Gas Transport in Porous Media: Simulations and Experiments on Partially Densified Aerogels

The experimental density dependence of gas (argon and nitrogen) permeability of partially densified silica aerogels in the Knudsen regime is quantitatively accounted for by a computer model. The model simulates both the structure of the sintered material and the random ballistic motion of a point particle inside its voids. The same model is also able to account for the densit y dependence of the specific pore surface as measured from nitrogen adsorption experiments.Comment: RevTex, 11 pages + 5 postscript figures appended using "uufiles". Published in Europhys. Lett. 29, p. 567 (1995

Molecular dynamics simulation of the early stages of the synthesis of periodic mesoporous silica

We present results of detailed atomistic modeling of the early stages of the synthesis of periodic mesoporous silica using molecular dynamics. Our simulations lead to the proposal of a mechanism that validates several previous experimental and modeling studies and answers many controversial issues regarding the synthesis of mesoporous silicas. In particular, we show that anionic silicates interact very strongly with cationic surfactants and, significantly adsorb on the surface of micelles, displacing a fraction of previously bound bromide counterions. This induces an increase in micelle size and also enhances silica condensation at the micelle surface. The presence of larger silica aggregates in solution further promotes the growth of micelles and, by binding to surfactant molecules in different micelles, their aggregation. This work demonstrates the crucial role played by silica in influencing, by way of a cooperative templating mechanism, the structure of the eventual liquid-crystal phase, which in turn determines the structure of the porous material

Co-Adsorption of Alkylphenols and a Non-Ionic Surfactant onto Bentonite

The influence of a non-ionic surfactant on the adsorption behaviour of alkylphenols onto bentonite was studied via measurements of their adsorption isotherms in the absence and presence of various concentrations of the surfactant. Irrespective of the presence of the surfactant, the alkylphenols were always adsorbed by the same mechanism. However, on adding the non-ionic surfactant TX100 to the medium, the adsorption properties were modified in a manner which depended on the alkyl chain length of the alkylphenol molecule. The longer the chain length, the weaker the influence of the TX100 surface aggregates on the adsorption behaviour. In terms of the relative hydrophobicity of these molecules, this behaviour is unexpected. However, it may be due in part to the adsorbing properties of the alkylphenols that are capable of self-aggregating at the bentonite/water interface and in part to the limited swelling capacity of the TX100 micelles or surface micelles. Partition coefficients between aqueous solution and surface aggregates were determined and compared with those calculated for the bulk micelles

Plastic densification in silica aerogels

Under isostatic pressure aerogels display an irreversible shrinkage caused by plastic deformation. As a consequence of this plastic shrinkage it is possible to densify, and thus to modify the elastic properties of aerogels. We show that the elasticity of the material is strongly influenced by structural transformation, and we observe a weakening of the network during the first stage of densification. The structural evolution, followed by Small Angle X-ray Scattering, shows that the densification mechanism is different from the one obtained by a sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, in contrast to viscous sintering

Hybrid materials for polymer electrolyte membrane fuel cells: Water uptake, mechanical and transport properties

The water uptake, transport and mechanical properties of organic-inorganic hybrid materials based on sulfonated and silylated, cross-linked polyetheretherketone (called SOSi-PEEK/N, where N is the molar percentage of silylated monomeric units) for application in fuel cells are reported. The mechanical properties are superior to those of S-PEEK, due to the presence of silicon. The water and proton diffusion coefficients are reported as function of temperature and relative humidity. The proton conductivity of the membranes increases nearly four orders of magnitude when relative humidity varies from 30% to 100% corresponding to an increase of water/equivalent of ion-exchange groups, λ, from 1 to 15. The activation energy for proton conduction is slightly higher than in Nafion. Proton diffusion coefficients obtained from the Nernst-Einstein equation are compared with water chemical diffusion coefficients determined from water sorption kinetics. The equivalence of both quantities for λ ≥ 4 indicates that proton conductivity takes place via Grotthuss-type mechanism

Crosslinked organic/inorganic hybrid proton exchange polymeric membranes

A general, efficient and experimentally simple method for the synthesis of covalently crosslinked class II hybrid polymers based on PolyEtherEtherKetone (PEEK) with a high degree of sulfonation (DS = 0.8) was performed. The synthetic strategy foreseen two steps: the first one, direct sulfonation, led to the introduction of sulfonic acid groups and to the formation of sulfone bridges among repeat units. The second step, lithiation and silylation, led to the formation of covalent bonds between the organic backbone and Si(OH)3 moieties. The materials were characterized by 1H NMR Spectroscopy, ATR/FTIR Spectroscopy, Thermogravimetric Analysis (TGA) and Mass Spectrometry. Preliminary proton conduction measurements in dry conditions were performed. The introduction of silicon groups in the system improved thermal stability

Contacteurs membranaires ceramiques pour l’extraction liquide-liquide

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