SAXS is an analytical technique that is based on the interaction of monochromatic X-rays with matter. It differs from XRPD in that it is used to examine substances with large d-spacings (1 to 100 nm) and therefore, according to Braggs Law, scatter monochromatic X-rays at low (small) angles. Examples include polymers, liquid crystals, nanoporous materials and microemulsions.

Due to their quasi-crystalline nature, surfactant meso-phases respond well to SAXS analysis. At low concentrations, information can be collected (gained?) describing the size and shape of the molecules and at higher concentrations (from above about 5%) the different mesophases give rise to different SAXS "fingerprints". For example La or "G" phase which has a SAXS profile (left below) - and a corresponding polarised light microscopy image (right below)

Whilst Lβ or "M" phase has a SAXS pattern and optical properties as below:

As we can see, at high concentrations, d-spacings (the size of the repeat unit - in this case the surfactant bi-layer + the solvent (e.g. water) schematic diagram) can be measured and information about phase and composition can be obtained, which is especially important if one is working near phase boundaries. Thermal stability can also be monitored.

Multi-lamellar vesicles (or spherulites) (schematic) can also display distinctive SAXS scatter plots:

This plot also demonstrates the thermal stability of the system. Temperature scans between 0° and 70°C can be made with variable hold and data acquisition times, especially important when products are being transported and stored at different ambient temperatures, or are intended to perform over a range of temperatures.

Below shows a polarised light micrograph of these spherulites together with a cryo-SEM image of similar:

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