Plastic ice in confined geometry: the evidence from neutron diffraction and NMR relaxation

Neutron diffraction and nuclear magnetic resonance (NMR) relaxation studies have been made of water/ice in mesoporous SBA-15 silica with ordered structures of cylindrical mesopores with a pore diameter ∼8.6 nm, over the temperature range 180-300 K. Both measurements show similar depressed freezing and melting points due to the Gibb-Thomson effect. The neutron diffraction measurements for fully filled pores show, in addition to cubic and hexagonal crystalline ice, the presence of a disordered water/ice component extending a further 50-80 K, down to around or below 200 K. NMR relaxation measurements over the same temperature range show a free induction decay that is partly Gaussian and characteristic of brittle ice but that also exhibits a longer exponential relaxation component. An argument has been made (Liu et al 2006 J. Phys:. Condens. Matter 18 10009-28; Webber et al 2007 Magn. Reson. Imag. 25 533-6) to suggest that this is an observation of ice in a plastic or rotationally mobile state, and that there is a fully reversible inter-conversion between brittle and plastic states of ice as the temperature is lowered or raised. More recent detailed NMR measurements are also discussed that allow the extraction of activation enthalpies and an estimate to be made of the equilibrium thickness, as a function of temperature, if the the assumption is made that the plastic component is in the form of a layer at the silica interface. The two different techniques suggest a maximum layer thickness of about 1.0-1.5 nm.