Time Resolved Small-Angle X-ray Scattering of Lyotropic Phase Evolution in Drying Levitated Aqueous Alpha Olefin Sulfonate Droplets

Abstract Aqueous surfactant dispersions exhibit rich lyotropic polymorphism as a function of concentration and temperature, yet their structural evolution under the nonequilibrium conditions encountered during drying remains poorly understood. Here, we tracked the phase behavior of acoustically levitated, drying droplets of the commercial anionic surfactant alpha olefin sulfonate (AOS) using in situ time-resolved small-angle X-ray scattering combined with cross-polarized microscopy. Droplets of 20 and 40 wt % AOS were dried at 40 and 60 °C, and each pattern was assigned against recently published equilibrium crystallographic data, allowing coexisting phases to be resolved from characteristic diffraction peaks. Evaporation established a radial concentration gradient that, for the 20 wt % droplet at 60 °C, induced the sequence micellar → hexagonal (within 16 s) → ribbon, with two hexagonal phases coexisting at ∼40 wt % (d10 = 4.83 nm) and ∼55 wt % (d10 = 4.33 nm) and a ribbon crust at ∼60 wt %. Formation of a dense lamellar crust then impeded water transport from the core, transiently swelling the interior hexagonal phase (d10 increasing from 4.80 to 4.95 nm at 20 wt % and 4.72 to 4.90 nm at 40 wt %) before its collapse. A 3D-hexagonal phase formed only at the lowest evaporation rate (20 wt %, 40 °C). In all cases the droplets dried within ∼1000 s into a fully lamellar shell (Bragg ratio 1:2:3) enclosing a hollow core, demonstrating how evaporation rate and initial concentration govern the nonequilibrium phase pathway relevant to surfactant spray-drying.