From ice to CO 2 hydrates and back-study of nucleation and initial growth using scanning electron microscopy

Gas hydrates (GH) form crystalline, non-stoichiometrid compounds belonging to the clathrate hydrate structural family in which gas molecules are trapped in hydrogen bonded water cages. While studies of the formation of GH from liquid water are numerous the formation from ice is less well covered and will be one of the subjects of the present study; the latter has considerable importance for hydrate formation and decomposition in Solar system bodies, in particular for Mars, on which hydrates could play an important role in geomorphological processes. Two main formation stages can be distinguished: (1) an initial nucleation and growth limited stage and (2) a later diffusion-controlled stage. The latter can be easily investigated by gas consumption methods, X-ray or neutron diffraction, which give a good quantitative account of the reactions in a bulk sample. Unfortunately, for the initial phase, the application of these techniques is somewhat limited by the insufficient sensitivity of the methods (with neutron diffraction still performing best); moreover, they do not resolve information on the ongoing nucleation and growth processes like nucleation sites density, shape, size of the formed crystals and their mutual arrangement. The application of an imaging technique like field-emission scanning electron microscopy (FE-SEM) can provide the key to understand these processes, as has been demonstrated in our earlier work in which we developed a shrinking core model for gas hydrate growth. Equally interesting is the microstructural arrangement of ice crystals formed upon clathrate decomposition, in particular for a better understanding of the phenomenon of anomalous(or self-) preservation. In this contribution we will discuss recent results concerning nucleation and initial growth processes on the ice / hydrate surface at various p-T conditions and various degrees of transformation. CryoSEM techniques will be mainly used for this purpose, performed on samples quenched to liq.N2 temperatures and recovered at various stages of the transformation process (socalled “interrupted runs”). Neutron and X-ray diffraction will be used in addition to control the averaged transformation degree as a function of time.