Structural characterization of water and ice in mesoporous SBA-15 silicas: II. The'almost-filled'case for 86 Å pore diameter (original) (raw)
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Journal of Physics: …, 2008
The diffraction results for the formation of ice in 86 °A diameter pores of a SBA-15 silica sample are analysed to provide information on the characteristics of the ice created in the pores. The asymmetric triplet at ∼1.7 °A−1, which involves several overlapping peaks, is particularlyrelevant to the different ice phases and contains a number of components that can be individually identified. The use of a set of three peaks with an asymmetric profile to represent the possibility of facetted growth in the pores was found to give an unsatisfactory fit to the data. The alternative method involving the introduction of additional peaks with a normal symmetric profile was found to give excellent fits with five components and was the preferred analytic procedure. Three peaks could be directly linked to the positions for the triplet of hexagonal ice, Ih, and one of the other two broad peaks could be associated with a form of amorphous ice. The variation of the peak intensity (and position) was systematic with temperature for both cooling and heating runs. The results indicate that a disordered state of ice is formed as a component with the defective crystalline ices. The position of a broad diffraction peak is intermediate between that of high-density and low-density amorphous ice. The remaining component peak is less broad but does not relate directly to any of the known ice phases and cannot be assigned to any specific structural feature at the present time.
Journal of Physics: …, 2006
Neutron diffraction and NMR relaxation measurements have been made of water/ice in SBA-15, a mesoporous silica constituting an ordered array of cylindrical mesopores of pore diameter ∼86 °A, over the temperature range 180–300 K in a cooling and heating cycle. The over-filled sample shows the initial formation of hexagonal ice on the outside of the silica grains, followed by the nucleation of cubic ice inside the pores at a lower temperature. Neutron scattering profiles for the cubic ice peaks are significantly broadened and indicate a defective structure, as observed in previous experiments on ice formation in sol–gel and MCM-type silicas. Below the pore freezing temperature the intensity of the cubic ice peaks exhibit a significant increase, down to the lowest experimental temperature, indicating a reversible conversion of defective ice to ordered ice crystals. The peak profile analysis for the two ice patterns indicates a systematic variation in the position as a function of temperature, giving values of the expansion coefficients that are slightly lower than other measurements for the bulk phase. NMR results on proton relaxation as a function of temperature indicate the presence of a mobile phase for temperatures below pore freezing that supports the view that there is interconversion between brittle and plastic phases of ice.
Physical Chemistry Chemical Physics, 2010
A study has been made as a function of temperature of the phase transformation of water and ice in two samples of mesoporous silica gel with pore diameters of B50 Å . One sample was modified by coating with a layer of trimethylchlorosilane, giving a predominantly hydrophobic internal surface, whereas the unmodified sample has a hydrophilic interface. The pore structure was characterised by nitrogen gas adsorption and NMR cryoporometry and the melting/freezing behaviour of water and ice in the pores was studied by DSC and neutron diffraction for cooling and heating cycles, covering a range of 200 to 300 K. Measurements were made for several filling-factors in the range 0.2 to 0.9. The results show a systematic difference in the form of ice created in each of the samples. The non-modified sample gives similar results to previous studies with hydrophilic silicas, exhibiting a defective form of cubic ice superimposed on a more disordered pattern that changes with temperature and has been characterised as 'plastic' ice . The modified sample has similar general features but displays important variability in the ice transformation features, particularly for the case of the low filling-factor (f = 0.2). The results exhibit a complex temperature-dependent variation of the crystalline and disordered components that are substantially altered for the different filling-factors.
Journal of Physics: Condensed Matter, 2004
Experimental techniques for studying the behaviour of water in confined geometry using neutron scattering and NMR methods are reviewed. A brief survey is given of earlier work on sol–gel silicas and these findings are updated by reference to current work on MCM- and SBA-type silicas. Particular attention is focused on the phase transitions and the relation between supercooled water and ice phases in the confined geometry of the mesopores. The characteristics are found to behave in a systematic manner although the nucleation phenomenon for partially filled pores reveals some unexpected complexity for the SBA silicas.
Nuclear Instruments and Methods in Physics …, 2008
Neutron diffraction is a standard method for determining the structure of matter on an atomic scale; NMR cryoporometry is a recent widely applicable technique for characterising structure on a 2nm to 2 mm scale. An extension of these techniques is described, Neutron Diffraction Cryoporometry (NDC). The information from a set of neutron diffraction measurements of liquids and their crystalline forms in meso-pores, as a function of temperature, is displayed as a cryoporometry graph. The data may then be conveniently interpreted using the Gibbs–Thomson relationship by analogy with the existing technique, NMR cryoporometry. Clear information is thus obtained on the relationship between phase and nano-structure, in a form well suited to further analysis. This method is applied to an equilibrium study of water/ice in SBA-15 templated silicas, as model nano- to meso-structured systems. The method described here uses global pattern matching (a one-dimensional morphing algorithm inside a linear least-squares fitting algorithm) applied to the full range of the diffraction data. This is a rapid method by comparison with the conventional method of fitting individual (overlapping) peaks, and has already led to NMR observations indicating plastic (rotator phase) ice in the same system. r 2007 Elsevier B.V. All rights reserved. PACS: 64.60.i; 81.07.b; 61.12.Ld; 61.18.Fs; 03.75.Hh; 05.70.Fh; 64.60.Qb; 64.70.Dv; 68.03.Cd; 68.08.p; 82.56.Na; 82.56.Ub; 61.43.Gt; 61.46.+w; 82.60.Qr Keywords: Confined geometry; Phase-change; Gibbs–Thomson; Neutron scattering; Neutron diffraction; Plastic ice; NMR; NMR relaxation; Cryoporometry; DSC; Thermoporosimetry; Global fitting; Pattern matching; Morphing; Porous silica
Water confined in a sol-gel network has been characterized by x-ray and neutron diffraction for two samples of mesoporous silica: one with a hydrophilic character (a nonmodified one) and another with a hydrophobic character (a modified one with a methylated internal pore surface). The pore size has been previously characterized [J. Jelassi et al., Phys. Chem. Chem. Phys. 134, 1039] to have a mean pore diameter of approximately 55 Å. The diffraction measurements presented in this paper have been made at room temperature [293 K] for a filling factor of 0.45, giving a mean thickness of 8-9 Å for the water layer. The results show that the local order of the confined water molecules in the intermediate region of 3-6 Å is significantly different from that of the bulk water and also for the two different environments. For the hydrophilic sample, the siloxyl groups at the surface modify the water structure through the effects of interfacial hydrogen-bonding, which influences the orientational configuration of local water molecules and creates a modified spatial arrangement in the pore. In the case of the hydrophobic sample, there is no specific interaction with the pore wall, which is primarily van der Waals type, and the water molecules at the interface are differently oriented to create a hydrogen-bonded network linked more directly to the rest of the water volume. In the present circumstances, the thickness of the water layer has a relatively small dimension so that the interpretation of the measured diffraction pattern is not as straightforward as for the bulk liquids, and it is necessary to consider the effects of diffraction-broadening from a distributed sample volume and also the contribution from cross-terms that remain after conducting a "wet-minus-dry" analysis procedure. These analytic difficulties are discussed in the context of the present measurements and compared with the work of other groups engaged in the study of water confined in different environments. The present results, again, emphasize the complexity influencing the properties of water in a confined geometry and the strong influence of surface interactions on its behavior.
Melting of ice in silica pores
Langmuir, 1989
sharing his data before publication. Most of the experimental work was carried out while W.D.H. was a summer Guest Researcher a t the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, NJ. We also thank Allen Tice of CRREL for valuable discussions and help with the DSC.
Structure of Ice in Confinement: Water in Mesoporous Carbons
Journal of Chemical & Engineering Data, 2016
In this study, the structure of nanoconfined ice and its behavior during the melting process have been investigated. For this purpose, deionized water was inserted into the pores of the ordered carbon structures CMK-3 and CMK-8 having pores of different diameters. The first set of experiments was performed using differential scanning calorimetry (DSC), from which the melting transition temperature of the confined ice was determined. In order to investigate the structure of ice formed inside the mesopores, wide-angle X-ray scattering was used. The measurements were performed at temperatures from 173 K up to and above the pore melting point for each system. The results of the XRD experiments showed features characteristic of both hexagonal, I h , and cubic, I c , ice at temperatures below the melting point. The structure of the confined ice corresponds to disordered stacking ice layers, ice I sd , and our results agree well with recent simulations of X-ray diffraction of such ice crystals by Murray and co-workers.
Journal of Applied Crystallography, 2010
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.