Structural analysis of the lyotropic phases in alpha-olefin sulfonate as a function of water content and temperature.

Alpha olefin sulfonate (AOS) is a widely used anionic surfactant, yet its phase behaviour has not previously been systematically mapped. Here, a comprehensive phase diagram is constructed over a concentration range of 2-70 wt% and temperatures from 25 to 80 °C using cross-polarized optical microscopy and small-angle X-ray scattering (SAXS). Polarized microscopy was used to delineate isotropic and anisotropic regions and identify phase boundaries, while SAXS provided quantitative structural characterisation, including lattice parameters, electron density reconstructions, and micellar size and shape analysis. Measurements were performed in sealed capillaries under controlled thermal conditions, with refined temperature increments used to resolve phase transitions. Five distinct phases were identified with decreasing water content: a micellar dispersion; a 2D hexagonal phase; a 2D ribbon phase; a previously unreported 3D hexagonal phase; and two lamellar phases. This temperature- and concentration-dependent sequence is structurally analogous to that of sodium dodecyl sulfate (SDS), though key distinctions arise from the molecular architecture of AOS, which promotes unique intermediate mesophases through differences in molecular packing and constrained hydration. Structural analysis reveals a systematic evolution of spatial water organisation, progressing from excess bulk water in the micellar phase, to tubular confinement within hexagonal and ribbon phases, to discrete water pockets embedded within surfactant bilayers in the 3D hexagonal phase, and finally to residual interfacial hydration in the lamellar phases. This work establishes the relationship between hydration state and mesophase structure, and provides the first complete structural phase diagram for this important surfactant system, enabling prediction of phase behaviour under the complex conditions encountered in industrial processing.