Unusual Structural Insights Revealed by Rheo–SAXS Studies of Nonaqueous Crystalline Gels

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Glycerol is a nonaqueous polar solvent and is of interest in many industrial areas due to its beneficial properties, such as green production and biocompatibility. Our previous works have shown the presence of a fibrillar phase on the microscale that consists of lamellar sodium dodecyl sulfate (SDS) crystals containing interstitial glycerol on the nanoscale. The phase is gel-like at room temperature and demonstrates shear-thinning behavior upon application of a shear. Initially, small-angle X-ray scattering coupled with rheology (rheo–SAXS) measurements were performed to elucidate the structural transition of the gel phase under an applied shear, but it became clear that the aging process of the gel has a profound impact on both the gel nanostructure and also the mechanical properties. For younger gels, both the dissolution of SDS crystallites and the alignment of the fibrillar phase were seen. However, in the aged gels, an unexpected foam was formed at shear rates $\dot{\gamma }$ > 700 s^–1. The microscopic structure of the foam phase was imaged using polarizing light microscopy and brightfield and darkfield optical microscopy. The nanostructure of the foam phase was investigated using rheo–SAXS. The foam phase was shown to be stabilized by the presence of SDS crystallites at the air–liquid interface, and the stability of the foam is high with foam persisting even t = 3 months after formation. These results highlight the importance of investigating green nonaqueous media and the gel aging process, both of which are interesting not only on a fundamental level but also for a range of industrial applications, from personal care products and cosmetics to food science.