Synchrotron radiation studies of hydrates under extreme conditions

ABSTRACT
First a basic introduction into the research with synchrotron radiation, i.e. with x-rays of ultra high intensity is given. Then the structure formation at the water-gas interface is monitored by high-energy x-ray scattering as an example. Hydrate formation, i.e. the formation of water cages with trapped guest molecules (see left figure below), is expected under certain conditions of temperature and pressure. Hydrates are important materials since most of the carbon on earth is stored as methane-hydrate in deep sea areas (so-called "burning ice", see right figure below). It is shown by x-ray reflectivity and scattering measurements that CO2-hydrate formation can only be observed at the liquid-liquid interface. For the first time, the microscopic nucleation process of such materials was directly observed. The measurements were performed at the ESRF in Grenoble and at the synchrotron radiation source DELTA on the campus of the Technische Universität Dortmund. At liquid-gas interfaces a condensation of liquid layers without any indications for hydrate pre-structures is measured. For other gases such as propane or xenon a completely different behaviour was found. Different microscopic nucleation models for hydrates will be discussed and computer simulations are compared with the data.