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Studies of nano-structured liquids in confined geometry and at surfaces.

By J. Beau W. Webber

Abstract

This is a progress report on elucidating the behaviour of liquids,\ud in particular water, in confined geometry on the nano- to mesoscale,\ud and at interfaces. There are important measurements still\ud to make, conclusions still to be drawn, and above all leaps of\ud understanding still to be made. However, a number of important\ud features in the behaviour of these systems have recently become\ud clearer.\ud Nano-structuring of liquids and their crystals changes their\ud Gibbs free energy, and hence their dynamics. This may most readily\ud be probed by monitoring the alteration of phase changes as a\ud function of temperature, together with changes in other parameters,\ud particularly the confinement diameter. Such studies may be\ud performed by monitoring the change in the pressure (at constant\ud temperature) of the liquid in its own vapour (Kelvin equation), or\ud by monitoring the change in the freezing/melting temperature\ud (at constant pressure) of a crystal in its own liquid (Gibbs–Thomson\ud equation).\ud In the latter case the melting and freezing temperatures of liquids\ud are modified by the changes in the volumetric Gibbs free energy\ud due to nanostructuring; this is related to the surface energy of\ud the curved interface between the crystal and its own liquid. This is\ud thus dependent on the geometry of the interface between the crystal\ud and its liquid. There is still discussion on this point as to the exact\ud geometric constants and functional forms that are applicable\ud for different confining geometries. Experimental evidence is presented\ud for the cases of cylindrical pores (SBA-15), and for pores\ud that on average are spherical (sol–gel). However, reconciling this\ud comparative data with melting/freezing temperatures in each of\ud these systems still pose a number of questions.\ud It is well known that bulk brittle ice has a hexagonal structure,\ud while brittle ice that forms in pores may be cubic in structure [1,2],\ud Figs. 10 and 11. Adjacent surfaces appear to further alter the\ud dynamics and structure of confined liquids and their crystals, leading\ud in the case of a water/ice system to a state of enhanced rotational\ud motion (plastic ice) just below the confined freezing/\ud melting transitions. This plastic ice layer appears to form at both\ud the ice–silica interface and the ice–vapour surface, and reversibly\ud transforms to brittle ice at lower temperatures. There is good evidence\ud to suggest that the plastic ice at a silica interface transforms\ud to cubic ice, while the plastic ice at vapour surfaces transforms to\ud hexagonal ice. That this plastic ice may correspond to a layer at the\ud crystal surface is suggested by the presence of only amorphous ice\ud in confined systems with small dimensions (<3 nm diameter),\ud whereas systems with larger dimensions (10 nm) contain brittle\ud cubic ice and also some hexagonal ice (if a vapour interface is present);\ud even larger systems (>30 nm) contain predominately hexagonal\ud ice. It is conjectured that this layer of plastic ice at vapour\ud surfaces may be present at the myriad of such interfaces in macroscopic\ud systems, such as snow-packs, glaciers and icebergs, and may\ud be an explanation for the need for plastic terms in the macroscopic\ud dynamical models of these systems [3].\ud These results also point the way forward for a wide-range of\ud cryoporometric metrology studies of systems that are ‘difficult’\ud for NMR, such as high iron content clays and rocks, as well as aged\ud concrete. Results are presented for cryoporometric measurements\ud on meteorite samples with a significant metallic content, exhibiting\ud T2 relaxation times down to 2.5 us

Topics: QC176.8.N35
Publisher: Elsevier
Year: 2010
OAI identifier: oai:kar.kent.ac.uk:25821

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