Low-field NMR for quality control on oils

Oil is a prominent, but multifaceted material class with a wide variety of applications. Technical oils, crude oils as well as edibles are main subclasses. In this review, the question is addressed how low-field NMR can contribute in oil characterization as an analytical tool, mainly with respect to quality control. Prerequisite in the development of a quality control application, however, is a detailed understanding of the oils and of the measurement. Low-field NMR is known as a rich methodical toolbox that was and is explored and further developed to address questions about oils, their quality, and usability as raw materials, during production and formulation as well as in use.     

Influence of the convective term in the Nernst-Planck equation on properties of ion transport through a layer of solution or membrane

The one-dimensional boundary-value problem of steady-state ion transport, which takes into account the convective component, is formulated and solved in terms of the Nernst-Planck model. This problem is investigated in connection with the diffusion layer, which is understood in a broad sense. This can be the diffusion layer as it is usually understood, i.e., located adjacent to a hydraulically permeable membrane. In another context it can be regarded as a capillary connecting two reservoirs filled with solutions of different concentration or as an uncharged macropore permeating the membrane and separating two solutions. Finally, the solution to the problem is applied to the membrane itself, which is represented as a quasi-homogeneous gel. In the latter case, a virtual electroneutral solution in local equilibrium with a small volume of membrane is considered. The problem is investigated in dimensionless form as a function of the Peclet number. It is shown that the Peclet number is numerically equal to the absolute value of the dimensionless convection velocity. The limiting current, concentration profiles, distributions of the field strength and potential, and effective transport numbers are analyzed as functions of the convective component.     

On the use of alumina in HPLC with aqueous mobile phases at extreme pH

Summary The applicability of alumina as a stationary phase in liquid chromatography is considered. Despite the presence of hydroxyl groups on wetted alumina, chemical modification with silanes by analogy to silica is not successful. Although depolarisation can be achieved with hexamethyldisilazane, the product is not stable to hydrolysis in aqueous media. The use of alumina as an ion-exchanger is more promising, especially because alumina appears to be stable over a very broad pH range. As a result of its amphoteric character alumina can be used either as an anion-exchanger or as a cation-exchanger. The latter possibility is more profitable in alkaline solvents and allows the separation of strongly basic compounds at pH values as high as 12. Retention can be controlled by judicious choice of ionic strength, counter ion and pH.     

Electrostatic energies and forces computed without explicit interparticle interactions: a linear time complexity formulation

A rapid method for the calculation of the electrostatic energy of a system without a cutoff is described in which the computational time grows linearly with the number of particles or charges. The inverse of the distance is approximated as a polynomial, which is then transformed into a function whose terms involve individual particles, instead of particle pairs, by a partitioning of the double sum. In this way, the electrostatic energy that is determined by the interparticle interactions is obtained without explicit calculation of these interactions. For systems of positive charges positioned on a face-centered cubic lattice, the calculation of the energy by the new method is shown to be faster than the calculation of the exact energy, in many cases by an order of magnitude, and to be accurate to within 1-2%. The application of this method to increase the accuracy of conventional truncation-based calculations in condensed-phase systems is also demonstrated by combining the approximated long-range electrostatic interactions with the exact short-range interactions in a "hybrid" calculation. For a 20-A sphere of water molecules, the forces are shown to be six times as accurate using this hybrid method as those calculated with conventional truncation of the electrostatic energy function at 12 A. This is accomplished with a slight increase in speed, and with a sevenfold increase in speed relative to the exact all-pair calculation. Structures minimized with the hybrid function are shown to be closer to structures minimized with an exact all-pair electrostatic energy function than are those minimized with a conventional 13-A cutoff-based electrostatic energy function. Comparison of the energies and forces calculated with the exact method illustrate that the absolute errors obtained with standard truncation can be very large. The extension of the current method to other pairwise functions as well as to multibody functions, is described.     

Regiodivergent C−H Acylation of Arenes by Switching from Ionic- to Radical-Type Chemistry Using NHC Catalysis

The Friedel–Crafts acylation reaction, which belongs to the class of electrophilic aromatic substitutions is a highly valuable and versatile reaction in synthesis. Regioselectivity is predictable and determined by electronic as well as steric factors of the (hetero)arene substrate. Herein, a radical approach for the acylation of arenes and heteroarenes is presented. C−H acylation is achieved through mild cooperative photoredox/NHC radical catalysis with the cross-coupling of an arene radical cation with an NHC-bound ketyl radical as a key step. As compared to the classical Friedel–Crafts acylation, a regiodivergent outcome is observed upon switching from the ionic to the radical mode. In these divergent reactions, aroyl fluorides act as the acylation reagents in both the ionic as well as the radical process.     

A simple model for calorimeters with temperature programming

We present a theoretical study of an RC-model constituted only by one heat capacity and one coupling with the thermostat. It is assumed that the thermostat temperature varies as a function of time, and the heat capacity variation is due to its dependence either on temperature or on the mass exchange with the exterior.The results are parallel to the corresponding RC-models where the thermostat temperature is constant. The variations of sensibility are shown, as well as a criterion for the applicability of inverse filtering as a deconvolution technique in calorimeters with temperature programming.     

Sequential injection with lab-at-valve (LAV) approach for potentiometric determination of chloride

Sequential injection with “Lab-at-Valve (LAV)” approach is demonstrated for potentiometric determination of chloride. The LAV flow-through electrode system consists of two Ag/AgCl electrodes: one as a reference electrode, silver chloride activated surface-silver wire soaked in a constant-concentration chloride ion solution in a small tube covered with a polymer-membrane, another as a working electrode (a similar silver chloride activated surface-silver wire) placed in a flow channel. The electrode system is attached at one port of a 10 port multiposition valve. A modified autoburette was used as a propelling device. Using SI operation via a program written in-house, based on LabVIEW^®, a standard/sample is inserted, via the selection valve, in potassium nitrate as an electrolyte and water is used as a carrier. The zones are transported from the holding coil to the flow cell to monitor the difference in potential due to concentration cell behavior. The potential difference is then recorded as a peak. Peak height is proportional to logarithm of chloride concentration. The SI-LAV for chloride determination is very simple, fast, precise, accurate, automatic and economical. Applications to mineral drinking water and surface water have been made. The results agree with those of IC and titrimetric methods.     

Chromatographic shape selectivity with carbon dioxide-acetonitrile mobile phases. Effect of mobile phase composition and density

Trends in chromatographic shape selectivity with mobile phases consisting of mixtures of carbon dioxide and acetonitrile are investigated. Selectivity is evaluated as a function of mobile phase composition, temperature, and column bonding chemistry. SRM (standard reference material) 869a is used as a probe of shape selectivity, while the selectivity between triphenylene and o-terphenyl is used to investigate planarity selectivity. Four molecular mass 228 polyaromatic hydrocarbon isomers are used to investigate shape selectivity based on differences in length-to-breadth ratio. Shape selectivity trends as a function of temperature and column type are found to be similar to what is seen in reversed-phase liquid chromatography, while the trend seen as the amount of acetonitrile in the mobile phase increases is found to be different than in reversed-phase liquid chromatography. In addition, the effect of mobile phase density, i.e., solvent strength, on shape selectivity is investigated by examining shape selectivity as a function of density with neat carbon dioxide as the mobile phase.     

Fully retarded van der Waals interaction between dielectric nanoclusters

The van der Waals (dispersion) interaction between an atom and a cluster or between two clusters at large separation is calculated by considering each cluster as a point particle, characterized by a polarizability tensor. For the extreme limit of very large separation, the fully retarded regime, one needs to know just the static polarizability in order to determine the interaction. This polarizability is evaluated by including all many-body (MB) intracluster atomic interactions self-consistently. The results of these calculations are compared with those obtained from various alternative methods. One is to consider each cluster as a collection of many atoms and evaluate the sum of two-body interatomic interactions, a common assumption. An alternative method is to include three-body atomic interactions as a MB correction term in the total energy. A comparison of these results reveals that the contribution of the higher-than-three-body MB interactions is always attractive and non-negligible even at such a large separation, in contrast to common assumptions. The procedure employed is quite general and is applicable, in principle, to any shape or size of dielectric cluster. We present numerical results for clusters composed of atoms with polarizability consistent with silica, for which the higher-than-three-body MB correction term can be as high as 42% of the atomic pairwise sum. This result is quite sensitive to the anisotropy and orientation of the cluster, in contrast to the result found in the additive case. We also present a power law expansion of the total van der Waals interaction as a series of n-body interaction terms.     

ZnO: material, physics and applications.

ZnO is presently experiencing a research boom with more than 2000 ZnO-related publications in 2005. This phenomenon is triggered, for example, by hope to use ZnO as a material for blue/UV optoelectronics as an alternative to GaN, as a cheap, transparent, conducting oxide, as a material for electronic circuits that are transparent in the visible or for semiconductor spintronics. Currently, however, the main problem is to achieve high, reproducible and stable p-doping. Herein, we critically review aspects of the material growth, fundamental properties of ZnO and ZnO-based nanostructures and doping as well as present and future applications with emphasis on the electronic and optical properties including stimulated emission.     

DFT study of the mechanisms of in water Au(I)-catalyzed tandem [3,3]-rearrangement/Nazarov reaction/[1,2]-hydrogen shift of enynyl acetates: a proton-transport catalysis strategy in the water-catalyzed [1,2]-hydrogen shift

A computational study with the Becke3LYP density functional was carried out to elucidate the mechanisms of Au(I)-catalyzed reactions of enynyl acetates involving tandem [3,3]-rearrangement, Nazarov reaction, and [1,2]-hydrogen shift. Calculations indicate that the [3,3]-rearrangement is a two-step process with activation free energies below 10 kcal/mol for both steps. The following Nazarov-type 4pi electrocyclic ring-closure reaction of a Au-containing dienyl cation is also easy with an activation free energy of 3.2 kcal/mol in CH2Cl2. The final step in the catalytic cycle is a [1,2]-hydride shift, and this step is the rate-limiting step (with a computed activation free energy of 20.2 kcal/mol) when dry CH2Cl2 is used as the solvent. When this tandem reaction was conducted in wet CH2Cl2, the [1,2]-hydride shift step in dry solution turned to a very efficient water-catalyzed [1,2]-hydrogen shift mechanism with an activation free energy of 16.4 kcal/mol. Because of this, the tandem reaction of enynyl acetates was found to be faster in wet CH2Cl2 as compared to the reaction in dry CH2Cl2. Calculations show that a water-catalyzed [1,2]-hydrogen shift adopts a proton-transport catalysis strategy, in which the acetoxy group in the substrate is critical because it acts as either a proton acceptor when one water molecule is involved in catalysis or a proton-relay stabilizer when a water cluster is involved in catalysis. Water is found to act as a proton shuttle in the proton-transport catalysis strategy. Theoretical discovery of the role of the acetoxy group in the water-catalyzed [1,2]-hydrogen shift process suggests that a transition metal-catalyzed reaction involving a similar hydrogen shift step can be accelerated in water or on water with only a marginal effect, unless a proton-accepting group such as an acetoxy group, which can form a hydrogen bond network with water, is present around this reaction's active site.     

Investigation of helium addition for laser-induced plasma spectroscopy of pure gas phase systems: Analyte interactions and signal enhancement

The role of helium addition on the analyte signal enhancement in laser-induced breakdown spectroscopy for analysis of pure gaseous systems was examined using carbon and hydrogen atomic emission lines. Increased analyte response, as measured by peak-to-base and signal-to-noise ratios, was observed with increasing helium addition, with maximum enhancement approaching a factor of 7. Additional measurements revealed a significant decrease in plasma electron density with increasing helium addition. To explore the mechanisms of analyte signal enhancement, the helium emission lines were also examined and found to be effectively quenched with nitrogen addition. In consideration of the data, it is concluded that the role of metastable helium is not as important as the overall changes in plasma properties, namely electron density and laser-plasma coupling. Helium addition is concluded to affect the electron density via Penning ionization, as well as to play a role in the initial plasma breakdown processes.     

Incubation Effect of Pre-Irradiation on Bubble Formation and Ablation in Laser Ablation in Liquids

Pulsed laser ablation in liquids (PLAL) is a multi-scale process, which is widely studied either in batch ablation with prolonged target irradiation as well as mechanistic investigations, in a defined (single-shot) process. However, fundamental studies on defined pulse series are rare. We have investigated the effect of a developing rough morphology of the target surface on the PLAL process with nanosecond pulses and, partially, picosecond pulses. At low fluence the cavitation bubble growth as well as the ablation yield depend on the irradiation history of the target. The bubble size increases with repeated irradiation on one spot for the first 2–30 pulses as well as with the applied dose. This is discussed within the framework of incubation effects. Incubation is found to be important, resulting in a bubble volume increase by a factor of six or more between pristine and corrugated targets. The target surface, changing from smooth to corrugated, induces a more efficient localization of laser energy at the solid-liquid interface. This is accompanied by a suppressed reflectivity and more efficient coupling of energy into the laser-induced plasma. Thus, the cavitation bubble size increases as well as ablation being enhanced. At high fluence, such incubation is masked by the rapid development of surface damage within the first shots, which eventually would lead to a reduction of bubble sizes.     

用大孔吸附树脂从发酵液中提取顺-1,2-二羟基-3,5-环己二烯

顺-1，2-二羟基-3，5-环己二烯(DHCD)是化学合戍高性能材料1．4-聚苯的单体。本文系统地研究了用大孔吸附树脂从发酵液中提取DHCD的方法，进行了树脂的筛选．考查了流速．溶液中葡萄糖浓度等因素对吸附能力的影响．选择极性的NKA-Ⅱ树脂作为吸附剂，采用12ml／mm的流速．用甲醇作为洗脱剂对DHCD发酵液进行提取使提取率达到82，2％．大大优于现有的用二氟甲烷萃取的工艺。     

Mapping electric fields generated by microelectrodes using optically trapped charged microspheres

In this work, we show a new technique to measure the direction and amplitude of the electric field generated by microelectrodes in a liquid environment, as often used in microfluidic devices. The method is based on the use of optical tweezers as a force transducer. A trapped, charged particle behaves as a probe. With this technique, it is possible to obtain a detailed map of the electric field, even for very complex electrode structures with a resolution below a micrometre and with a sensitivity as low as a few hundreds of V m(-1).     

Noninvasive fluid flow measurements in microfluidic channels with backscatter interferometry

The ability to measure fluid velocity within picoliter volumes or on-chip noninvasively, is important toward fully realizing the potential of microfluidics and micrototal analysis systems, particularly in applications such as micro-high-performance liquid chromatography (HPLC) or in metering mixing where the flow rate must be quantified. Additionally, these measurements need to be performed directly on moving fluids in a noninvasive fashion. We presented here the proof of principle experiments showing nonintrusive fluid flow measurements can be accomplished on-chip using a pump and probe configuration with backscattering interferometry. The on-chip interferometric backscatter detector (OCIBD) is based on a fiber-coupled HeNe laser that illuminates a portion of an isotropically etched 40 microm radius channel and a position sensitive transducer to measure fringe pattern shifts. An infrared laser with a mechanical shutter is used to heat a section of a flowing volume and the resulting refractive index (RI) change is detected with the OCIBD downstream as a time-dependent RI perturbation. Fluid velocity is quantified as changes in the phase difference between the shutter signal and the OCIBD detected signal in the Fourier domain. The experiments are performed in the range of 3-6 microL/h with 3sigma detection limits determined to be 0.127 nL/s. Additionally, the RI response of the system is calibrated using temperature changes as well as glycerol solutions.     

Quantum Mechanical/Molecular Mechanical Study of the HDV Ribozyme: Impact of the Catalytic Metal Ion on the Mechanism

A recent crystal structure of the precleaved HDV ribozyme along with biochemical data support a mechanism for phosphodiester bond self-cleavage in which C75 acts as a general acid and bound Mg(2+) ion acts as a Lewis acid. Herein this precleaved crystal structure is used as the basis for quantum mechanical/molecular mechanical calculations. These calculations indicate that the self-cleavage reaction is concerted with a phosphorane-like transition state when a divalent ion, Mg(2+) or Ca(2+), is bound at the catalytic site but is sequential with a phosphorane intermediate when a monovalent ion, such as Na(+), is at this site. Electrostatic potential calculations suggest that the divalent metal ion at the catalytic site lowers the pK(a) of C75, leading to the concerted mechanism in which the proton is partially transferred to the leaving group in the phosphorane-like transition state. These observations are consistent with experimental data, including pK(a) measurements, reaction kinetics, and proton inventories with divalent and monovalent ions.     

Melt crystallized polyamide 6.6 and its copolymers, Part II. Crystallization mechanisms in the homopolymer

The crystallization kinetics of polyamide 66 have been studied using polarized optical microscopy. The growth rate data for positively birefringent spherulites in polyamide 66 show a distinct change of slope, which would be identified as a regime I/II transition based on secondary nucleation theory. However, recent data for the same specimens crystallized isothermally, from small angle X-ray scattering found the lamellar thickness to be constant at approximately 2.0 chemical repeat units, but with an internal crystalline core and a substantial innerlayer. The crystal core increases in size to 2 chemical repeat units with both time and temperature at the expense of the inner layer. This evidence is totally inconsistent with secondary nucleation theory, where a lamellar thickness which varies significantly with supercooling is an integral part of the derivations.A calculation of the dimensions of the critical nucleus is reported here, using surface free energies, which found it to be impossibly large at a value between 14 and 360 stems in size, further suggesting that another crystallization mechanism is operating. Calculations of the surface free energy of the hydrogen-bonded surface suggest that it is the high energy surface, rather than the folded surface, which normally occurs as the high energy surface in polymers. As the high energy surface, the hydrogen-bonded surface would be expected to be the growth face, as occurs in non-polymeric materials. An earlier model of Lovinger, which placed the fold direction into the melt, generating a rough surface, is consistent with these results.It is suggested that crystallization in polyamide 66, if not in all polyamides, occurs through a surface roughening mode of growth. This is a natural consequence of the presence of H-bonding in the direction of growth. In one sense, polyamide 66 is conventional in its growth behavior, relative to non-polymeric materials, as the growth face is the highest energy surface. As such, the lamellar thickness would no longer be a morphological variable related to the supercooling in any direct way as an essential part of any crystallization theory for polymers. Such behavior is impossible in other polymeric systems as the fold surface is the highest energy surface and the presence of folds prohibits growth on that surface. However, models of surface roughening, which were developed as an alternative explanation for the behavior of, principally, polyethylene, predict the conventional lamellar thickness - supercooling relation to exist, which is inconsistent with the observed existence of a constant lamellar thickness with variable crystal core dimensions.It is suggested that polyamide 66 be taken as a paradigm for a different kind of polymer crystallization than that normally encountered. That is crystallization in which the growth face is the highest energy surface, not the folded chain surface, having much in common with the behavior of non-polymeric materials. The energetic changes occurring in this case, however, are governed by a combination of entropic and enthalpic barriers to crystallization, rather than being dominated by enthalpic considerations, as in metals. This is a direct result of the entropic effects of the long chain nature of polymers combined with the enthalpic effects of the hydrogen-bonding.     

Bending elasticity of charged surfactant layers: the effect of mixing

Expressions have been derived from which the spontaneous curvature (H(0)), bending rigidity (k(c)), and saddle-splay constant (k(c)) of mixed monolayers and bilayers may be calculated from molecular and solution properties as well as experimentally available quantities such as the macroscopic hydrophobic-hydrophilic interfacial tension. Three different cases of binary surfactant mixtures have been treated in detail: (i) mixtures of an ionic and a nonionic surfactant, (ii) mixtures of two oppositely charged surfactants, and (iii) mixtures of two ionic surfactants with identical headgroups but different tail volumes. It is demonstrated that k(c)H(0), k(c), and k(c) for mixtures of surfactants with flexible tails may be subdivided into one contribution that is due to bending properties of an infinitely thin surface as calculated from the Poisson-Boltzmann mean field theory and one contribution appearing as a result of the surfactant film having a finite thickness with the surface of charge located somewhat outside the hydrophobic-hydrophilic interface. As a matter of fact, the picture becomes completely different as finite layer thickness effects are taken into account, and as a result, the spontaneous curvature is extensively lowered whereas the bending rigidity is raised. Furthermore, an additional contribution to k(c) is present for surfactant mixtures but is absent for k(c)H(0) and k(c). This contribution appears as a consequence of the minimization of the free energy with respect to the composition of a surfactant layer that is open in the thermodynamic sense and must always be negative (i.e., k(c) is generally found to be brought down by the process of mixing two or more surfactants). The magnitude of the reduction of k(c) increases with increasing asymmetry between two surfactants with respect to headgroup charge number and tail volume. As a consequence, the bending rigidity assumes the lowest values for layers formed in mixtures of two oppositely charged surfactants, and k(c) is further reduced in anionic/cationic surfactant mixtures where the surfactant in excess has the smaller tail volume. Likewise, the reduction of k(c) is enhanced in mixtures of an ionic and a nonionic surfactant where the ionic surfactant has the smaller tail. The effective bilayer bending constant (k(bi)) is also found to be reduced by mixing, and as a result, k(bi) is seen to go through a minimum at some intermediate composition. The reduction of k(bi) is expected to be most pronounced in mixtures of two oppositely charged surfactants where the surfactant in excess has the smaller tail in agreement with experimental observations.     

Analytical chemistry and inorganic chemistry — an unhappy marriage?

Summary Analytical chemistry is a discipline which has a large impact in other fields of chemistry and natural sciences as well as in technology and society. Traditionally, analytical chemistry has been grouped together with inorganic chemistry to such an extent that they are often viewed as a single discipline. While this has been beneficial for the development of both inorganic and analytical chemistry, it is increasingly important that the need of analytical education by the organic and biochemists as well as by chemical engineers is clearly recognized. The tightening environmental protection requires the analyst to be conversant with more sensitive, more accurate, and more reliable techniques in novel chemical surroundings, but at the same time he has to have as thorough knowledge in every field of chemistry as possible.