Percolation model for selective dissolution of multi-component glasses

A percolation model is developed which accounts for most known features of the process of porous glass membrane preparation by selective dissolution of multi-component glasses. The model is founded within tile framework of the classical percolation theory, wherein the components of a glass are represented by random sites on a suitable lattice. Computer simulation is used to mirror the generation of a porous structure during the dissolution process, reproducing many of the features associated with the phenomenon. Simulation results evaluate the effect of the initial composition of the glass on the kinetics of the leaching process as well as the morphology of the generated porous structure. The percolation model establishes the porous structure as a percolating cluster of unreachable constituents in the glass. The simulation algorithm incorporates removal of both, the accessible leachable components in the glass as well as the independent clusters of unreachable components not attached to the percolating cluster. The dissolution process thus becomes limited by the conventional site percolation thresholds of the unreachable components (which restricts the formation of the porous network), as well as the leachable components (which restricts the accessibility of the solvating medium into the glass). The simulation results delineate the range of compositional variations for successful porous glass preparation and predict the variation of porosity, surface area, dissolution rates and effluent composition with initial composition and time. Results compared well with experimental studies and improved upon similar models attempted in die past

Fundamental studies for sol-gel derived gas-separation membranes

We prepared silica membranes using sol-gel techniques and explored the effects of postdeposition sintering, capillary stresses developed during drying, and surface derivatization of the membranes with titanium iso-propoxide. We observed that both partial sintering of membranes and development of larger-capillary stresses during membrane formation lead to consolidation of the membrane structure as evidenced by increased ideal separation factors, e.g. {alpha}CO{sub 2}2/CH{sub 4} > 250 over the temperature range of 160 to 220 C. Surface derivatization was also shown to be an effective tool to reduce the membrane pore size in an angstrom by angstrom fashion, resulting in comparable separation factors. What`s more, the altered pore surface chemistry of TiO{sub 2} derivatized membranes may lead to improved stability and impart catalytic properties to the membrane surface

Shrinkage and microstructural development during drying of organically modified silica xerogels

We have studied the different driving forces behind syneresis in MTES/TEOS gels by aging them in different H{sub 2}O/EtOH pore fluids. We show using shrinkage, density, contact angle, and N{sub 2} sorption measurements, the influence of gel/solvent interactions on the microstructural evolution during drying. Competing effects of syneresis (that occurs during aging) and drying shrinkage resulted in the overall linear shrinkage of the organically modified gels to be constant at {approximately}50%. Increasing the hydrophobicity of the gels caused the driving force for syneresis to change from primarily condensation reactions to a combination of condensation and solid/liquid interfacial energy. In addition the condensation driven shrinkage was observed to be irreversible, whereas the interfacial free energy driven shrinkage was observed to be partially reversible. Nitrogen sorption experiments show that xerogels with the same overall extent of shrinkage can have vastly different microstructures due to the effects of microphase separation

Influence of surfactants on the structure of titanium oxide gels : experiments and simulations

We report here on experimental and numerical studies of the influence of surfactants on mineral gel synthesis. The modification of the gel structure when the ratios water-precursor and water-surfactant vary is brought to the fore by fractal dimension measures. A property of {\em polydispersity of the initial hydrolysis} is proposed to explain these results, and is successfuly tested through numerical experiments of three dimensional chemically limited aggregation.Comment: 12 pages, 4 Postscript figures, uses RevTe

Ion conducting and paramagnetic d-PCL(530)/siloxane-based biohybrids doped with Mn 2+ ions

Amorphous α,ω-hidroxylpoly(ε-caprolactone) (PCL(530))/siloxane ormolytes doped with manganese perchlorate (Mn(ClO4)2) (d-PCL(530)/siloxanenMn(ClO4)2) with n = 20, 50, and 100), thermally stable up to at least 200 ºC, were synthesized by the sol-gel method. Ionic conductivity values up to 4.8×10−8 and 2.0×10−6 S cm−1 at about 25 and 100 ºC, respectively, where obtained for n = 20. FT-IR data demonstrated that the hydrogen bonding interactions present in the non-doped d-PCL(530)/siloxane host hybrid matrix were significantly influenced by the inclusion of Mn(ClO4)2 which promoted the formation of more oxyethylene/urethane and urethane/urethane aggregates. In addition, the Mn2+ ions bonded to all the “free” C=O groups of the urethane cross-links and to some of the “free” ester groups of the amorphous PCL(530) chains. In the electrolytes, the ClO4 − ions were found “free” and bonded to the Mn2+ ions along a bidentate configuration. The magnitude of the electron paramagnetic resonance (EPR) hyperfine constant of the analyzed samples (A ≈ 90×10-4 cm−1 ) suggested that the bonding between Mn2+ ions and the surrounding ligands is moderately ionic. The synthetized d-PCL(530)/siloxanenMn(ClO4)2 biohybrids have potential application in paramagnetic, photoelectrochemical and electrochromic devices.This work was supported by Fundacao para a Ciencia e a Tecnologia (FCT) and Feder (contracts PTDC/CTM-BPC/112774/2009, PEst-OE/QUI/UI0616/2014 and PEst-C/QUI/UI0686/2013) and COST Action MP1202 "Rational design of hybrid organic-inorganic interfaces". R.F.P.P. acknowledges FCT for a grant (SFRH/BPD/87759/2012). M.M.S. acknowledges CNPq (PVE grant 406617/2013-9), for a mobility grant. The financial support of the Brazilian agencies Capes and CNPq are gratefully acknowledged. Research was partially financed by the CeRTEV, Center for Research, Technology and Education in Vitreous Materials, FAPESP 2013/07793-6.info:eu-repo/semantics/publishedVersio