The concentration interval of 48−62 wt % hexaethylene glycol n-hexadecyl ether (C16EO6) in deuterium oxide (2H2O) has been investigated using optical microscopy, 2H nuclear magnetic resonance, and small angle X-ray scattering and the detailed phase behavior established. On cooling from the lamellar phase (Lα), the phase sequence defected lamellar (Lα H), mesh intermediate (Int.), Ia3d cubic (V1), and hexagonal (H1) or hexagonal plus gel biphase (H1 + Lβ) was observed. On heating from the hexagonal or two-phase region, the region of intermediate and cubic phases formed on cooling is replaced by an Ia3d cubic phase. The mesh intermediate phase, formed on cooling, is demonstrated to have a rhombohedral structure, assigned the space group R3̄m. It represents a phase in which the water-filled defects of the higher temperature Lα H phase become correlated between layers. The formation of the phase is driven by the need to introduce more curvature into the interface as the temperature is decreased because of increased ethylene oxide head group (EO) hydration. The interlayer correlation arises from the strong, repulsive head group overlap interaction in this system.

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A section of t h e composition-temperature phase prism of a ternary non-ionic surfactant-water-oil system defined by a constant surfactant to oil ratio has been investigated. The structural sequence of normal spheres to planar bilayers, via cylinders, is observed a s a function of decreasing water concentration at fixed temperature.As a function of increasing temperature at fixed surfactant and oil concentration, we note the commonly observed reversal in mean curvature of the polar/apolar interface. The mean curvature progressively alters from positive values in t h e low-temperature microemulsion phase to negative mean curvature in t h e hightemperature L, phase, with an intermediate lamellar phase where t h e mean curvature passes through zero. The phase diagram of this section is similar to that of, the binary surfactant-water system. The average area per surfactant at the polar/apolar interface is 46 & 2 A2. A micellar cubic phase is observed between t h e microemulsion and the hexagonal phase while no bicontinuous cubic phase is observed.

Intermediate phases in lyotropic liquid crystalline systems provide fascinating topological links between the hexagonal and lamellar phases. The formation of intermediate phases has been shown to result from a delicate balance of interaction in amphiphilic systems. Their structures fall into one of three broad classes: ribbon, mesh, or bicontinuous. In concentrated binary mixtures of nonionic surfactants and water, the intermediate phase is observed to replace the bicontinuous cubic phase under certain circumstances. In this paper we present more detailed scattering results from the mesh intermediate phases in the C30EO9/ 2 H2O and C16EO6/ 2 H2O systems. We show that the space group is indeed R3 hm and consists of a three-connected mesh structure with no interlayer connections. Three different mathematical models for the structure, a minimal curvature surface, a rod-box model, and a simple rod model, are fitted to the revised lattice parameters. All give good values for the surface area per molecule and the aggregate thickness parameter. They show that the progressive decrease in the ethylene oxide hydration with increasing temperature causes the progressive decrease in surface curvature. Also in this paper we address the structural relationship between the R3 hm mesh phase and adjacent phases in both surfactant systems.