Scientific Highlight September 2007

X-ray Reflectivity from Free-Liquid Surfaces

Andrew Nelson and Tracey Hanley (ANSTO)

Our X-ray reflectometer has recently been commissioned to examine reflectivity from free-liquid surfaces. This will allow the instrument to perform exciting work in many more areas, such as protein / lipid / surfactant adsorption at interfaces. Problems with evaporation, vibration and acoustic noise have been solved in the pragmatic Heath-Robinson tradition with a specially designed sample can. The sand is necessary to damp low frequency vibrations passed up through the instrument.

The sample and instrument are stable enough to measure both specular and off-specular scattering, from the air-liquid interface in a quick and easy manner. The diagrams above show the reflectivity (and fits) from pure water and a sodium dodecyl sulphate (SDS) solution. As expected the pure water has a roughness of 2.8A - the literature value. SDS adsorbs at the air-water interface and forms a 21A thick layer. Each of the specular curves were measured in around 2 minutes, with the off-specular scattering taking about an hour. Future advances in this sample environment will include temperature control and variable gas/humidity atmospheres. We aim to access temperature ranges between 0 and 150 degrees.