Permeability reduction in porous materials by in situ

The effect of in situ formed silica gel on the permeability of a porous material was investigated experimentally. Gelling solutions of tetra-methyl-ortho-silicate (TMOS) and methanol in water were imbibed into dry sandstone plates and cured for several days. The permeability of the untreated sandstone is on the order of 1 µm^2, whereas the intrinsic permeability of the silica alcogel is 5–6 orders of magnitude lower. The method of beam bending was employed to measure concurrently the permeability D and Young’s modulus Ep of cylindrical gel rods, prepared from the TMOS-based sol-gel solutions. Second, the permeabilities and moduli of the treated sandstones were measured. For both types of samples the gel structure was varied by varying the concentration of the TMOS in a solution and the pH of the water used. The parameters D and Ep follow from a detailed analysis of the measured relaxation of the load that is applied to the sample under constant deflection. In case of the gels, the relaxation was interpreted using common expressions for hydrodynamic relaxation and viscoelastic (VE) relaxation. It was found that the permeability of the gels decreases with increasing silica content and that acid-catalyzed gels exhibit a significantly lower permeability than base-catalyzed gels. The modulus Ep increases with increasing silica content and aging time. The relaxation data of the sandstone—treated with gel—exhibited a more complex behavior. The normalized load curves showed hydrodynamic relaxation as well as strong and fast VE relaxation. The relaxation data for the rock samples treated with the lowest concentration gel was fitted successfully with the predictions. For higher concentrations the fit was less accurate, but the permeability estimates were within an order of magnitude. The overall permeability of the treated rock is higher than the intrinsic permeability of the gels; this indicates that the gel does not completely fill the pore space. Nevertheless, the permeability is reduced by a factor 10^4 with respect to untreated sandstone, and therefore the gel adequately blocks the pores

Structure of nanoparticles embedded in micellar polycrystals

We investigate by scattering techniques the structure of water-based soft composite materials comprising a crystal made of Pluronic block-copolymer micelles arranged in a face-centered cubic lattice and a small amount (at most 2% by volume) of silica nanoparticles, of size comparable to that of the micelles. The copolymer is thermosensitive: it is hydrophilic and fully dissolved in water at low temperature (T ~ 0{\deg}C), and self-assembles into micelles at room temperature, where the block-copolymer is amphiphilic. We use contrast matching small-angle neuron scattering experiments to probe independently the structure of the nanoparticles and that of the polymer. We find that the nanoparticles do not perturb the crystalline order. In addition, a structure peak is measured for the silica nanoparticles dispersed in the polycrystalline samples. This implies that the samples are spatially heterogeneous and comprise, without macroscopic phase separation, silica-poor and silica-rich regions. We show that the nanoparticle concentration in the silica-rich regions is about tenfold the average concentration. These regions are grain boundaries between crystallites, where nanoparticles concentrate, as shown by static light scattering and by light microscopy imaging of the samples. We show that the temperature rate at which the sample is prepared strongly influence the segregation of the nanoparticles in the grain-boundaries.Comment: accepted for publication in Langmui

Facile control of silica nanoparticles using a novel solvent varying method for the fabrication of artificial opal photonic crystals

In this work, the Stöber process was applied to produce uniform silica nanoparticles (SNPs) in the meso-scale size range. The novel aspect of this work was to control the produced silica particle size by only varying the volume of the solvent ethanol used, whilst fixing the other reaction conditions. Using this one-step Stöber-based solvent varying (SV) method, seven batches of SNPs with target diameters ranging from 70 to 400 nm were repeatedly reproduced, and the size distribution in terms of the polydispersity index (PDI) was well maintained (within 0.1). An exponential equation was used to fit the relationship between the particle diameter and ethanol volume. This equation allows the prediction of the amount of ethanol required in order to produce particles of any target diameter within this size range. In addition, it was found that the reaction was completed in approximately 2 h for all batches regardless of the volume of ethanol. Structurally coloured artificial opal photonic crystals (PCs) were fabricated from the prepared SNPs by self-assembly under gravity sedimentation

Isolation of a wide range of minerals from a thermally treated plant: Equisetum arvense, a Mare’s tale

Silica is the second most abundant biomineral being exceeded in nature only by biogenic CaCO3. Many land plants (such as rice, cereals, cucumber, etc.) deposit silica in significant amounts to reinforce their tissues and as a systematic response to pathogen attack. One of the most ancient species of living vascular plants, Equisetum arvense is also able to take up and accumulate silica in all parts of the plant. Numerous methods have been developed for elimination of the organic material and/or metal ions present in plant material to isolate biogenic silica. However, depending on the chemical and/or physical treatment applied to branch or stem from Equisetum arvense; other mineral forms such glass-type materials (i.e. CaSiO3), salts (i.e. KCl) or luminescent materials can also be isolated from the plant material. In the current contribution, we show the chemical and/or thermal routes that lead to the formation of a number of different mineral types in addition to biogenic silica

Stress-corrosion mechanisms in silicate glasses

The present review is intended to revisit the advances and debates in the comprehension of the mechanisms of subcritical crack propagation in silicate glasses almost a century after its initial developments. Glass has inspired the initial insights of Griffith into the origin of brittleness and the ensuing development of modern fracture mechanics. Yet, through the decades the real nature of the fundamental mechanisms of crack propagation in glass has escaped a clear comprehension which could gather general agreement on subtle problems such as the role of plasticity, the role of the glass composition, the environmental condition at the crack tip and its relation to the complex mechanisms of corrosion and leaching. The different processes are analysed here with a special focus on their relevant space and time scales in order to question their domain of action and their contribution in both the kinetic laws and the energetic aspects.Comment: Invited review article - 34 pages Accepted for publication in J. Phys. D: Appl. Phy

Biorefining of wheat straw:accounting for the distribution of mineral elements in pretreated biomass by an extended pretreatment–severity equation

BACKGROUND: Mineral elements present in lignocellulosic biomass feedstocks may accumulate in biorefinery process streams and cause technological problems, or alternatively can be reaped for value addition. A better understanding of the distribution of minerals in biomass in response to pretreatment factors is therefore important in relation to development of new biorefinery processes. The objective of the present study was to examine the levels of mineral elements in pretreated wheat straw in response to systematic variations in the hydrothermal pretreatment parameters (pH, temperature, and treatment time), and to assess whether it is possible to model mineral levels in the pretreated fiber fraction. RESULTS: Principal component analysis of the wheat straw biomass constituents, including mineral elements, showed that the recovered levels of wheat straw constituents after different hydrothermal pretreatments could be divided into two groups: 1) Phosphorus, magnesium, potassium, manganese, zinc, and calcium correlated with xylose and arabinose (that is, hemicellulose), and levels of these constituents present in the fiber fraction after pretreatment varied depending on the pretreatment-severity; and 2) Silicon, iron, copper, aluminum correlated with lignin and cellulose levels, but the levels of these constituents showed no severity-dependent trends. For the first group, an expanded pretreatment-severity equation, containing a specific factor for each constituent, accounting for variability due to pretreatment pH, was developed. Using this equation, the mineral levels could be predicted with R(2) > 0.75; for some with R(2) up to 0.96. CONCLUSION: Pretreatment conditions, especially pH, significantly influenced the levels of phosphorus, magnesium, potassium, manganese, zinc, and calcium in the resulting fiber fractions. A new expanded pretreatment-severity equation is proposed to model and predict mineral composition in pretreated wheat straw biomass

) Maceil, C. E.; In The Encyclopedia of Nuclear Magnetic Resonance

Evanescent-wave cavity ring-down spectroscopy has been applied to a planar fused-silica surface covered with crystal violet (CV + ) cations to characterize the silanol groups indirectly. A radiation-polarization dependence of the adsorption isotherm of CV + at the CH 3 CN/silica interface is measured and fit to a two-site Langmuir equation to determine the relative populations of two different types of isolated silanol groups. CV + binding at type I sites yields a free energy of adsorption of -29.9 ( 0.2 kJ/mol and a saturation surface density of (7.4 ( 0.5) × 10 12 cm -2 , whereas the values of -17.9 ( 0.4 kJ/mol and (3.1 ( 0.4) × 10 13 cm -2 are obtained for the type II sites. The CV + cations, each with a planar area of ∼120 Å 2 , seem to be aligned randomly while lying over the SiOtype I sites, thereby suggesting that this type of site may be surrounded by a large empty surface area (>480 Å 2 ). In contrast, the CV + cations on a type II sites are restricted with an average angle of ∼40°tilted off the surface normal, suggesting that the CV + cations on these sites are grouped closely together. The average tilt angle increases with increasing concentration of crystal violet so that CV + cations may be separated from each other to minimize the repulsion of nearby CV + and SiOH sites. Adsorption behavior of organic molecules on silica surfaces has been the major theme of interface studies for improving the efficiency of chromatographic separations. When cationic molecules are involved, the strong electrostatic interaction with the negatively charged silanol (SiOH) groups on the surface of the stationary-phase silica may cause unwanted peak broadening and tailing, mainly from a slow kinetic response of the electrostatic adsorption. [1][2][3][4][5][6] The surface charge density is one of the primary factors influencing the strength of electrostatics. Accordingly, insight into how the cationic molecules interact with the local silanol groups of the silica surface should aid in the improvement of the design of surface modifications. Silanol groups play the main role in influencing the interfacial adsorption behavior, possessing an average surface density of ∼4.9 × 10 14 cm -2 on the silica surface 7-9 or an average surface area of 20.4 Å 2 per silanol group. As compared to silica sol particles, which have higher surface areas of (0.1-5) × 10 22 Å 2 /g, 7-9 only a few studies focus on characterization of silanol groups on a planar silica surface. 10-12 Ong et al. 10 first reported that isolated and vicinal silanol groups both exist at the water/silica interface possessing different pK a values of 4.9 and 8.5, with corresponding surface populations of 19 and 81%, respectively. These results were confirmed by means of cross-polarization magic angle spinning NMR 13 and fluorescence microscopy. 14 The isolated silanol groups with pK a ) 4.9 are anticipated to be separated far from each other (>5.5 Å), permitting proton dissociation. The vicinal silanol groups are located so closely as to form hydrogen bonds directly with their neighbors (<3.3 Å), which share 46% of the surface population, or through a water-molecule bridge (3.5-5.5 Å), which covers ∼35% of the surface population. 12,[15][16][17] By using second harmonic generation (SHG) with a cationic crystal violet (CV + ) molecular probe to investigate the local density distribution of the isolated silanols (pK a ) 4.9) on the planar fusedsilica surface, Xu and co-workers 12 classified them into two types. The first type of silanol group is anticipated to be surrounded by a large empty surface area (g120 Å 2 ) with a surface density o