabilized entropically.