(614d) Structure of Ice in Confinement; Water in Mesopores
We report X ârays diffraction studies of water adsorbed in nanoporous activated carbon fibres (ACFs) and CMK-3 and CMK-8 carbon mesopores of different pore sizes. The fibres are built of turbostratic nanoparticles separated by quasi two-dimensional voids, forming narrow slit-shaped pores; CMK-3 and CMK-8 are the reverse carbon replica of silica SBA-15 and KIT-6 porous matrices. In order to determine the structure of water within the pores and its influence on the fibresâ structure, mean interatomic and intermolecular distances have been estimated from the positions of the maxima of the normalized angular distribution functions obtained by X-ray diffraction . We observe significant changes in the interlayer distance of the carbon nanoparticles; the results suggest that very high pressures arise within the pores,of the order a few hundreds MPa, as has been observed in molecular simulations [1,2]. Such results are confirmed by observation of high pressure forms of ice in cylindrical nanocarbons , using neutron and X âray diffraction . For water in multi-walled carbon nanotubes (MWCN), below the pore melting point cubic ice was observed. This structure of ice can be obtained during re-crystallisation from high pressure phases at low temperature for bulk water. For CMK-3 and CMK-8 mesopores we observed the existence of a stacking-disordered ice, Isd [4,5]. This metastable ice is neither cubic nor hexagonal, and is not a simple mixture of the two, but a combination of cubic sequences intertwined with hexagonal sequences, which was identified as having the space group p3m1 . Moreover, the stacking disorder can vary in complexity depending on the way the ice is formed and on the prevailing thermal conditions during this process. An analysis of the kind of Isd formed in CMK-3 and CMK-8 of different pore sizes will be presented. These crystal forms, which occur in bulk water only at temperatures below 180 K in the case of cubic ice, and at pressures of hundreds or thousands of MPa are stabilized by the confinement.
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