Neutron / X-ray Scattering in Thin Layers at Interfaces

Neutron/X-Ray Scattering in Thin Layers at Interfaces

To study features smaller than the wavelength of light, we use two experimental techniques that utilize the scattering of x-rays and neutrons: X-ray/neutron reflectivity and X-ray grazing incidence diffraction (GID). Parameters such as layer thickness, density, and interfacial roughness can be determined by characterizing the surface’s reflectivity-the intensity ratio of neutrons or X-rays scattered from the surface relative to the intensity of the incident beam. X-ray GID allows the characterization of thin crystalline layers at an interface and can provide in-plane (lateral) structural information. When supported by a smooth interface, these techniques allow structural determination in layers 5Å to 1mm thick. Systems we have studied with these techniques include:

  • Neutron Scattering Measurements of Confined Complex Fluids-In this work, we use a novel apparatus, the Neutron Confinement Cell, to measure the molecular density and orientation of confined, ultra-thin complex fluids under static and dynamic flow conditions. Our studies focus on determining the structure of adsorbed polymer diblock films (polymer brush layers) at the solid-solution interface.
  • Cholera Toxin’s Assault on Lipid Monolayers-The toxin’s effect on membrane structure is modeled using phospholipid monolayers at the air-water interface through X-ray reflectivity and GID, and correlated with changes in protein structure during activation as determined by neutron reflectivity.
  • Characterization of Lipid Bilayers at the Solid-Liquid Interface-In these studies, we deposit model membranes in the form of lipid bilayers on ultra polished substrates. We then study the properties of lipid rafts and gel-phase domains.
scattering1 Reflectivity Geometry:
An incoming beam reflects from the interface of each thin film at an angle equal to the incident angle. Beams reflected from each interface can then constructively and destructively interfere with one another to produce minima and maxima in the reflectivity curve. These features are then analyzed to determine the thickness and density of each layer.

Interfacial forces and molecular structure are simultaneously characterized using the Neutron Confinement Cell; i.e., for polymer brush layers as shown.