The instrument*, unique in the "French Great-West", makes it possible to study materials from atomic to mesoscopic lengthscales. The X-ray generator is a rotating anode (Cu) having by construction two beam lines, both equipped with large two-dimensional detectors (image plates). The small and medium angle X-ray scattering beam line is dedicated to the study of soft matter (detergency, cosmetics, pharmaceutics...), colloids, polymer materials, etc... Once finished (during the next months) this beam line can be used in two configurations, one with high beam intensity (curved mirror, already set up) and one with high resolution (curved mirror + germanium monochromator). The two-dimensional detector is mobil, allowing variations in the sample-detector distance, D, from 0.2 to 2m (WAXS to SAXS). A vaccum tube placed between the sample and the detector limits the beam absorption by air for large distances D. Equipment for sample environment is under construction and will evoluate with the needs.
This high performance research instrument is accessible on request to researchers from the public or private sector. For more information please contact the scientist in charge of the facility: Prof. P. Méléard, Tel. (33) 2 23 23 80 78 or firstname.lastname@example.org
* The facility was financed by a government-region contract of the ENSCR and by supplements from the CNRS, the university Rennes 1 and the university of South Brittany. Under the scientific authority of P. Méléard, the construction of this instrument is done with the scientific and technical help of several laboratories: the UMR CNRS 6052 "Synthesis and Activation of Biomolecules" based at the ENSCR, UMR CNRS 6626 "Group of condensed matter and materials" of the University Rennes 1 and the EA 2592 "Laboratory of Polymers and Proceedings" of the University of South Brittany.
Self-assembly is frequently encountered with soft matter. Amphiphilic molecules (surfactants, phospholipids, ect...) self assemble to form micelles, hexagonal (A), lamellar (B), cubic phases, etc... As the characteristic distances of such phases (red arrows) are large compared to the wavelength (1.54Å), a specific small angle equipment is necessary to do X-ray scattering. For example, the first order diffraction of a lamellar phase having a repeat distance of about 100Å is observed at an angle lower than 1°, explaining the need for large sample-detector distances.
Another peculiarity of soft matter is related to the weak variation in the
electronic density of dispersed aqueous/fatty matter, resulting in weak X-ray
scattering intensities. The facility, conceived for the study of such
materials, is thus equipped with a high beam intensity set-up (low
resolution) for low contrast systems and a high resolution set-up for
medium contrast systems (example: phospholipids).
| Generally, supramolecular complex
systems possess large characteristic distances. In the case of
the system seen at the left, a lamellar phase constrains the
cylinders that are intercalated between the bilayers. The cylinders are
further sufficiently concentrated so that their long axes are on average
parallel: both, a smectic order and a nematic order are observed in this
mixture of phospholipids and DNA fragments (see Pott
& Roux FEBS Letters 551 (2002) 150-154).
The precise analysis of small angle X-ray images gives important information about local molecular organisation like the repeat distance (red arrow), a correlation distance (blue arrow), the electron density (superposed green curve)...
|The gathered information will depend on the exact type of the sample with characteristic distances reaching from tenths of Anströms to tenths of nanometers. One might for instant deal with composite polymer materials to get an idea about the intercaled mineral organisation. The field of application is huge covering numerous areas of fundamental or applied research: formulation in all domains of innovative applied reseach (ink and paint, food science, detergency, cosmetics...), nanostructured materials, synthetic vectors, etc...|
|Quite often supramolecular complex systems are also organized on the molecular scale. This is what one can see on the X-ray image (left) obtained with a dispersion of multilamellar liposomes. One detects a correlation peak in the wide angle region (towards the outside of the detector) indicating a liquid organization of the aliphatic chains in the hydrophobic bilayer interior (typically in the range of 5Å). A single experiment in the small to mean angle region (SMAXS : about 1-25°) allows to study the whole set of different degrees of organization.|
|Soft matter has often an organization that leads to powder type diffractions (spherical distribution). Yet, one may obtain macroscopically oriented samples as a function of sample preparation (mechanical or thermal treatments...). Such an orientation can extend over several microns (in the case of a phospholipid lamellar phase, several million bilayers). We then have a "soft" monocrystal whose orientation with respect to the incident beam can be changed by the user.|
|In our group we possess a vast expertise in the preparation and characterization (SAXS and SMAXS) of oriented samples based on phospholipids. The principal advantage of this orientation is related to the tiny quantities of material necessary to obtain high quality diffractions that can be used for the reconstruction of the electronic density profile. The use of thin films on a curve substrate is a technique of noteworthy advantage as it limits the acquisition time and allows high resolution diffraction measurments. It is then possible to get more precise information on the molecular organization like, for example, the bilayer thickness (HTH), the unit cell of a rippled phase, etc...|
|The image on the left side (SAXS) is obtained with a phospholipid system characterized by the coexistence of a lamellar and a hexagonal phase. The sample contains about 1mg of material in the form of a thin layer and the peak resolution corresponds to the intrinsic resolution of the system. On may further note that the long axes of the hexagonal phase tubes are parallel to the membrane plane of the lamellar phase.|