8 Neutron and X-ray diffraction studies on complex liquids

Abstract

The above examples illustrate the extent to which present day neutron and X-ray diffraction methods are being used to determine interatomic structure in a wide range of liquid and amorphous systems. The determination of pair radial distribution functions not only offers a means to characterise different structures in liquids, but also provides theorists with information to construct realistic model potentials that can be used to calculate macroscopic behaviour and structural properties in regimes not currently accessible to experiment. The well-established NDIS difference methods remain superior to all other methods for the determination of interatomic pairwise structure. The relatively new AXD (or DAS) difference methods have the potential to answer long-standing questions about the structure around species with mass number greater than about 30. However, the relatively low X-ray scattering power from light elements such as hydrogen, carbon, nitrogen etc. means that it will never be possible to resolve completely structures of biologically important liquids by X-ray methods alone. EXAFS spectroscopy has the distinct advantage over both diffraction techniques as it can be used to study local structure around particular species at high dilution. Therefore studies which combine reference data from AXD or NDIS, with extensive EXAFS data, are likely to be useful in studies of structure in regimes which prove difficult for AXD and NDIS. It is clear that no one method will be sufficient to resolve structure at the required level of detail around all species in a complex liquid. Instead one must rely on a full complement of diffraction and other techniques including computer simulation to determine the complete atomic structure of a complex liquid or amorphous system. On the technical front, the construction and commissioning of new neutron diffractometers with higher count rates, such as D20 and D4C at ILL, and GEM at ISIS with an optimised sample environment for work at non-ambient conditions, will enable new and more extensive research to be undertaken. Additionally, the new custom-built X-ray diffractometer for liquids proposed for the DIAMOND synchrotron being established at RAL will provide a much-needed boost for wide-ranging AXD and EXAFS investigations of complex liquids. Besides the many studies of immediate interest suggested at the end of some sections, there are several investigations that will become feasible in the longer term as the technology develops. These include 1. the use of isotopes such as 12C and 13C and 33S and 32S which will enable detailed and extensive structural studies to be carried out on a wide range of biologically significant materials, and 2. the exploitation of higher neutron and X-ray count rates to facilitate real time experiments to investigate changes of structure as a chemical or biochemical reaction occurs. The one strong theme which emerges from all the work described in this paper is that diffraction, especially that based on difference techniques, remains the best means to determine structure at atomic resolution in complex liquids.