Confined water (blue) in a silica (red and yellow) pore. Water near the pore edge has reduced density, caused by the strong orientational surface interaction.
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The structure of confined water
Water confinement occurs widely in nature, in areas ranging from biophysics (protein folding, enzyme activity) to geology (water in clays, hydrate formation in sediments). Confinement substantially modifies the bulk properties of water, e.g. lowering the freezing point and raising the boiling point. Hitherto, surprisingly little was known about water structure in confinement. We have therefore investigated the structure of confined water, using vycor, a silica glass with a random network of ~40Å pores, as the confining medium. Earlier neutron diffraction measurements have been coupled with a new Empirical Potential Structure Refinement simulation which includes a model of both the confined water and the surrounding amorphous silica. There are on average only ~2.2 hydrogen bonds per water molecule in confined water, as compared to ~3.6 in bulk water. The second water coordination shell also moves markedly inwards by ~0.6Å. It appears that confined water structure is dominated by the surface layer, where hydrogen bonding to the vycor OH groups causes a dramatic reorientation of the water molecules.
H Thompson (ISIS and UCL), AK Soper (ISIS), MA Ricci (Universitá di Roma III), F Bruni (Universitá di Roma III) and NT Skipper (UCL)
Research date: December 2006
Dr H Thompson, [H.Thompson@rl.ac.uk] F Bruni et al., J Chem Phys 109 (1998) 1478
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