118 research outputs found
Atomistic Simulation and Theory of Nanoporous Carbons and Nanostructures
International audienc
Freezing point depression and freeze-thaw damage by nano-fuidic salt trapping
A remarkable variety of organisms and wet materials are able to endure
temperatures far below the freezing point of bulk water. Cryo-tolerance in
biology is usually attributed to "anti-freeze" proteins, and yet massive
supercooling (C) is also possible in porous media containing only
simple aqueous electrolytes. For concrete pavements, the common wisdom is that
freeze-thaw damage results from the expansion of water upon freezing, but this
cannot explain the large pressures (~MPa) required to damage concrete,
the observed correlation between pavement damage and de-icing salts, or the
damage of cement paste loaded with benzene (which contracts upon freezing). In
this Letter, we propose a different mechanism -- nanofluidic salt trapping --
which can explain the observations, using simple mathematical models of
dissolved ions confined to thin liquid films between growing ice and charged
surfaces. Although trapped salt lowers the freezing point, ice nucleation in
charged pores causes enormous disjoining pressures via the rejected ions, until
their removal by precipitation or surface adsorption at a lower temperatures
releases the pressure and allows complete freezing. The theory is able to
predict the non-monotonic salt-concentration dependence of freeze-thaw damage
in concreter and provides a general framework to understand the origins of
cryo-tolerance.Comment: 5 figure
Multiscale Poromechanics of Wet Cement Paste
Capillary effects such as imbibition-drying cycles impact the mechanics of
granular systems over time. A multiscale poromechanics framework was applied to
cement paste, that is the most common building material, experiencing broad
humidity variations over the lifetime of infrastructure. First, the liquid
density distribution at intermediate to high relative humidities is obtained
using a lattice gas density functional method together with a realistic
nano-granular model of cement hydrates. The calculated adsorption/desorption
isotherms and pore size distributions are discussed and compare well to
nitrogen and water experiments. The standard method for pore size distribution
determination from desorption data is evaluated. Then, the integration of the
Korteweg liquid stress field around each cement hydrate particle provided the
capillary forces at the nanoscale. The cement mesoscale structure was relaxed
under the action of the capillary forces. Local irreversible deformations of
the cement nano-grains assembly were identified due to liquid-solid
interactions. The spatial correlations of the nonaffine displacements extend to
a few tens of nm. Finally, the Love-Weber method provided the homogenized
liquid stress at the micronscale. The homogenization length coincided with the
spatial correlation length nonaffine displacements. Our results on the solid
response to capillary stress field suggest that the micronscale texture is not
affected by mild drying, while local irreversible deformations still occur.
These results pave the way towards understanding capillary phenomena induced
stresses in heterogeneous porous media ranging from construction materials,
hydrogels to living systems.Comment: 6 figures in main text, 4 figures in the SI appendi
Molecular Simulations of Supercritical Fluid Permeation through Disordered Microporous Carbons
International audienceFluid transport through microporous carbon-based materials is inherent in numerous applications, ranging from gas separation by carbon molecular sieves to natural gas production from coal seams and gas shales. The present study investigates the steady-state permeation of supercritical methane in response to a constant cross-membrane pressure drop. We performed dual control volume grand canonical molecular dynamics (DCV-GCMD) simulations to mimic the conditions of actual permeation experiments. To overcome arbitrary assumptions regarding the investigated porous structures, the membranes were modeled after the CS1000a and CS1000 molecular models, which are representative of real microporous carbon materials. When adsorption-induced molecular trapping (AIMT) mechanisms are negligible, we show that the permeability of the microporous material, although not significantly sensitive to the pressure gradient, monotonically decreases with temperature and reservoir pressures, consistent with diffusion theory. However, when AIMT occurs, the permeability increases with temperature in agreement with experimental data found in the literature
Freezing point depression and freeze-thaw damage by nanofluidic salt trapping
A remarkable variety of organisms and wet materials are able to endure temperatures far below the freezing point of bulk water. Cryotolerance in biology is usually attributed to “antifreeze” proteins, and yet massive supercooling (10 MPa) required to damage concrete, the observed correlation between pavement damage and deicing salts, or the FT damage of cement paste loaded with benzene (which contracts upon freezing). In this work, we propose a different mechanism—nanofluidic salt trapping—which can explain the observations, using simple mathematical models of dissolved ions confined between growing ice and charged pore surfaces. When the transport time scale for ions through charged pore space is prolonged, ice formation in confined pores causes enormous disjoining pressures via the ions rejected from the ice core, until their removal by precipitation or surface adsorption at lower temperatures releases the pressure and allows complete freezing. The theory is able to predict the nonmonotonic salt-concentration dependence of FT damage in concrete and provides some hint to better understand the origins of cryotolerance from a physical chemistry perspective
Docking 90 Sr radionuclide in cement: An atomistic modeling study
a b s t r a c t Cementitious materials are considered to be a waste form for the ultimate disposal of radioactive materials in geological repositories. We investigated by means of atomistic simulations the encapsulation of strontium-90, an important radionuclide, in calcium-silicate-hydrate (C-S-H) and its crystalline analog, the 9 Å-tobermorite. C-S-H is the major binding phase of cement. Strontium was shown to energetically favor substituting calcium in the interlayer sites in C-S-H and 9 Å-tobermorite with the trend more pronounced in the latter. The integrity of the silicate chains in both cementitious waste forms were not affected by strontium substitution within the time span of molecular dynamics simulation. Finally, we observed a limited degradation of the mechanical properties in the strontium-containing cementitious waste form with the increasing strontium concentration. These results suggest the cement hydrate as a good candidate for immobilizing radioactive strontium
Ion specificity of confined ion-water structuring and nanoscale surface forces in clays
Ion specificity and related Hofmeister effects, ubiquitous in aqueous
systems, can have spectacular consequences in hydrated clays, where
ion-specific nanoscale surface forces can determine large scale cohesive,
swelling and shrinkage behaviors of soil and sediments. We have used a
semi-atomistic computational approach and examined sodium, calcium and aluminum
counterions confined with water between charged surfaces representative of clay
materials, to show that ion-water structuring in nanoscale confinement is at
the origin of surface forces between clay particles which are intrinsically
ion-specific. When charged surfaces strongly confine ions and water, the
amplitude and oscillations of the net pressure naturally emerge from the
interplay of electrostatics and steric effects, which can not be captured by
existing theories. Increasing confinement and surface charge densities promote
ion-water structures that increasingly deviate from the ions' bulk hydration
shells, being strongly anisotropic and persistent, and self-organizing into
optimized, nearly solid-like assemblies where hardly any free water is left. In
these conditions, strongly attractive interactions can prevail between charged
surfaces, due to the dramatically reduced dielectric screening of water and the
highly organized water-ion structures. By unravelling the ion-specific nature
of these nanoscale interactions, we provide evidence that ion-specific
solvation structures determined by confinement are at the origin of ion
specificity in clays and potentially a broader range of confined aqueous
systems.Comment: Main text: 14 pages and 6 figures. Supporting information: 5 figures.
Submitted to The Journal of Physical Chemistry
Capillary stress and structural relaxation in moist granular materials
We propose a theoretical framework to calculate capillary stresses in complex
mesoporous materials, such as moist sand, nanoporous hydrates, and drying
colloidal films. Molecular simulations are mapped onto a phase-field model of
the liquid-vapor mixture, whose inhomogeneous stress tensor is integrated over
Voronoi polyhedra in order to calculate equal and opposite forces between each
pair of neighboring grains. The method is illustrated by simulations of
moisture-induced forces in small clusters and random packings of spherical
grains using lattice-gas Density Functional Theory. For a nano-granular model
of cement hydrates, this approach reproduces the hysteretic water
sorption/desorption isotherms and predicts drying shrinkage strain isotherm in
good agreement with experiments. We show that capillary stress is an effective
mechanism for internal stress relaxation in colloidal random packings, which
contributes to the extraordinary durability of cement paste.Comment: 4 figure
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