Keywords: SAXS, WAXS, emulsions, surfactants, microphase separation, nanoparticles
Summary:Many personal care products, cosmetics and edibles are often concentrated surfactant systems and stabilized emulsions (which may contain solid nanoparticles). A common feature is that they have an associated multiphase microstructure, often related to function (texture, spreadability, taste, etc.). To tune the product performance and control properties during the production process, an intimate knowledge of the associated long and short range molecular structure is required. Furthermore, evolution during the use of the product, or stability under storage, also requires structural characterization to ensure or quantify limited degradation. We will present Small Angle and Wide Angle X-ray Scattering (SAXS/WAXS) as a powerful technique to elucidate the hierarchical structures observed in formulations over length scales from 1 to several hundred nanometers. The strength of the technique over optical or electron-based methods is that no special sample preparation is required, and optically opaque or embedded structures can be studied; for example, even dry dense powders. Over the past decade, new developments in hardware (x-ray sources, detectors and collimation) and software have made the technique accessible to laboratory users interested in nanoscale structure without the need for prior x-ray scattering experience. In this short talk, we will present a heuristic view of SAXS/WAXS to understand the nature of the measurement and how a researcher can 1) extract characteristic sizes and even shapes of microphase-separated regions, 2) simultaneous packing order and phase within lipid bilayer structures, 3) observation of structural hierarchies inherent throughout the length scales interrogated by SAXS. Canonical examples will be provided in personal care (shampoo) and an edible (chocolate). To extend the range of length scale, we briefly describe UltraSAXS which can measure structure sizes in the micron range, thus overlapping with optical probes.