High-resolution structural elucidation of extremely swollen lyotropic phases

Abstract

Self-assembled lyotropic phases are important in a variety of applications, in particular microemulsions are essential for formulation science. A spectacular situation arises when microemulsions are made to swell by systematically increasing the bending modulus of the surfactant film separating the oil and water regions. In an attempt to realize such extremely swollen microemulsion phases, Peter et al. [Phys. Rev. Lett., 76 (1996) 3866] found a variety of lyotropic phases including a long-range ordered three-dimensional cubic phase over a narrow section of the complex phase diagram of a pseudo-quaternary system composed of decane, brine, octanol, and sodium dodecyl sulfate. In this work, the same region of the phase diagram was reinvestigated using high-resolution small-angle X-ray scattering (HR-SAXS) and rheo-SAXS, which is an important technical aspect for homogenizing the sample and orienting the structural units. Whilst the formation of a swollen two-dimensional hexagonal phase was observed, the structural features of a cubic phase were not detected. The long correlation lengths noted prior were also seen here, over 2000 nm for the hexagonal phase, taken from rheo-SAXS measurements. Based on the measurements covering more than three orders of magnitude in scattering vector, the structure appeared to be an organization of elongated swollen emulsion droplets, which could form an interconnected structure, dense liquid-like order, or further order into a hexagonal morphology with unusually large lattice spacings for a surfactant system.