Novel method for the measurement of xenon gas solubility using 129Xe NMR spectroscopy

A novel method is presented for determining xenon partitioning between a gas phase and a liquid phase. An experimental setup which permits the simultaneous measurement of the 129Xe chemical shift in both the gas and the liquid phases, that is, under the same experimental conditions, has been designed. Xenon solubility is obtained via 129Xe chemical shift measurements in the gas phase. The method was validated against xenon solubility data from the literature; in general, the agreement is found to be within 3%. The solubility of xenon in three solvents for which data have not been previously reported (acetone, acetonitrile, and 1,1,2,2-tetrachloroethane) was determined using this novel method. 129Xe chemical shifts for dissolved xenon are also reported; it is found that xenon-xenon interactions may play a significant role in the liquid phase even at low equilibrium xenon pressures.     

Determination of pore sizes and volumes of porous materials by 129Xe NMR of xenon gas dissolved in a medium

In our previous paper (J. Phys. Chem. B 2005, 109, 757) it was illustrated that the 129Xe NMR spectra of xenon dissolved in acetonitrile confined into mesoporous materials give detailed information on the system, especially about the pore sizes. A resonance signal originating from xenon atoms sited in very small cavities built up inside the pores during the freezing transition (referred to as signal D) turned out to be highly sensitive to the pore size. The emergence of this signal reveals the phase transition temperature of acetonitrile inside the pores, which can also be used to determine the size of the pores. In addition, the difference in the chemical shifts of two other signals arising from xenon dissolved in bulk and confined acetonitrile (B and C) provides another method for determining the pore sizes. In the present work, the observed correlations have been investigated using an extensive set of measurements with a variety of porous materials (silica gels and controlled pore glasses) with the mean pore diameters ranging from 43 to 2917 A. The usefulness of the correlations has been demonstrated by calculating the pore size distributions from the spectral data. The distributions are in agreement with those reported by the manufacturers, when the mean pore diameter is smaller than approximately 500 A. In addition, it has been shown that the porosity of the materials can be determined by comparing the intensities of the signals arising from the bulk and confined liquid. When acetonitrile is replaced by cyclohexane in the sample, the dependence of the chemical shift difference between the B and C signals on the pore size becomes more sensitive, but no D signal appears below the freezing point. In addition, the influence of xenon gas on the melting points of bulk and confined acetonitrile has been studied by 1H NMR cryoporometry. The measurements show that the temperature of the latter transition lowers slightly more, and consequently affects the pore sizes calculated by means of the difference in the phase transition temperatures. Hysteresis in the phase transitions in a cooling-warming cycle has also been studied as a function of the temperature stabilization time by 129Xe NMR of xenon dissolved in acetonitrile.     

Diffusion aspects of lithium intercalation as applied to the development of electrode materials for lithium-ion batteries

The creation of new electrode materials and the modification of existing ones are important trends in the development of lithium-ion batteries. Of special significance is to evaluate their diffusivity, i.e., the ability of providing transfer of the electroactive component. Such electrochemical techniques as cyclic voltammetry, electrochemical impedance spectroscopy, potentiostatic intermittent titration technique, and galvanostatic intermittent titration technique are used for this purpose. The values of chemical diffusion coefficient D estimated in similar electrode materials are shown to scatter by several orders of magnitude. Principal causes of this rather considerable scattering are discussed, including the uncertainty of diffusion area estimations and the use of various approaches to deriving equations to calculate D. Our conclusions are illustrated by examples of D estimations in the electrode materials Li _x C₆, Li _x Sn, Li _x TiO₂, Li _x WO₃, LiM _y Mn_2?y O₄, and LiFePO₄.     

A CP-MAS NMR study of some deactivation methods in capillary gas chromatography

The effect of temperature, water content, and the type of reagent on the silylation of fused silica capillaries was studied by ²⁹Si and ¹³C CP-MAS NMR. Fumed silica (Cab-O-Sil M5), which is essentially a highly dispersed vitreous quartz with a surface comparable to that of fused silica capillary columns, was selected as a model material. Hexamethyldisilazane (HMDS) and 1,2-diphenyl-1,1,3,3-tetraphenyldisilazane (DPTMDS), which were used as silylation reagents, yielded trimethyl- and dimethylphenylsilyl surface groups respectively at lower temperatures (< 350°C and <250°C respectively). At higher temperatures, increasingly more dimethylsilyl groups are formed, with the silicon bound to two oxygen atoms. This process occurs for DPTMDS at a considerably lower temperature than for HMDS. The formation of silyl groups on the surface and the disappearance of hydroxyl groups are followed independently. The ¹³C NMR and GC-MS of the reaction products showed that with DPTMDS, the formation of two Si-O-Si links is accompanied by a loss of phenyl groups rather than of methyl groups. After the Cab-O-Sil had been dried over P₂O₅, the formation of these double links occurred for HMDS only at temperatures above 460°C and for DPTMDS at 400°C. Thus we concluded that water supplies oxygen atoms for double Si-O-Si links (possibly crosslinks) necessary for efficient deactivation. This may explain the less successful silanization of fused silica capillaries because their water content is lower than that of glass capillaries.     

Condition monitoring methods applied to degradation of chlorosulfonated polyethylene cable jacketing materials

Three promising polymer material condition monitoring (CM) methods were applied to eight commercial chlorosulfonated polyethylene cable jacket materials aged under both elevated temperature and high-energy radiation conditions. The CM methods examined, cross-sectional modulus profiling, solvent uptake and NMR T₂ relaxation time measurements of solvent-swelled samples, are closely related since they are all strongly influenced by the changes in overall crosslink density of the materials. Each approach was found to correlate well with ultimate tensile elongation measurements, the most widely used method for following degradation of elastomeric materials. In addition approximately universal failure criteria were found to be applicable for the modulus profiling and solvent uptake measurements, independent of the CSPE material examined and its degradation environment. For an arbitrarily assumed elongation “failure” criterion of 50% absolute, the CSPE materials typically reached “failure” when the modulus increased to ∼35 MPa and the uptake factor in p-xylene decreased to ∼1.6.     

Spectral transformations for deconvolution methods applied on gas chromatography-mass spectrometry data

In hyphenated chromatography, overlapping chromatographic peaks can be resolved into pure spectra and pure chromatographic profiles by several multivariate deconvolution techniques. In general, these methods require bilinearity, which implies that the spectrum of each analyte is constant. The slow scan speeds normally used in gas chromatography-mass spectrometry (GC-MS) will destroy bilinearity and introduce systematic noise in the data because the concentration in the detector changes during the scan. This effect, described as the scan effect, may hinder successful resolution by multivariate deconvolution. In selected ion monitoring (SIM) GC-MS, the scan effect may be removed by simple transformations of the mass spectra. The effects of different transformations are demonstrated both on pure chromatographic peaks and on difficult resolution problems where there are small differences between the spectra of the analytes.     

Reverse micelles dissolved in supercritical xenon: an NMR spectroscopic study

Reverse micelles currently gain increasing interest in chemical technology. They also become important in biomolecular NMR due to their ability to host biomolecules such as proteins. In the present paper, a procedure for the preparation of high-pressure NMR samples containing reverse micelles dissolved in supercritical xenon is presented. These reverse micelles are formed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT). For the first time, NMR spectroscopy could be applied to reverse micelles in supercritical xenon. The AOT/H(2)O/Xe system was studied as a function of experimental parameters such as xenon pressure, water content, and salt concentration. Optimum conditions for reverse micelle formation in supercritical xenon could be determined. It is, furthermore, demonstrated that biomolecules such as amino acids and proteins can be incorporated into the reverse micelles dissolved in supercritical xenon.     

Scientific methods applied to the study of art objects

Scientific methods applied to the study of works of art are varied (examination, analysis and dating) and must be chosen with care in accordance with the object of the research. This paper briefly sets out the state of the art highlighting the development and applications of non-destructive methods. Such methods come under either the use of electromagnetic or particle radiation for the formation of images based on shape, development technique, or, on the other hand, the quantitative determination of elements down to and including their traces, isotopes or microsurfaces for the purpose of identifying the matter, its date, origin and development.     

Chemometric methods applied to the calibration of a Vis-NIR sensor for gas engine's condition monitoring

This paper describes the calibration process of a Visible-Near Infrared sensor for the condition monitoring of a gas engine's lubricating oil correlating transmittance oil spectra with the degradation of a gas engine's oil via a regression model. Chemometric techniques were applied to determine different parameters: Base Number (BN), Acid Number (AN), insolubles in pentane and viscosity at 40 °C. A Visible-Near Infrared (400-1100 nm) sensor developed in Tekniker research center was used to obtain the spectra of artificial and real gas engine oils. In order to improve sensor's data, different preprocessing methods such as smoothing by Saviztky-Golay, moving average with Multivariate Scatter Correction or Standard Normal Variate to eliminate the scatter effect were applied. A combination of these preprocessing methods was applied to each parameter. The regression models were developed by Partial Least Squares Regression (PLSR). In the end, it was shown that only some models were valid, fulfilling a set of quality requirements. The paper shows which models achieved the established validation requirements and which preprocessing methods perform better. A discussion follows regarding the potential improvement in the robustness of the models.     

NMR spectroscopy applied to photochromism investigations

Despite its poor time-resolution and low sensitivity compared with classical spectrophotometric methods, modern NMR is now a highly developed spectroscopy technique, appropriate for photochromism studies owing to its high spectral resolution and two-dimensional (2D) NMR techniques that offer detailed structural and quantitative information. NMR spectroscopy can therefore be applied to answer questions concerning which compounds are produced, how they are formed and how they evolve and behave within the photochromic reaction. In this review, we have chosen among the wide variety of photochromic molecules, some specific examples in the family of spirocompounds, naphthopyrans, dihydropyrenes, diarylethenes and aryldiketones to illustrate the ways in which NMR has proved useful in the kinetic and structural studies of photochromism.     

NMR spectroscopic study of xenon fluorides in the gas phase and of XeF2 in the solid state

The xenon fluorides, XeF₂, XeF₄, and XeF₆, were characterized by gas-phase and NMR spectroscopy, providing the first gas-phase NMR spectroscopic data for the xenon fluorides. Xenon difluoride was also characterized in the solid state by static and magic angle spinning (MAS) and NMR spectroscopy, providing experimental values for the (4260±10 ppm) and (125±5 ppm) shielding anisotropies in XeF₂. A method for encapsulating reactive fluoride samples for study by MAS NMR spectroscopy is also described.     

A study of some deactivation methods for fused silica capillary columns by CP-MAS NMR and capillary gas chromatography

The effect of deactivating a fused silica surface by silylation with 1,1,3,3-tetraphenyl-1,3-dimethylilazane (TPDMDS), triphenylsilylamine (TPSA), and octamethylcyclotetrasiloxane (D4) and by polydimethylsiloxane degradation (PSD) is studied. Rehydrated, dried, and deactivated Cab-O-Sil M5 samples are used as model materials for ²⁹Si CP-MAS NMR analysis. At about 350 °C, TPDMDS yelds mainly diphenylmethylsiloxysilane, dimethyldisiloxysilane, and triphenylsiloxysilane groups. TPSA yields phenyltrisiloxysilane, diphenyldisiloxysilane, and triphenylsiloxysilane groups. At 400°C, the products formed initially are eventually replaced by methyltrisiloxysilane or phenyltrisiloxysilane groups, while a substantial number of silanol groups still remains. The possible consequences for wettability are discussed. D4 reacts with Cab-O-Sil even at 200°C, but a large number of silanol groups remains. This number decreases gradually at higher temperatures and becomes negligible above 400°C. The formation of methyltrisiloxysilane groups, which starts at 425°C, is predominant at 490°C.     

Time-Resolved Diffusion NMR Measurements for Transient Processes

A general procedure for measurement of time-resolved diffusion coefficients of molecular species by NMR is described, including the use of methanol for fast temperature-independent gradient calibration.     

Comparative study of DFT methods applied to small titanium/oxygen compounds

The performance of a number of different local and nonlocal density functional theory (DFT) methods has been investigated for some small titanium—oxygen systems. Equilibrium geometries, ionization potentials, dipole moments, atomization energies, and harmonic vibrational frequencies have been calculated for the TiO, TiO₂, and Ti₂ molecules, and the results are compared with experimental data and ab initio calculations. It is shown that most DFT methods perform much better than the ab initio Hartree—Fock (HF), second-order perturbation theory (MP2), and configuration interaction including single and double excitations (CISD) treatments. For good agreement with experimental data, gradient corrections to the exchange part of the DFT functional are needed, as well as some type of correction for the errors in the calculated energy splittings between different atomic states of titanium. Hybrid methods including a mixture of HF exchange with DFT exchange correlation do not perform as well as “pure” DFT methods for the studied systems. © 1996 John Wiley & Sons, Inc.     

Modern microscopy methods for the structural study of porous materials

This article describes a number of important recent microscopy tools and their application in particular to the study of porous inorganic materials. The authors believe that these new techniques are on the threshold of delivering enormous new power in the chemist's arsenal for understanding new and complex behaviour in multi-component, hierarchical or composite materials. In particular we consider the contribution of electron crystallography, three-dimensional electron tomography, ultra-high resolution scanning electron microscopy as well as the combined application of high-resolution electron microscopy and atomic force microscopy to the study of surfaces and crystal growth. Much of this work has taken on a particular significance owing to the ground breaking work of scientists at Mobil and in Japan 10 years ago in the successful synthesis of materials with porosity on many length scales achieved through the cooperative self-assembly between inorganic and organic phases. This resulted in a series of materials known as M41S of which MCM-41 and MCM-48 were two of the first and most important structures to be synthesised. This has led to a wealth of new porous structures with order over many length scales and has presented new problems in characterisation. Microscopy methods properly executed are particularly important in the study of this new class of material.     

NMR diffusion and relaxation correlation methods: New insights in heterogeneous materials

Heterogeneous materials, such as biological tissues, foodstuffs, and rocks, contain a range of microscopic environments where the molecular constituents often have different NMR relaxation time constants and self-diffusion coefficients. Multidimensional correlation methods have greatly improved the possibility for separating and assigning the NMR responses from distinct environments, thereby allowing for a more complete characterization of structure, dynamics, and molecular exchange in heterogeneous materials. Here, we review recent developments in experimental methodology and data analysis approaches.     

A comparative study of methods for determining gas hold-up time

It has been proved mathematically that there exists an identity between the various published methods for determining the hold-up times in gas chromatography. All of these methods are based on the equation: ln t'(R) = ln (t(R) - t(M)) = a + bZ, where t(R), t'(R), and t(M) are retention times, adjusted retention times, and hold-up times, respectively; a and b are coefficients that depend on the experimental conditions, and Z is the numbers of carbon atom of a homologue. So long as there are no errors in the values of the retention times, the hold-up times obtained by different methods based on this equation will be the same. The calculated results of experimental data reported in the literature support the viewpoints presented here.