The temperature dependence of water confined in the ordered cylindrical nanopores of MCM-41 and SBA-15 materials is studied by means of Raman scattering for different pore sizes covering a diameter range from 2.0 to 8.9 nm. The liquid-solid phase transition temperature of water in confinement can be determined by the analysis of the mode contribution in the OH-stretching region. For pore sizes down to 3 nm, the freezing/melting point depression with decreasing pore size can be consistently described by a modified Gibbs-Thomson equation, with a nonfreezable water layer of 0.6 nm (about two monolayers) close to the pore walls. When the pore size is 2.5 nm or smaller, indication for a first-order phase transition can no longer be found that is in agreement with previously reported differential scanning calorimetry measurements on the same samples. The Raman data further suggest that two spatially separated water phases exist in the smallest pores, i.e., the nonfreezable wall layer and a structurally different water phase in the core of the pores. A distinct tetrahedral hydrogen-bonded network of water molecules is found only in the core part of the pores. In the weakest confinement (8.9-nm pore diameter), the core water is shown to be compatible with crystalline ice with a spectral fingerprint similar to bulk ice. In strong confinement (2.0-nm pore diameter), the core water shows a spectral fingerprint identical to low-density amorphous ice, and there is a gradual transition between these two extremes. These findings suggest that the core part of confined water undergoes considerable structural changes with decreasing pore size, leading us to question recent proposals that aim to extract information about the state of bulk liquid water in the "no man's land" from water in confinement.

A simple explanation is given for the low-temperature density minimum of water confined within cylindrical pores of ordered nanoporous materials of different pore size. The experimental evidence is based on combined data from in-situ small-angle scattering of X-rays (SAXS) and neutrons (SANS), corroborated by additional wide-angle X-ray scattering (WAXS). The combined scattering data cannot be described by a homogeneous density distribution of water within the pores, as was originally suggested from SANS data alone. A two-step density model reveals a wall layer covering approximately two layers of water molecules with higher density than the residual core water in the central part of the pores. The temperature-induced changes of the scattering signal from both X-rays and neutrons are consistent with a minimum of the average water density. We show that the temperature at which this minimum occurs depends monotonically on the pore size. Therefore we attribute this minimum to a liquid-solid transition of water influenced by confinement. For water confined in the smallest pores of only 2 nm in diameter, the density minimum is explained in terms of a structural transition of the surface water layer closest to the hydrophilic pore walls.

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Abstract:Experimental results on sorption-induced deformation during npentane desorption were obtained by in-situ dilatometry and in-situ small angle X-ray scattering (SAXS). The sample investigated was a silica-based monolith with hierarchical pore structure comprising a macroporous network of struts, each strut containing well-defined cylindrical mesopores ordered on a 2D hexagonal lattice. In-situ dilatometry and in-situ SAXS measurements revealed strain isotherms of similar shape, which are qualitatively in good agreement with recent theoretical predictions. From the relative pressure range of the liquid filled mesopores a pore load modulus of 1.5 GPa is determined. The relative pressure region of mono-and multilayer formation, however, reveals differences between the two independent methods. In particular, the net strain at saturation pressure is considerably larger for in-situ dilatometry. We attribute this observation to the different sensitivity of the two methods to anisotropic deformation in the hierarchical solid framework. While in-situ SAXS measures the mesopore lattice strain and is therefore exclusively sensitive to radial deformation of the struts, dilato-*Corresponding authors: Oskar Paris,

The capillary condensation and capillary emptying of water and perfluoropentane in ordered mesoporous SBA‐15 silica is studied by in‐situ small‐angle neutron scattering (SANS). The SANS data can be perfectly described by a simple analytical model for spatially random pore filling (Laue scattering) for the entire range of pore‐filling fractions. From this it is concluded that recently proposed pore correlations due to elastic interactions between neighbouring pores upon capillary condensation do not play a role in this system. The pores fill randomly according to their size distribution, in perfect agreement with the classical Kelvin equation. The relation between the overall pore‐filling fraction as determined from the volumetric sorption isotherm, and the fraction of completely filled pores as obtained from the fit of the SANS data, allows conclusions to be drawn about the thermodynamic metastability of the adsorption process.