Dynamic Monte-Carlo simulations of diffusion limited reactions in rough nanopores

It has recently been demonstrated that self- (or tracer) diffusivities and transport diffusivities of gases in nanoporous materials may differ in the Knudsen regime, when the pore surface is rough on molecular scales. Self-diffusion is roughness dependent, while transport diffusion is not. The impact of this result on diffusion-controlled reactions, catalyzed by porous catalysts with a rough internal surface, is now investigated. We consider the steady-state problem of a species diffusing in a three-dimensional pore with a fractal surface, on which the species reacts. Dynamic Monte-Carlo simulations and analytical calculations show that the self-diffusivity plays the fundamental role in determining the effective catalytic efficiency of a rough pore. The transport diffusivity, on the other hand, is not the fundamental quantity of interest to surface phenomena such as heterogeneous catalysis, and can only be used for porous materials with a smooth internal surface, in which case the self- and transport diffusivities coincide.     

Ion mobility in Nafion-117 membranes

Transport phenomena in Nafion-117 perfluorosulphonic membranes have been studied by different techniques. Membrane electric conductivity has been measured under alternative current as a function of the equilibrating concentrations of MCI (M = Li, Na, K) and HCl solutions. Conductivities were found to increase for lithium and sodium membrane forms and decrease for the potassium form with an electrolyte concentration increase. The obtained data were compared with the results of diffusion permeability, ion-exchange kinetics, ⁷Li and ³⁵Cl NMR measurements. In all cases permeability increased with electrolyte concentration growth. Diffusion coefficients and transport numbers for different counter- and co-ions were determined. The obtained data are discussed with the use of the two-phase membrane structure model. The selectivity of membrane sorption was found to increase with growth of cation radius.     

Effect of molecular architecture on the self-diffusion of polymers in aqueous systems: A comparison of linear, star, and dendritic poly(ethylene glycol)s

Star polymers with a hydrophobic cholane core and four poly(ethylene glycol) (PEG) arms, CA(EG_n)₄, have been synthesized by anionic polymerization. Pulsed-gradient spin-echo NMR spectroscopy was used to study the diffusion behavior of the star polymers, ranging from 1000 to 10,000 g/mol, in aqueous solutions and gels of poly(vinyl alcohol) (PVA) at 23 °C. The star polymers have a lower self-diffusion coefficient than linear PEGs at equivalent hydrodynamic radius. In water alone, the star polymers and their linear homologues have a similar diffusion behavior in the dilute regime, as demonstrated by the similar concentration dependence of the self-diffusion coefficients. In the semidilute regime, the star polymers tend to aggregate due to their amphiphilic properties, resulting in lower self-diffusion coefficients than those of linear PEGs. ¹H NMR T₁ measurements at 10-70 °C revealed that the PEG arms of the star polymers are more mobile than the core, suggesting the star polymers in solution have a conformation similar to that of poly(propylene imine) dendrimers.     

Influence of the migration of chloride ions on the electrochemical impedance spectroscopy of mortar paste

During the last decade, electrochemical impedance spectroscopy (EIS) started to be effectively exploited to investigate the electrical properties of hydrated cement pastes.This paper is devoted to the study of the high- and medium-frequency regions of the impedance spectrum and focused on determining the relationship between chloride migration and impedance spectroscopy in accelerated diffusion tests carried out on samples of mortar.After introducing an experimental protocol based on a four-electrode arrangement, we present the results of a parametric study which is related to the thickness and water-cement ratio (W/C) of the samples.Firstly, all our measurements show that the presence of chloride ions modifies the impedance response of mortar and reveals a small loop as of modification of the composition of the upstream solution. This loop is probably due to the development of interfacial phenomena between material and the solution.Secondly, as the migration of chloride process progresses, an increase followed by a decrease of the bulk electrical resistance is observed, whereas the second loop, due to the presence of chlorides, remains constant.An equivalent electrical circuit is then proposed to fit the different experimental data.     

Jörg Kärger: Diffusion Is His Life

Jörg Kärger is a pioneer in the field of diffusion and his work has contributed significantly to the understanding of molecular mass transfer in porous systems. He has been actively researching diffusion in nanoporous materials for more than 50 years and, according to the Web of Science, is the most productive author in the field of diffusion, with 247 entries (search for diffusion in the title). He also has the highest total local citation score in the field of diffusion, which indicates not only the high number but also the high quality of his publications.     

Theoretical and Experimental Spectroscopic Analysis of Cyano-Substituted Styrylpyridine Compounds

A combined theoretical and experimental study on the structure, infrared, UV-Vis and ¹H NMR data of trans-2-(m-cyanostyryl)pyridine, trans-2-[3-methyl-(m-cyanostyryl)] pyridine and trans-4-(m-cyanostyryl)pyridine is presented. The synthesis was carried out with an efficient Knoevenagel condensation using green chemistry conditions. Theoretical geometry optimizations and their IR spectra were carried out using the Density Functional Theory (DFT) in both gas and solution phases. For theoretical UV-Vis and ¹H NMR spectra, the Time-Dependent DFT (TD-DFT) and the Gauge-Including Atomic Orbital (GIAO) methods were used, respectively. The theoretical characterization matched the experimental measurements, showing a good correlation. The effect of cyano- and methyl-substituents, as well as of the N-atom position in the pyridine ring on the UV-Vis, IR and NMR spectra, was evaluated. The UV-Vis results showed no significant effect due to electron-withdrawing cyano- and electron-donating methyl-substituents. The N-atom position, however, caused a slight change in the maximum absorption wavelengths. The IR normal modes were assigned for the cyano- and methyl-groups. ¹H NMR spectra showed the typical doublet signals due to protons in the trans position of a double bond. The theoretical characterization was visibly useful to assign accurately the signals in IR and ¹H NMR spectra, as well as to identify the most probable conformation that could be present in the formation of the styrylpyridine-like compounds.     

Moisture transport in coated wood

Moisture accumulation inside wood causes favorable conditions for decay. Application of a coating alters the moisture sorption of wood and prevents accumulation of moisture. This paper presents the results of a nuclear magnetic resonance (NMR) study on the influence of a coating on the moisture absorption of wood.NMR allows to determine both local wood moisture content and rate of water absorption during water absorption and desorption of coated and uncoated wood. In contrast to weighing, both quantities are measured dynamically and non destructively with high spatial and temporal resolution in relatively short experiments. In addition, NMR relaxometry distinguishes between moisture in lumina and moisture in cell walls, which allows to accurately characterize sorption processes in wood. In the present study, samples with a diameter of 20 mm and a height of 10 mm, are studied in a 4.7 T NMR scanner with a spatial resolution of 33 ± 3 μm.Several commonly used wood–coating combinations are studied. Water is placed on the tangential side of samples equilibrated at 22% relative humidity while the wood moisture content (MC) is monitored for 24 h. This research shows that the sorption behavior of coated wood depends on the specific combination of wood and coating. Additionally, the amount of water that is absorbed in a coating may have a strong influence on the moisture content of the coated wood.We found that the water absorption of a hardwood dark red meranti sample is diffusion dominated. Application of a waterborne acrylic coating has no influence on this absorption process, which is attributed to the large water uptake of the coating. A solvent borne alkyd coating that absorbs very little water is found to strongly reduce the water uptake of the meranti studied. The waterborne coating reduces the water uptake of pine by preventing capillary water uptake of rays present in this softwood. The solvent borne alkyd coating further reduces uptake. Uncoated spruce also absorbs water by capillary suction. On this wood, the acrylic coating strongly reduces the water uptake; comparable to alkyd coated spruce. This is a result of the pits of spruce which became aspirated during drying. Application of a coating might fixate the aspirated pits, resulting in a structure with low permeability.     

红外光谱法测定二元乙丙橡胶中乙烯、丙烯组成

阐述了测定乙丙橡胶中乙烯、丙烯组成的意义和方法,借助核磁共振数据建立了红外光谱法的标准工作曲线,并对其精确性进行了评价。     

Diffusion in Nanoporous Solids in the Focus of IUPAC – A Tribute to Jens Weitkamp

In bidding farewell to Prof. Dr.-Ing. Jens Weitkamp, we remember his many activities and his beneficial influence on the development of science, notably in the various fields of chemical technology and engineering. These activities include his engagement in support of the development of techniques dedicated to improvement of our understanding of the complex phenomenon of mass transfer in nanoporous materials, as both an ingenious scientist and a brilliant organizer and gifted leader of scientific alliances. The present contribution provides a brief introduction to the many facets of research in this field, with particular recognition of the most recent developments and Prof. Weitkamp's contributions.     

红外光谱法定量测定聚异戊二烯橡胶顺式-1，4-结构含量

研究红外光谱法测定聚异戊二烯橡胶(IR)的顺式-1,4-结构含量。结果表明,通过测定红外光谱中顺式-1,4和3,4-结构双键上碳-氢键变角振动吸收峰的积分面积,并与核磁共振氢谱测定的二者比值相比较,确定二者红外光谱特征峰出峰效率校正因子,计算得出IR的顺式-1,4-结构含量。红外光谱法定量分析IR顺式-1,4-结构含量操作简单、快捷,结果准确。