The present thesis presents a systematic study of the influence of confinement on structural properties of water at low temperatures. Mesoporous silica materials with cylindrical pores on a two-dimensional hexagonal lattice (MCM-41 and SBA-15) represent ideal model systems for analysing the phase behaviour of water in nano-confinement. Using materials with seven different pore sizes ranging between 2 - 9 nm, the low-temperature behaviour of confined water was studied by means of X-ray, neutron, and Raman scattering. The obtained results show that water confined in silica mesopores represents an exceedingly complex system. The data suggest a non-homogeneous water occupancy of the available confinement volume. At least two spatially separated water phases exist in hydrophilic mesopores, i.e. a non-freezable wall layer, and a structurally different water phase in the core of the pores. The development of a distinct tetrahedral hydrogen-bonded water network upon cooling was found only in the core part of the pores. This effect was found to result in significantly different water density values attributed to the respective phase. Moreover, the inner part of confined water was shown to undergo considerable structural changes with decreasing pore size. Additional cooling and heating measurements revealed that the liquid-solid phase transition of the inner water is accompanied by a density minimum situated at the freezing/melting temperature which depends on confinement dimensions. This density minimum was interpreted as the crossover between the anomalous density behaviour for liquid water and the normal density change with temperature for ice. The observed temperature-induced mesopore lattice deformation was described by a novel approach, introducing the negative Laplace pressure as the pressure difference between the liquid and the solid phase. The influence of water on the mesoporous structure of the used materials was analysed within the first application of a newly developed laboratory apparatus allowing long-term in situ phase transition studies by means of small-angle X-ray scattering. This study revealed distinct silica matrix modification of SBA-15 material upon repeated water sorption. The obtained data suggest that pore lattice deformation occurring during water capillary condensation and evaporation irreversibly changes the silica matrix by partially enclosing water within collapsed pore regions.
|Translated title of the contribution||Wassereigenschaften in begrenzter Geometrie|
|Publication status||Published - 2012|
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