Gas adsorption in mcm-41 porous silicas dynamic measurements using sans

The European Physical Journal E - The isothermal gas adsorption of two hexane isomers (n-hexane and cyclohexane) in the mesopores of MCM-41 silica have been investigated by small angle neutron...     

Sol-Gel Routes to Zeolite Membranes and Thin Films

The synthesis of zeolite membranes and thin films using the secondary growth process is briefly described. In this process colloidal zeolite particles (sols) are prepared hydrothermally and then subsequently deposited on substrates to produce uniform layers of controlled thickness, as illustrated with silicalite and zeolite-A. The formation and growth of the zeolite sols has been investigated in situ by small angle neutron scattering (SANS). SANS measurements on silicalite sols at progressively higher concentrations have provided details of the colloid interactions which lead to zeolite gel-layer structures which are uniform and free of defects.     

Gas adsorption in mcm-41 porous silicas dynamic measurements using SANS

The isothermal gas adsorption of two hexane isomers (n-hexane and cyclohexane) in the mesopores of MCM-41 silica have been investigated by small angle neutron scattering (SANS). This has been achieved by performing SANS measurements under contrast matching conditions for the silica matrix and condensed hydrocarbon confined in the mesopores. Insight into the kinetics of adsorption have been derived from changes in the intensity of the (100) diffraction peak associated with the ordered hexagonal mesoporous structure of the MCM-41 silica.     

Upper Bound on the Packing Density of Regular Tetrahedra and Octahedra

Aristotle contended that (regular) tetrahedra tile space, an opinion that remained widespread until it was observed that non-overlapping tetrahedra cannot subtend a solid angle of 4π around a point if this point lies on a tetrahedron edge. From this 15th century argument, we can deduce that tetrahedra do not tile space but, more than 500 years later, we are unaware of any known non-trivial upper bound to the packing density of tetrahedra. In this article, we calculate such a bound. To this end, we show the existence, in any packing of regular tetrahedra, of a set of disjoint spheres centered on tetrahedron edges, so that each sphere is not fully covered by the packing. The bound on the amount of space that is not covered in each sphere is obtained in a recursive way by building on the solid angle argument. The argument can be readily modified to apply to other polyhedra. The resulting lower bound on the fraction of empty space in a packing of regular tetrahedra is 2.6…×10⁻²⁵ and reaches 1.4…×10⁻¹² for regular octahedra.     

Data-driven robust optimization

Mathematical Programming - The last decade witnessed an explosion in the availability of data for operations research applications. Motivated by this growing availability, we propose a novel schema...     

Carbohydrates as Multifunctional Chiral Scaffolds in Combinatorial Synthesis

Four orthogonally stable protecting groups and a selectively cleavable anchor that are stable under basic conditions are required in order that carbohydrates can be employed as chiral polyfunctional scaffolds in combinatorial solid-phase syntheses of high diversity. The schematic representation shows the combinatorial synthesis with a carbohydrate scaffold (SG=protecting group, A=anchor, P=polymer carrier), which proceeds by sequential selective deprotection, functionalization, washing of the solid phase, and cleavage of the anchor.     

Characterization of polymer dispersions by Fourier transform rheology

Fourier transform rheology is a very sensitive technique to characterize non-linear rheological fluid properties. It has been applied here for the first time to polymer dispersions in water and the results are compared to those from conventional rheology, namely steady and small amplitude oscillatory shear experiments. The investigated systems are mainly based on styrene and n-butylacrylate. A first attempt was made to evaluate how far colloidal parameters like particle volume fraction and ionic strength as well as chemical composition and surface characteristics of the dispersed particles are reflected in FT-rheology spectra. Significantly different non-linearities are observed for highly concentrated dispersions of particles with different T_g. These differences are not detected in linear oscillatory shear and show up in steady shear only at significantly higher shear rates. Particle surface characteristics influence the non-linear response in oscillatory shear significantly and the intensity of the overtones is found to be higher for a dispersion of particles with a “hairy” swollen surface layer as compared to a system of smooth particles, although the solids content was adjusted to match the steady shear viscosity. The intensity of the overtones in FT-rheology strongly decrease upon dilution. At a solid content below 35% no differences are observed in the FT-experiments for the systems investigated here, whereas the differences in steady shear are very pronounced in this concentration range. A significant influence of added salt onto the non-linear response is detected for some systems, which might be correlated to the stability of these systems. The observed phenomena certainly cannot be explained in terms of constitutive equations or microstructural statistical mechanical models at present. Thus, FT-rheology yields information complementary to classical steady or linear oscillatory shear experiments. Received: 11 December 2000 Accepted: 8 April 2001     

Robust sample average approximation

Mathematical Programming - Sample average approximation (SAA) is a widely popular approach to data-driven decision-making under uncertainty. Under mild assumptions, SAA is both tractable and enjoys...