Use of a capacitance measurement device for surrogate noncontact conductance measurement

A capacitance to digital converter (AD7746) is used in the same mode as noncontact conductance detectors. The detector output is linearly proportional to specific conductance (sigma) at low sigma values but becomes nonlinear and reaches a plateau value at sigma approximately >0.75 mS/cm, regardless of the nature of the electrolyte. For all applications at sub- to low-mM concentrations, the device, available as an evaluation board, provides a very affordable nothing-else-required means of contactless surrogate conductivity detection from capillary scale to larger bore conduits. For the same measurement cell volume, the detector provides virtually the same limits of detection (LODs) as a standard galvanic contact conductivity detector in conventional scale suppressed conductometric ion chromatography. The detection limits deteriorate as the conduit inner diameter decreases.     

A study of the crystallization kinetics of PVF2 by DSC and dilatometry

Poly(vinylidene fluoride) was crystallized isothermally from the melt at several temperatures. Crystallization rates have been measured using dilatometry and differential scanning calorimetry. The results were analysed in terms of the Avrami equation. The different methods yielded similar values for the kinetic parameters. The surface free energy of the lamellar crystallites has been determined.     

Thermodynamic assessment of the variation of the surface areas of two synthetic swelling clays during adsorption of water

Two synthetic smectites (montmorillonite and beidellite) are studied by a water adsorption technique in order to assess their specific surface areas under atmospheric conditions. A route recently proposed for extracting the thermodynamic data from experimental adsorption isotherms is used. The variation of the specific surface area during water adsorption is obtained, which can be linked to the enlargement of the interlayer space determined using X-ray diffraction. This variation is compared to an idealized specific surface area obtained from TEM and X-ray measurements in agreement with crystallographic models. All these results are also compared with those obtained previously for a natural montmorillonite. A simple view of swelling is proposed.     

Microstructure and physico-chemical transformation of some common woods from Cameroon during drying

Journal of Thermal Analysis and Calorimetry - The influence of drying on the microstructure, physical and chemical properties of some tropical wood species has been investigated using...     

Expansion during the reaction sintering of PZT

Expansion during the reaction sintering of PZT from PT and PZ constituents was studied at different molar ratios, for different particle sizes, and different temperatures: compositional and particle size effects were observed. The expansion is related to the size of the PT particles present and exhibits a maximum at 50 mole% PT for equal sizes of PT and PZ.     

Successive adsorption of polyacrylamide compounds from electrolyte solutions on the surface of kaolinitic clay particles

Successive adsorption of oppositely charged polyelectrolytes, namely, cationic and anionic acrylamide copolymers, on a solid phase surface from solutions with high ionic strength is investigated. The constants of the Freundlich equation are calculated for the adsorption of different polymers. The interrelation between the adsorption values of polymers and their flocculation activity with respect to clay-salt suspensions is determined. The successive adsorption of oppositely charged polyelectrolytes strongly affects the flocculation due to the formation of polyelectrolyte complexes on the surface of clay particles. The mechanism for complexation is proposed.     

An assessment of theoretical procedures for predicting the thermochemistry and kinetics of hydrogen abstraction by methyl radical from benzene

The reaction enthalpy (298 K), barrier (0 K), and activation energy and preexponential factor (600-800 K) have been examined computationally for the abstraction of hydrogen from benzene by the methyl radical, to assess their sensitivity to the applied level of theory. The computational methods considered include high-level composite procedures, including W1, G3-RAD, G3(MP2)-RAD, and CBS-QB3, as well as conventional ab initio and density functional theory (DFT) methods, with the latter two classes employing the 6-31G(d), 6-31+G(d,p) and/or 6-311+G(3df,2p) basis sets, and including ZPVE/thermal corrections obtained from 6-31G(d) or 6-31+G(d,p) calculations. Virtually all the theoretical procedures except UMP2 are found to give geometries that are suitable for subsequent calculation of the reaction enthalpy and barrier. For the reaction enthalpy, W1, G3-RAD, and URCCSD(T) give best agreement with experiment, while the large-basis-set DFT procedures slightly underestimate the endothermicity. The reaction barrier is slightly more sensitive to the choice of basis set and/or correlation level, with URCCSD(T) and the low-cost BMK method providing values in close agreement with the benchmark G3-RAD value. Inspection of the theoretically calculated rate parameters reveals a minor dependence on the level of theory for the preexponential factor. There is more sensitivity for the activation energy, with a reasonable agreement with experiment being obtained for the G3 methods and the hybrid functionals BMK, BB1K, and MPW1K, especially in combination with the 6-311+G(3df,2p) basis set. Overall, the high-level G3-RAD composite procedure, URCCSD(T), and the cost-effective DFT methods BMK, BB1K, and MPW1K give the best results among the methods assessed for calculating the thermochemistry and kinetics of hydrogen abstraction by the methyl radical from benzene.     

Calculating the first-order kinetics of three coupled,reversible processes

Calculation of the concentration-time profile in four consecutive, well-mixed compartments that are connected by diffusional transport is a frequently occurring problem for chemists and engineers. Mathematically this is equivalent to many other problems such as the concentration profiles of a parent compound and its three consecutive reaction products resulting from reversible first-order kinetics. Here we present an analytical solution to this problem implemented in a Microsoft Excel spreadsheet (available for download) and we discuss various examples of how this simple-to-use tool can be applied to very different scenarios from various fields of science.     

Optimization of acidic treatment of bentonitic clays from the national layers

Investigations in the field of the development of inexpensive catalysts for the production of environmentally safe fuels, based on synthesis-gas from biomass, have been performed. The influence of acidic (sulphuric and phosphorous acids) treatment on the specific surface of Russian (Voronezh and Krasnodar) origins of bentonitic clays was investigated. For the determination of the optimal conditions of acidic treatment, the method of extremal experiment planning has been applied. The full factor experiment was chosen as the plan; in such an experiment, all factor combinations are implemented on all levels used for investigations. The variable parameters were as follows: the acid concentration, and the temperature and time of the acid treatment. The average pore diameter and the specific surface of clays were chosen as the response functions. The response surface appearance is very complex.     

Synthesis,characterization, and challenges faced during the preparation of zirconium pillared clays

In this study, Zr-pillared montmorillonite clays (Zr-PILCs) were synthesized using two different precursor materials: raw montmorillonite (CM) and sodium ion-saturated montmorillonite (Na-CM) at different Zr/clay ratios (2.5, 5 and 10 mmol/g). To study the effect of Zr concentration and clay pre-treatment with NaCl on pillaring, the modified clay samples were characterized in detail using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (STEM-EDX). The XRD analysis showed the increase of basal spacing of Zr-PILCs prepared from both precursor materials: from 1.26 to 1.74 nm in the case of CM, and from 1.13 to 1.93 nm for Na-CM. Results from FT-IR revealed new bands ascribed to Zr-O bonds in the range of 400–500 cm^?1 in Zr-pillared samples obtained from Na-CM at Zr/clay ratios of 2.5 and 5 mmol/g. The distribution and nature of Zr species in between the silicate layers were studied using STEM-EDX and HAADF imaging. They were found to be separated by a distance of 1.5–3 nm and their thickness lies in the range of 1–2 nm. Pillared clays prepared from pre-treatment with NaCl were more thermally stable at higher temperatures.     

Modeling the kinetics of step homopolymerization reactions--an assessment of different approaches

A critical analysis of two models used to describe the kinetics of step homopolymerization has been undertaken. The classical second-order kinetic model and the more widely adopted Mathew et al. oligomer precipitation model were tested against data published in the literature. The classical model, which predicts an identical molecular weight distribution to that obtained by Flory using a statistical approach, is based on a single rate constant and provided an excellent fit (R²>0.99) to the experimental data. Derived rate constants exhibited logical trends. The Mathew et al. oligomer precipitation model, on the other hand, requires three fitting parameters. This model also fitted the data well but in many cases yielded either negative rate constants for the purported termination step or illogical parametric trends. It was concluded that the classical model was superior to the precipitation model in describing the kinetics and reaction mechanisms of step homopolymerization.     

Use of a generalized fisher equation for global optimization in chemical kinetics

A new approach for parameter estimation in chemical kinetics has been recently proposed (Ross et al. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 12777). It makes use of an optimization criterion based on a Generalized Fisher Equation (GFE). Its utility has been demonstrated with two reaction mechanisms, the chlorite-iodide and Oregonator, which are computationally stiff systems. In this Article, the performance of the GFE-based algorithm is compared to that obtained from minimization of the squared distances between the observed and predicted concentrations obtained by solving the corresponding initial value problem (we call this latter approach "traditional" for simplicity). Comparison of the proposed GFE-based optimization method with the "traditional" one has revealed their differences in performance. This difference can be seen as a trade-off between speed (which favors GFE) and accuracy (which favors the traditional method). The chlorite-iodide and Oregonator systems are again chosen as case studies. An identifiability analysis is performed for both of them, followed by an optimal experimental design based on the Fisher Information Matrix (FIM). This allows to identify and overcome most of the previously encountered identifiability issues, improving the estimation accuracy. With the new data, obtained from optimally designed experiments, it is now possible to estimate effectively more parameters than with the previous data. This result, which holds for both GFE-based and traditional methods, stresses the importance of an appropriate experimental design. Finally, a new hybrid method that combines advantages from the GFE and traditional approaches is presented.     

Analysis of the nonisothermal crystallization kinetics in three linear aromatic polyester systems

Melt or cold crystallization kinetics has a strong bearing on morphology and the extent of crystallization, which significantly affects the physical properties of polymeric materials. Nonisothermal crystallization kinetics are often analyzed by the classical Johnson–Mehl–Avrami–Kolmogorov (JMAK) model or one of its variants, even though they are based on an isothermal assumption. As a result, during the nonisothermal (e.g. constant heating or cooling rate) crystallization of polymeric material, different sets of model parameters are required to describe crystallization at different rates, thereby increasing the total number of model parameters. In addition, due to the uncorrelated nature of these model parameters with the cooling or heating rate, accurate modeling at any intermediate condition is not possible. In the present work, these two limitations of the conventional approach have been eliminated by exhibiting the existence of a functional relationship between cooling or heating rate and effective activation energy during nonisothermal melt or cold crystallization in three linear aromatic polyesters. Furthermore, it has been shown that when the JMAK model is used in conjunction with this functional relationship, it is possible to precisely predict the experimental nonisothermal melt or cold crystallization kinetics at any linear cooling or heating rate with a single set of model parameters.     

Criteria for the dilatometric analysis to determine the transformation kinetics during continuous heating

Journal of Thermal Analysis and Calorimetry - A method was established to determine the amount of austenite formed, by stages, in a low-carbon steel through dilatometric analysis. Based on length...     

Development of novel pillared clays for the encapsulation of inorganic complexes

Several pillared clays were prepared by using a polyalcohol (ethylene glycol or poly(vinyl alcohol)) or a poly(ethylene oxide) surfactant as an interlayer gallery template and an aluminum oligomer species as the pillaring agent. The use of polyalcohols or nonionic surfactants, such as Tergitol, gave materials which, in general, presented larger basal spacing than those found for the solids prepared by a similar procedure but without additives. The initial positive effect in the expansion of the clay interlayers was not totally retained after calcination of the materials; most probably, at the end, the basal spacing is still ruled by the intercalating aluminum species. The pillared clay with the largest basal spacing and specific surface area was used to encapsulate copper(II) complexes with pentadentate N3O2 Schiff base ligands derived from copper(II) acetylacetonate by in situ synthesis. The characterization made (X-ray diffraction, X-ray photoelectron spectroscopy, FTIR spectroscopy, chemical analysis, and low-temperature N2 adsorption) provided evidence that copper(II) complexes with pentadentate N3O2 Schiff base ligands were efficiently entrapped within the lower dimension pores of the pillared clay and that they interact strongly with the pillared clay matrix.     

Kinetic features of phase transformation of kaolinite into metakaolinite for kaolin clays from different deposits

The rate of the endothermic process of kaolinite transformation into metakaolinite was examined by X-ray phase and thermogravimetric analyses, as well as by means of differential scanning calorimetry, in relation to the particle size distribution and crystal structure characteristics of kaolinite in kaolin clays from different deposits.     

The kinetics of homogeneous and two-step nucleation during protein crystal growth from solution

Many experimental reports for the kinetics of crystal nucleation and growth, from an isothermal solution, point to a sigmoidal-like behavior for the process. Here we consider three different nucleation models from the literature and show that all lead to sigmoidal or sigmoidal-like behavior for the kinetics of nucleation. A two-step nucleation process is known to occur within certain supersaturated protein solutions, and it is demonstrated in this report how the sigmoidal law yields kinetic information for the two-step and homogeneous nucleation modes. We propose here that two-step solute-rich associates form in the solution around seed nuclei that are already present at or near the point in time when the solution is prepared. Using this hypothesis, we are able to model the time-dependent volume of the two-step phase per unit volume of solution and show that this compares well with reported experimental data. A kinetic model is given for the proposed process, which leads to a sigmoidal rate law. Additionally, a relation between the initial and final nuclei densities and the induction time is derived. As a result of this study, the conclusion is that two-step activity increases with increasing initial supersaturation or increasing salt concentration.