Cellulose consolidated with polyethylene glycol: The nanoscale mechanisms revealed by hybrid Monte Carlo/molecular dynamics modeling.

Polyethylene glycol (PEG) consolidation treatment is a widely used conservation strategy for wooden culture relics. However, the consolidation mechanism of PEG is still open to interpretation. PEG-cellulose, the representative component of wood cell wall, interactions are governed by various coupled multi-scale mechanisms which require nano-scale investigation.

Molecular Simulation of Cyclohexane in Nanoporous Materials: Adsorption of Conformers and Coadsorption with Water and Carbon Dioxide.

Molecular simulation is used to investigate the adsorption of an organic molecule and its different conformers into various nanoporous adsorbents. In more detail, we perform grand canonical Monte Carlo simulations to determine the adsorption isotherms for cyclohexane with its three conformers (chair, boat, and twisted boat) at three different temperatures into a molecular model of active carbons and two metal-organic frameworks (MOFs) (Cu-BTC and Al-Fum). By considering the adsorption of each conformer separately, we show that the adsorption isotherms are weakly dependent on the molecular conformation.

Bridging Microscopic Dynamics and Hydraulic Permeability in Mechanically-Deformed Nanoporous Materials.

In the field of nanoconfined fluids, there are striking examples of deformation/transport coupling in which mechanical solicitation of the confining solid and dynamics of the confined fluid impact each other. While this intriguing behavior can be harnessed for applications (e.g.

Nanostructure and interfacial mechanical properties of PEG/cellulose nanocomposites studied with molecular dynamics.

Our starting hypothesis is that Polyethylene glycol (PEG) can be utilized to mix with the biopolymers for consolidating fiber-reinforced composites without deteriorating their hygro-mechanical properties. The effect of PEG on the shear strength during pull-out of crystalline cellulose (CC) fiber out of an amorphous cellulose matrix is simulated with molecular dynamics. The interfacial shear stress shows a stick-slip behavior and is weakened with increasing moisture content.

Strong Swelling and Symmetrization in Siliceous Zeolites due to Hydrogen Insertion at High Pressure.

Hydrogen and helium saturate the 1D pore systems of the high-silica (Si/Al>30) zeolites Theta-One (TON), and Mobile-Twelve (MTW) at high pressure based on x-ray diffraction, Raman spectroscopy and Monte Carlo simulations. In TON, a strong 22 % volume increase occurs above 5 GPa with a transition from the collapsed P2 to a symmetrical, swelled Cmc2 form linked to an increase in H content from 12 H/unit cell in the pores to 35 H/unit cell in the pores and in the framework of the material. No transition and continuous collapse of TON is observed in helium indicating that the mechanism of H insertion is distinct from other fluids.

Apparent Anomalous Temperature Dependence of Self-Diffusion Studied by Pulsed-Field Gradient Nuclear Magnetic Resonance and Thermodynamic Modeling.

The self-diffusivity of cyclohexane and -octane adsorbed in hierarchical zeolite monoliths has been investigated by using PFG-NMR. In these samples, the intrinsic FAU-X zeolite microporosity combines with a complex macroporous network composed of aggregated zeolite nanocrystals. As temperature is increased, cyclohexane self-diffusivity apparently decreases, reaches a minimum, and then starts increasing upon further increasing the temperature.

Thermal Conductivity of a Fluid-Filled Nanoporous Material: Underlying Molecular Mechanisms and the Effect.

Nanoporous materials are central to the energy and environmental crisis, with key applications in adsorption, separation, and catalysis. While confinement and surface effects on fluids severely confined in their porosity are well documented, the thermal behavior of nanoporous solids subjected to fluid adsorption remains puzzling in many aspects. With striking phenomena such as the so-called effect, through which fluid/solid collisions decrease the overall thermal conductivity, the solid thermal conductivity and, more generally, heat transfer and dispersion in these complex systems challenge classical approaches (e.

Anisotropic Deformation in a Polymer Slab Subjected to Fluid Adsorption.

Nanoporous adsorbents can mechanically swell or shrink once upon the accumulation of guest fluid molecules at their internal surfaces or in their cavities. Existing theories in this field attribute such sorption-induced swelling to a tensile force, while shrinkage is always associated with a contractive force. In this study, however, we propose that the sorption-induced deformation of a porous architecture is not solely dictated by the stress conditions but can also be largely influenced by its mechanical anisotropy.

On De Gennes narrowing of fluids confined at the molecular scale in nanoporous materials.

Beyond well-documented confinement and surface effects arising from the large internal surface and severely confining porosity of nanoporous hosts, the transport of nanoconfined fluids remains puzzling in many aspects. With striking examples such as memory, i.e.

Soap-Film Membranes for CO/Air Separation.

Thin liquid films are a potential game changer in the quest for efficient gas separation strategies. Such fluid membranes, which are complementary to their solid counterparts involving porous materials, can achieve complex separation by combining permeability and adsorption mechanisms in their liquid core and at their surface. In addition, unlike porous solid membranes that must be regenerated between separation steps to recover a gas-free porosity, thus preventing continuous operation, liquid membranes can be regenerated using continuous liquid flow through the fluid film.

Mixture Adsorption in Nanoporous Zeolite and at Its External Surface: In-Pore and Surface Selectivity.

Using toluene, ethylene, and water as gas compounds with different representative molecular interactions, we perform atom-scale simulations for their mixtures to investigate the selectivity of the core nanoporosity and external surface in a prototypical zeolite. As expected, the overall behavior suggests that increasing the pressure of a given component promotes the desorption of the coadsorbing species. However, for water-toluene mixtures, we identify that the pseudohydrogen bonding between water and toluene leads to beneficial coadsorption as toluene adsorption in the low-pressure range promotes water adsorption.

Sorption-Deformation Interplay in Hierarchical Porous Polymeric Structures Composed of a Slit Pore in an Amorphous Matrix.

Prevailing absorbents like wood-derived porous scaffolds or polymeric aerogels are normally featured with hierarchical porous structures. In former molecular simulation studies, sorption, deformation, and coupled sorption-deformation have been studied for single-scale materials, but scarcely for materials where micropores (<2 nm) and mesopores (2-50 nm) coexist. The present work, dealing with a mesoscopic slit pore between two slabs of microporous amorphous cellulose (AC), aims at modeling sorption-deformation interplay in hierarchical porous cellulosic structures inspired by polymeric modern adsorbents.

Low-Temperature Dynamics of Water Confined in Unidirectional Hydrophilic Zeolite Nanopores.

The dynamical properties of water molecules confined in the unidirectional hydrophilic nanopores of AlPO-54 are investigated with quasi-elastic neutron scattering as a function of temperature down to 118 K. AlPO-54 has among the largest pores known for aluminophosphates and zeolites (about 1.3 nm), though they are small enough to prevent water crystallization due to the high degree of confinement.

Sorption-Deformation-Percolation Model for Diffusion in Nanoporous Media.

Diffusion of molecules in porous media is a critical process that is fundamental to numerous chemical, physical, and biological applications. The prevailing theoretical frameworks are challenged when explaining the complex dynamics resulting from the highly tortuous host structure and strong guest-host interactions, especially when the pore size approximates the size of diffusing molecule. This study, using molecular dynamics, formulates a semiempirical model based on theoretical considerations and factorization that offer an alternative view of diffusion and its link with the structure and behavior (sorption and deformation) of material.

From Transient to Stationary Transport in Porous Networks under Various Adsorption Conditions and Kinetics.

We investigate the interplay between adsorption and transport in a two-dimensional porous medium by means of an extended Lattice Boltzmann technique within the Two-Relaxation-Time framework. We focus on two canonical adsorption thermodynamics and kinetics formalisms: (1) the Henry model in which the adsorbed amount scales linearly with the free adsorbate concentration and (2) the Langmuir model that accounts for surface saturation upon adsorption. We simulate transport of adsorbing and nonadsorbing particles to investigate the effect of the adsorption/desorption ratio , initial free adsorbate concentration , surface saturation Γ, and Peclet numbers Pe on their dispersion behavior.

How Reproducible are Surface Areas Calculated from the BET Equation?

Porosity and surface area analysis play a prominent role in modern materials science. At the heart of this sits the Brunauer-Emmett-Teller (BET) theory, which has been a remarkably successful contribution to the field of materials science. The BET method was developed in the 1930s for open surfaces but is now the most widely used metric for the estimation of surface areas of micro- and mesoporous materials.

Gas Adsorption in Zeolite and Thin Zeolite Layers: Molecular Simulation, Experiment, and Adsorption Potential Theory.

Molecular simulations and experiments are used to investigate methane adsorption in bulk and thin layers of MFI zeolite (silicalite-1). After comparing the theoretical adsorption data obtained using Grand Canonical Monte Carlo simulations for bulk MFI at various temperatures against experiments, zeolite layers with different crystalline orientations and levels of surface flexibility are considered. The data obtained for such prototypical systems allow us to rationalize both the qualitative and quantitative impact of external surface in nanoporous solids.

Surface Protolysis and Its Kinetics Impact the Electrical Double Layer.

Surface conductivity in the electrical double layer (EDL) is known to be affected by proton hopping and diffusion at solid-liquid interfaces. Yet, the role of surface protolysis and its kinetics on the thermodynamic and transport properties of the EDL are usually ignored as physical models consider static surfaces. Here, using a novel molecular dynamics method mimicking surface protolysis, we unveil the impact of such chemical events on the system's response.

Impact of surface diffusion on transport through porous materials.

The peak parking method was applied to evaluate the surface diffusivity D of polystyrenes dissolved in a THF/heptane mixture and transported through porous silica materials with various morphologies. With this method, the overall effective diffusivity D is measured experimentally with coarse-grained models like Maxwell equation allowing one to infer the particle diffusivity D. Such particle diffusivity has two main contributions: in-pore diffusivity D and surface diffusivity D.

Electronic screening using a virtual Thomas-Fermi fluid for predicting wetting and phase transitions of ionic liquids at metal surfaces.

Of relevance to energy storage, electrochemistry and catalysis, ionic and dipolar liquids display unexpected behaviours-especially in confinement. Beyond adsorption, over-screening and crowding effects, experiments have highlighted novel phenomena, such as unconventional screening and the impact of the electronic nature-metallic versus insulating-of the confining surface. Such behaviours, which challenge existing frameworks, highlight the need for tools to fully embrace the properties of confined liquids.

Hygromechanical mechanisms of wood cell wall revealed by molecular modeling and mixture rule analysis.

Despite the thousands of years of wood utilization, the mechanisms of wood hygromechanics remain barely elucidated, owing to the nanoscopic system size and highly coupled physics. This study uses molecular dynamics simulations to systematically characterize wood polymers, their mixtures, interface, and composites, yielding an unprecedented micromechanical dataset including swelling, mechanical weakening, and hydrogen bonding, over the full hydration range. The rich data reveal the mechanism of wood cell wall hygromechanics: Cellulose fiber dominates the mechanics of cell wall along the longitudinal direction.

Lattice Boltzmann method for adsorption under stationary and transient conditions: Interplay between transport and adsorption kinetics in porous media.

A numerical method based on the Lattice Boltzmann formalism is presented to capture the effect of adsorption kinetics on transport in porous media. Through the use of a general adsorption operator, canonical models such as Henry and Langmuir adsorption as well as more complex adsorption mechanisms involving collective behavior with lateral interactions and surface aggregation can be investigated using this versatile model. By extending the description of adsorption phenomena to kinetic regimes with any underlying adsorption model, this effective technique allows assessing the coupled dynamics resulting from advection, diffusion, and adsorption in pores not only in stationary conditions but also under transient conditions (i.

Carbon dioxide as a line active agent: Its impact on line tension and nucleation rate.

By considering a water capillary bridge confined between two flat surfaces, we investigate the thermodynamics of the triple line delimiting this solid-liquid-vapor system when supplemented in carbon dioxide. In more detail, by means of atom-scale simulations, we show that carbon dioxide accumulates at the solid walls and, preferably, at the triple lines where it plays the role of a line active agent. The line tension of the triple line, which is quantitatively assessed using an original mechanical route, is shown to be driven by the line excess concentrations of the solute (carbon dioxide) and solvent (water).

Unexpected Order-Disorder Transition in Diacetylene Alcohol Langmuir Films.

The phase diagram of the Langmuir film of diacetylene alcohol-henicosa-5,7-diyn-1-ol-is investigated by means of surface pressure versus surface area isotherms, Brewster angle microscopy, X-ray reflectivity, and grazing incident X-ray diffraction. Among the usual phases described in the generic phase diagram of small head group molecules, one observes an unexpected reversible transition from an ordered condensed phase to a disordered one upon increasing the surface pressure. We postulate that the origin of this unusual, unprecedented transition results from the competition between the interactions between the diacetylene blocks in the hydrophobic chain and the hydrogen bonds between head groups and water.

Reduced phase stability and faster formation/dissociation kinetics in confined methane hydrate.

The mechanisms involved in the formation/dissociation of methane hydrate confined at the nanometer scale are unraveled using advanced molecular modeling techniques combined with a mesoscale thermodynamic approach. Using atom-scale simulations probing coexistence upon confinement and free energy calculations, phase stability of confined methane hydrate is shown to be restricted to a narrower temperature and pressure domain than its bulk counterpart. The melting point depression at a given pressure, which is consistent with available experimental data, is shown to be quantitatively described using the Gibbs-Thomson formalism if used with accurate estimates for the pore/liquid and pore/hydrate interfacial tensions.