70 research outputs found
Hydraulic Transport Across Hydrophilic and Hydrophobic Nanopores: Flow Experiments with Water and n-Hexane
We experimentally explore pressure-driven flow of water and n-hexane across
nanoporous silica (Vycor glass monoliths with 7 or 10 nm pore diameters,
respectively) as a function of temperature and surface functionalization
(native and silanized glass surfaces). Hydraulic flow rates are measured by
applying hydrostatic pressures via inert gases (argon and helium, pressurized
up to 70 bar) on the upstream side in a capacitor-based membrane permeability
setup. For the native, hydrophilic silica walls, the measured hydraulic
permeabilities can be quantitatively accounted for by bulk fluidity provided we
assume a sticking boundary layer, i.e. a negative velocity slip length of
molecular dimensions. The thickness of this boundary layer is discussed with
regard to previous capillarity-driven flow experiments (spontaneous imbibition)
and with regard to velocity slippage at the pore walls resulting from dissolved
gas. Water flow across the silanized, hydrophobic nanopores is blocked up to a
hydrostatic pressure of at least 70 bar. The absence of a sticking boundary
layer quantitatively accounts for an enhanced n-hexane permeability in the
hydrophobic compared to the hydrophilic nanopores.Comment: 15 pages, 7 figures, in press, Physical Review E 201
Capillary rise of water in hydrophilic nanopores
We report on the capillary rise of water in three-dimensional networks of
hydrophilic silica pores with 3.5nm and 5nm mean radii, respectively (porous
Vycor monoliths). We find classical square root of time Lucas-Washburn laws for
the imbibition dynamics over the entire capillary rise times of up to 16h
investigated. Provided we assume two preadsorbed strongly bound layers of water
molecules resting at the silica walls, which corresponds to a negative velocity
slip length of -0.5nm for water flow in silica nanopores, we can describe the
filling process by a retained fluidity and capillarity of water in the pore
center. This anticipated partitioning in two dynamic components reflects the
structural-thermodynamic partitioning in strongly silica bound water layers and
capillary condensed water in the pore center which is documented by sorption
isotherm measurements.Comment: 4 pages, 3 figure
Molecular dynamics of n-hexane: A quasi-elastic neutron scattering study on the bulk and spatially nanochannel-confined liquid
We present incoherent quasi-elastic neutron scattering measurements in a
wavevector transfer range from 0.4 AA^{-1} to 1.6AA^{-1} on liquid n-hexane
confined in cylindrical, parallel-aligned nanochannels of 6 nm mean diameter
and 260 micrometer length in monolithic, mesoporous silicon. They are
complemented with, and compared to, measurements on the bulk system in a
temperature range from 50K to 250K. The time-of-flight spectra of the bulk
liquid can be modeled by microscopic translational as well as fast localized
rotational, thermally-excited, stochastic motions of the molecules. In the
nano-confined state of the liquid, which was prepared by vapor condensation, we
find two molecular populations with distinct dynamics, a fraction which is
immobile on the time scale of 1ps to 100ps probed in our experiments and a
second component with a self-diffusion dynamics slightly slower than observed
for the bulk liquid. No hints of an anisotropy of the translational diffusion
with regard to the orientation of the channels' long axes have been found. The
immobile fraction amounts to about 5% at 250K, gradually increases upon cooling
and exhibits an abrupt increase at 160K (20K below bulk crystallization), which
indicates pore freezingComment: 10 pages, 7 figure
Melting and freezing of argon in a granular packing of linear mesopore arrays
Freezing and melting of Ar condensed in a granular packing of template-grown
arrays of linear mesopores (SBA-15, mean pore diameter 8 nanometer) has been
studied by specific heat measurements C as a function of fractional filling of
the pores. While interfacial melting leads to a single melting peak in C,
homogeneous and heterogeneous freezing along with a delayering transition for
partial fillings of the pores result in a complex freezing mechanism
explainable only by a consideration of regular adsorption sites (in the
cylindrical mesopores) and irregular adsorption sites (in niches of the rough
external surfaces of the grains, and at points of mutual contact of the powder
grains). The tensile pressure release upon reaching bulk liquid/vapor
coexistence quantitatively accounts for an upward shift of the
melting/freeezing temperature observed while overfilling the mesopores.Comment: 4 pages, 4 figures, to appear as a Letter in Physical Review Letter
In-situ-SANS investigations of C5F12 condensation in mesoporous silicas with a hierarchical pore structure
Neutron diffraction on methane and hydrogen hydrates under high pressure
Gas hydrates are crystalline solids composed of water and gas. They have attracted considerable attention over the past decade both for their geophysical relevancy [1] and for their possible application to gas storage [2]. Pressure is a key parameter in the study of these systems as gas hydrates are believed to exist at pressure in nature and the gas content is found to increase in gas hydrates as their crystalline structure rearranges upon compression. In addition, high-pressure studies on gas hydrates offer new possibilities to explore water-gas interactions.
We will present recent work on methane and hydrogen hydrates at high pressure performed by neutron diffraction in the GPa range [3]. Several issues including the gas content in the different high-pressure structures will be discussed
Effects of Cr Doping and Water Content on the Crystal Structure Transitions of Baâ‚‚Inâ‚‚Oâ‚…
Temperature-dependent crystal structure alterations in the brownmillerite-type material Ba₂In₂O₅ play a fundamental role in its applications: (i) photocatalytic CO₂ conversion; (ii) oxygen transport membranes; and (iii) proton conduction. This is connected to a reversible uptake of up an equimolar amount of water. In this study, in situ X-ray and neutron diffraction were combined with Raman spectroscopy and solid-state nuclear magnetic resonance experiments to unravel the effects of Cr doping and water content on the crystal structure transitions of Ba₂In₂O₅(H₂O)x over a wide temperature range (10 K ≤ T ≤ 1573 K, x < 1). A mixture of isolated and correlated protons was identified, leading to a highly dynamic situation for the protons. Hence, localisation of the protons by diffraction techniques was not possible. Cr doping led to an overall higher degree of disorder and stabilisation of the tetragonal polymorph, even at 10 K. In contrast, a further disordering at high temperatures, leading to a cubic polymorph, was found at 1123 K. Cr doping in Ba₂In₂O₅ resulted in severe structural changes and provides a powerful way to adjust its physical properties to the respective application
Illuminating solid gas storage in confined spaces – methane hydrate formation in porous model carbons
Methane hydrate nucleation and growth in porous model carbon materials illuminates the way towards the design of an optimized solid-based methane storage technology. High-pressure methane adsorption studies on pre-humidified carbons with well-defined and uniform porosity show that methane hydrate formation in confined nanospace can take place at relatively low pressures, even below 3 MPa CH4, depending on the pore size and the adsorption temperature. The methane hydrate nucleation and growth is highly promoted at temperatures below the water freezing point, due to the lower activation energy in ice vs. liquid water. The methane storage capacity via hydrate formation increases with an increase in the pore size up to an optimum value for the 25 nm pore size model-carbon, with a 173% improvement in the adsorption capacity as compared to the dry sample. Synchrotron X-ray powder diffraction measurements (SXRPD) confirm the formation of methane hydrates with a sI structure, in close agreement with natural hydrates. Furthermore, SXRPD data anticipate a certain contraction of the unit cell parameter for methane hydrates grown in small pores.L. B. gratefully acknowledges the Federal Ministry of Education and Research (Bundesministerium fĂĽr Bildung und Forschung, BMBF) for support of the Mechanocarb project (award number 03SF0498). J. S. A. acknowledges financial support from MINECO (project MAT-2013-45008-p) and Generalitat Valenciana (PROMETEOII/2014/004). V. B. thanks the Federal Ministry of Education and Research (Bundesministerium fĂĽr Bildung und Forschung, BMBF) for financial support (project No. 05K13OD3)
Cooperative light-induced breathing of soft porous crystals via azobenzene buckling
Although light is a prominent stimulus for smart materials, the application of photoswitches as light-responsive triggers for phase transitions of porous materials remains poorly explored. Here we incorporate an azobenzene photoswitch in the backbone of a metal-organic framework producing light-induced structural contraction of the porous network in parallel to gas adsorption. Light-stimulation enables non-invasive spatiotemporal control over the mechanical properties of the framework, which ultimately leads to pore contraction and subsequent guest release via negative gas adsorption. The complex mechanism of light-gated breathing is established by a series of in situ diffraction and spectroscopic experiments, supported by quantum mechanical and molecular dynamic simulations. Unexpectedly, this study identifies a novel light-induced deformation mechanism of constrained azobenzene photoswitches relevant to the future design of light-responsive materials
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