Separation of acidic and neutral compounds by strong anion-exchange capillary electrochromatography dynamically modified with sodium dodecylsulfate

Summary Acidic and neutral compounds have been separated by strong anion-exchange capillary electrochromatography (SAXCEC) dynamically modified by addition of sodium dodecylsulfate (SDS). It was found that separation of neutral solutes by SAXCEC without addition of SDS is difficult because of the weak interaction of the solutes with the hydrophilic packing surface. The hydrophobicity of the packing surface increases on addition of SDS to the mobile phase, however, and the capacity of separating neutral solutes increases. Acidic solutes are retained, mainly because of the ion-exchange properties of this system. The influence of mobile phase composition, e.g. SDS concentration, ionic strength, and organic modifier fraction, on the retention of acidic and neutral solutes was investigated. Three acids and five neutral solutes were separated in only 5 min under optimized conditions, because the direction of the strong electroosmotic flow (EOF) was the same as that of electrophoretic migration of the acids under the conditions used. The repeatability of this system in terms of migration time relative standard deviation (RSD) is good—less than 0.48% for 10 consecutive runs of all the solutes tested. Column efficiencies for acids were > 125000 plates m⁻¹; those for neutral solutes varied from 25000 to 100000 plates m⁻¹.     

Excimer laser ablation of gold-loaded inverse polystyrene-block-poly (2-vinylpyridine) micelles

Diblock-copolymers (PS(1700)-b-P2VP(450) or PS(1350)-b-P2VP(400)) forming spherical micelles, can be loaded with a Au-salt and deposited on top of various substrates. Such polymer films have been exposed to a pulsed ArF excimer laser in order to remove the polymer matrix and, in parallel, to chemically reduce the salt into metallic Au nanodots. To analyze this process in detail, it was subdivided into three steps: (a) laser ablation of thick and thin diblock-copolymer films; (b) laser irradiation of Au-salt loaded diblock-copolymer films; and (c) laser irradiation of arrays of metallic Au nanodots. In (a) it was found that a complete removal of the polymer by laser ablation is only possible in air under ambient conditions while identical laser irradiations under vacuum result in a residual layer of approximately 14 nm. Substep (b) revealed a nucleation process of the resulting metallic Au within the micellar core leading to clusters of small Au dots. Furthermore, this substep provided evidence for an asymmetric interplay between the macroscopic temperature of a polymer film during laser treatment and the energy density per laser pulse. In (c) it could be demonstrated that metallic Au nanodots on mica are stable against laser irradiation under conditions leading to a polymer removal. Received: 7 August 2000 / Accepted: 2 November 2000 / Published online: 3 April 2001     

Effects of Different Drying Processes on the Material Properties of Bacterial Cellulose Membranes

Summary: Membranes of bacterial cellulose produced by Gluconacetobacter xylinus show a high water and gas permeability that can be altered by different drying techniques. It could be shown that freeze-drying reduces the swellability of the polymer membranes by a factor of 5 while evaporation drying causes a reduction by a factor of 50. The strong decrease of swellability for an evaporation dried membrane could be correlated with a reduction of the absolute number of polymer strands that form the network structure of the membrane, determined with oscillatory shear rheological experiments. The removal of network meshes by a complete aggregation of polymer strands could be confirmed by IR-spectroscopy with an increased degree of intramolecular hydrogen bonding of cellulose strands. In contrast to this, the freeze-drying process shows a slight increase of the number of network meshes due to partial aggregation of free polymer strands. Freeze-dried membranes show a gas permeability two orders of magnitude higher then evaporation dried membranes. The absolute permeability strongly depends on the bacterial strain used for the polymer membrane synthesis and varies by up to 1.5 orders of magnitude for the same drying process. The Young's modulus of the polymer membranes varies with the bacterial strain used, but does not show the same trends as the permeability. Finally, a comparison of the characterized properties shows that only one of the tested strains shows the capability to synthesize membranes that meets the requirements for an application as a wet wound dressing.     

Esterification reaction kinetics of acetic acid and n-pentanol catalyzed by sulfated zirconia

This study reports experimental data and kinetic modeling of acetic acid esterification with n-pentanol using sulfated zirconia as a catalyst. Reactions were carried out in an isothermal well-mixed batch reactor at different temperatures (50-80°C), n-pentanol to acid molar ratios (1:1-3:1), and catalyst loadings (5-10 wt% in relation to the total amount of acetic acid). The reaction mechanism regarding the heterogeneous catalysis was evaluated considering pseudo-homogeneous, Eley–Rideal, and Langmuir–Hinshelwood model approaches. The reaction mixture was considered a nonideal solution and the UNIQUAC thermodynamic model was used to take into account the nonidealities in the liquid phase. The results obtained indicated that increases in the temperature and catalyst loading increased the product formation, while changes in the n-pentanol to acetic acid molar ratio showed no significant effect. The estimated enthalpy of the reaction was −8.49 kJ mol⁻¹, suggesting a slightly exothermic reaction. The Eley–Rideal model, with acetic acid adsorbed on the catalyst as the limiting step, was found to be the most significant reaction mechanism.     

Kinetics, products, and mechanisms of secondary organic aerosol formation

Secondary organic aerosol (SOA) is formed in the atmosphere when volatile organic compounds (VOCs) emitted from anthropogenic and biogenic sources are oxidized by reactions with OH radicals, O(3), NO(3) radicals, or Cl atoms to form less volatile products that subsequently partition into aerosol particles. Once in particles, these organic compounds can undergo heterogenous/multiphase reactions to form more highly oxidized or oligomeric products. SOA comprises a large fraction of atmospheric aerosol mass and can have significant effects on atmospheric chemistry, visibility, human health, and climate. Previous articles have reviewed the kinetics, products, and mechanisms of atmospheric VOC reactions and the general chemistry and physics involved in SOA formation. In this article we present a detailed review of VOC and heterogeneous/multiphase chemistry as they apply to SOA formation, with a focus on the effects of VOC molecular structure on the kinetics of initial reactions with the major atmospheric oxidants, the subsequent reactions of alkyl, alkyl peroxy, and alkoxy radical intermediates, and the composition of the resulting products. Structural features of reactants and products discussed include compound carbon number; linear, branched, and cyclic configurations; the presence of C[double bond, length as m-dash]C bonds and aromatic rings; and functional groups such as carbonyl, hydroxyl, ester, hydroxperoxy, carboxyl, peroxycarboxyl, nitrate, and peroxynitrate. The intention of this review is to provide atmospheric chemists with sufficient information to understand the dominant pathways by which the major classes of atmospheric VOCs react to form SOA products, and the further reactions of these products in particles. This will allow reasonable predictions to be made, based on molecular structure, about the kinetics, products, and mechanisms of VOC and heterogeneous/multiphase reactions, including the effects of important variables such as VOC, oxidant, and NO(x) concentrations as well as temperature, humidity, and particle acidity. Such knowledge should be useful for interpreting the results of laboratory and field studies and for developing atmospheric chemistry models. A number of recommendations for future research are also presented.     

Dynamic Capacity Acquisition and Assignment under Uncertainty

Given a set of m resources and n tasks, the dynamic capacity acquisition and assignment problem seeks a minimum cost schedule of capacity acquisitions for the resources and the assignment of resources to tasks, over a given planning horizon of T periods. This problem arises, for example, in the integrated planning of locations and capacities of distribution centers (DCs), and the assignment of customers to the DCs, in supply chain applications. We consider the dynamic capacity acquisition and assignment problem in an environment where the assignment costs and the processing requirements for the tasks are uncertain. Using a scenario based approach, we develop a stochastic integer programming model for this problem. The highly non-convex nature of this model prevents the application of standard stochastic programming decomposition algorithms. We use a recently developed decomposition based branch-and-bound strategy for the problem. Encouraging preliminary computational results are provided.     

Use of polyethylene glycol in the process of sol–gel encapsulation of Burkholderia cepacia lipase

Lipases from Burkholderia cepacia were encapsulated using polyethylene glycol (PEG, M _w 1500) at various concentrations (0.5–3.0 %, w/v) as an additive during the sol–gel immobilisation process. Matrixes immobilized in the presence and absences of additives were characterized by thermal analysis [thermogravimetric (TG) and differential scanning calorimetry (DSC)], scanning electron microscopy (SEM), enzymatic activity, and total activity recovery yield (Ya). The addition of PEG increased the activity values, with Ya just above 1.0 % (w/v) in the presence of PEG. The additional of 1.0 % (w/v) PEG increased enzyme activity from 33.98 to 89.91 U g^?1 and the values of recovery yield were 43.0–91.4 %, compared to values of the samples without PEG. PEG enhanced the thermal stability of the matrix structure in the temperature range 50–200 °C, as confirmed by TG and DSC analyses. This was influenced by the presence of water bound to the matrix. The SEM micrographs clearly showed an increase in the number of deposits on the material surface, producing matrices with greater porosity.     

Anisotropy of a vortex instability observed at high current densities in YBa2Cu3O7δ superconducting films

Experimental data are provided for YBaCuO films that an instability of the vortex system, which manifests itself by a voltage jump at a critical current I*, exhibits strong anisotropy if the magnetic field is tilted from parallel to perpendicular to the c-axis. The angular dependence of I* can be well described by a model emphasizing the component of the magnetic field parallel to the c-axis. If the current range is restricted to values close to I*, the current-voltage characteristics below the instability show a satisfactory agreement with the prediction of the theory of ‘Self-Organized Criticality’ (SOC). In terms of this theory it is possible to relate the critical vortex velocity v* to the temperature and field dependent characteristic size of the underlying vortex avalanches. If, however, standard Larkin-Ovchinnikov theory is applied to describe the instability, this critical velocity is related to the scattering rate of quasiparticles. Analyzed in this way and assuming an isotropic diffusion constant of the quasiparticles, an anisotropic scattering rate and its temperature dependence can be extracted.     

Puzzling Reaction of Imidazole with Methyl Parathion: P=S versus P=O Mechanistic Shift Dilemma in Organophosphates

Organophosphates (OPs) constitute many toxic agrochemicals and warfare and can undergo a wide spectrum of mechanisms, some which are fairly unexplored. In this sense, concise mechanistic elucidation stands out as a strategic tool for achieving efficient detoxification and for monitoring processes. Particularly intriguing is the effect of substituting the oxygen atom of the phosphoryl moiety (P=O) in OPs with a sulfur atom to give the thio-derived OPs (i.e., OTPs, P=S). In general, imidazole (IMZ) reacts very efficiently with OPs by targeting the phosphorus atom, although herein we evidence a thio-driven shift with OTPs: IMZ undergoes unusual nucleophilic attack at the aliphatic carbon atom of methyl parathion. Alkylation of IMZ under mild conditions (aqueous weakly basic medium) is also novel and should be applicable to other novel IMZ-based architectures, and thereby, it can be a great ally for organic synthesis. Overall, a broader understanding of the mechanistic trend involved in such highly toxic agents is provided.