Fouling behavior of microstructured hollow fiber membranes in dead-end filtrations: critical flux determination and NMR imaging of particle deposition

The fouling behavior of microstructured hollow fibers was investigated in constant flux filtrations of colloidal silica and sodium alginate. It was observed that the fouling resistance increases faster with structured fibers than with round fibers. Reversibility of structured fibers' fouling was similar during silica filtrations and better in sodium alginate filtrations when compared with round fibers. The deposition of two different silica sols on the membranes was observed by NMR imaging. The sols had different particle size and solution ionic strength and showed different deposition behaviors. For the smaller particle-sized sol in deionized solution (Ludox-TMA), there was more deposition within the grooves of the structured fibers and much less on the fins. For the alkali-stabilized sol Bindzil 9950, which had larger particles, the deposition was homogeneous across the surface of the structured fiber, and the thickness of the deposit was similar to that on the round fiber. This difference between the deposition behavior of the two sols is explained by differences in the back diffusion, which creates concentration polarization layers with different resistances. The Ludox sol formed a thick polarization layer with very low resistance. The Bindzil sol formed a slightly thinner polarization layer; however, its resistance was much higher, of similar magnitude as the intrinsic membrane resistance. This high resistance of the polarization layer during the Bindzil sol filtration is considered to lead to quick flow regulation toward equalizing the resistance along the fiber surface. The Ludox particles were trapped at the bottom of the grooves as a result of reduced back diffusion. The fouling behavior in sodium alginate filtrations was explained by considering the size-dependent deposition within the broad alginate size distribution. The better reversibility of fouling in the structured fibers is thought to be the result of a looser deposit within the grooves, which is more easily removed than a compressed deposit on the round fibers.     

Centri-voltammetry for biosensing systems: biocentri-voltammetric xanthine detection

Centri-voltammetry is a method that combines centrifugation and voltammetry. This method, developed by our group in 2003, yielded promising results when applied to trace analysis of metal ios. We demonstrate here the first application of centri-voltammetry to biosensing systems. A xanthine biosensor was constructed by immobilizing xanthine oxidase on a planar platinum electrode which then was placed at the bottom of a centri-voltammetric cell. The experimental parameters were optimized to give two linear ranges. The first is from 0.1 to 1???M, and the second from 5 to 50???M. The RSD is 3.4 (n?=?5). The biosensor was applied to the determination of xanthine in wine and in urine. Calculated recoveries are 101?±?0.61% (n?=?3) for wine samples, and 102?±?0.556% (n?=?3) for urine samples.

Dynamic pricing and scheduling in a multi-class single-server queueing system

This paper investigates an optimal sequencing and dynamic pricing problem for a two-class queueing system. Using a Markov Decision Process based model, we obtain structural characterizations of optimal policies. In particular, it is shown that the optimal pricing policy depends on the entire queue length vector but some monotonicity results prevail as the composition of this vector changes. A numerical study finds that static pricing policies may have significant suboptimality but simple dynamic pricing policies perform well in most situations.     

Synthesis and characterization of a new soluble metal-free and metallophthalocyanines bearing biphenyl-4-yl methoxy groups

The synthesis and characterization of peripherally tetra-biphenyl-4-yl-methoxy substituted metal-free (4), Ni(II) (5), Cu(II) (6), Zn(II) (7), Co(II) (8) and Pb(II) (9) phthalocyanine derivatives are reported. These new phthalocyanine derivatives show the enhanced solubility in organic solvents and they have been characterized by a combination of IR, ¹H NMR, ¹³C NMR, UV–vis, mass spectral data, elemental analysis and thermal analysis methods (TG/DTA). The photophysical (fluorescence quantum yield and lifetime) and photochemical (singlet oxygen generation and photodegradation quantum yield) properties of tetra-biphenyl-4-yl-methoxy substituted zinc (II) phthalocyanine derivative (7) are also investigated. The fluorescence of this phthalocyanine derivative (7) is effectively quenched by addition of 1,4-benzoquinone (BQ).     

Local coordination of Fe3+ in Li[Co(0.98)Fe(0.02)]O2 as cathode material for lithium ion batteries-multi-frequency EPR and Monte-Carlo Newman-superposition model analysis

The local coordination of the Fe(3+)-centers in Li[Co(0.98)Fe(0.02)]O(2) cathode materials for lithium-ion batteries has been investigated by means of XRD and multi-frequency EPR spectroscopy. EPR clearly showed the Fe(3+) being in a high-spin state with S = 5/2. The set of spin-Hamiltonian parameters obtained from multi-frequency EPR experiments with Larmor frequencies ranging between 9.8 and 406 GHz was transformed into structural information by means of an expansion to standard Newton-superposition modeling, termed as Monte-Carlo Newman superposition modeling. Based on this analysis, an isovalent incorporation of the Fe(3+)-ions on the Co(3+)-sites, i.e. Fe(x)(Co), has been shown. With that respect, the positive sign of the axial second-order fine-structure interaction parameter B(0)(2) is indicative of an elongated oxygen octahedron, whereas B(0)(2) < 0 points to a compressed octahedron coordinated about the Fe(3+)-center. Furthermore, the results obtained here suggest that the oxygen octahedron about the Fe(3+)-ion is slightly distorted as compared to the CoO(6) octahedron, which in turn may impose mechanical strain to the cathode material.     

Identifying Symmetry Classes of Elasticity Tensors Using Monoclinic Distance Function

We present a method to identify the symmetry class of an elasticity tensor whose components are given with respect to an arbitrarily oriented coordinate system. The method is based on the concept of distance in the space of tensors, and relies on the monoclinic or transversely isotropic distance function. Since the orientation of a monoclinic or transversely isotropic tensor depends on two Euler angles only, we can plot the corresponding distance functions on the unit sphere in ℝ³ and observe the symmetry pattern of the plot. In particular, the monoclinic distance function vanishes in the directions of the normals of the mirror planes, so the number and location of the zeros allows us to identify the symmetry class and the orientation of the natural coordinate system. Observing the approximate locations of the zeros on the plot, we can constrain a numerical algorithm for finding the exact orientation of the natural coordinate system.     

Subspace methods for three‐parameter eigenvalue problems

We propose subspace methods for three‐parameter eigenvalue problems. Such problems arise when separation of variables is applied to separable boundary value problems; a particular example is the Helmholtz equation in ellipsoidal and paraboloidal coordinates. While several subspace methods for two‐parameter eigenvalue problems exist, their extensions to a three‐parameter setting seem challenging. An inherent difficulty is that, while for two‐parameter eigenvalue problems, we can exploit a relation to Sylvester equations to obtain a fast Arnoldi‐type method, such a relation does not seem to exist when there are three or more parameters. Instead, we introduce a subspace iteration method with projections onto generalized Krylov subspaces that are constructed from scratch at every iteration using certain Ritz vectors as the initial vectors. Another possibility is a Jacobi–Davidson‐type method for three or more parameters, which we generalize from its two‐parameter counterpart. For both approaches, we introduce a selection criterion for deflation that is based on the angles between left and right eigenvectors. The Jacobi–Davidson approach is devised to locate eigenvalues close to a prescribed target; yet, it often also performs well when eigenvalues are sought based on the proximity of one of the components to a prescribed target. The subspace iteration method is devised specifically for the latter task. The proposed approaches are suitable especially for problems where the computation of several eigenvalues is required with high accuracy. MATLAB implementations of both methods have been made available in the package MultiParEig (see http://www.mathworks.com/matlabcentral/fileexchange/47844-multipareig ).     

Electrochemical adsorption properties and inhibition of copper corrosion in chloride solutions by ascorbic acid: experimental and theoretical investigation

In this study, the effect of ascorbic acid in the electrochemical behavior of copper has been investigated in 3.5 % NaCl solution using potentiodynamic polarization and electrochemical impedance spectroscopy. Current-potential curve and Nyquist diagrams were obtained in different concentrations of ascorbic acid. The surface morphology of copper after its exposure to 3.5 % NaCl solution with and without of ascorbic acid was examined by scanning electron microscopy. The obtained results show that ascorbic acid inhibits corrosion of copper in 3.5 % NaCl solution. The inhibition activity of ascorbic acid increases with a decrease in the concentration of ascorbic acid.     

Flux trapping in a macroscopic cylinderical hole drilled in Bi-Sr-Ca-Cu-O

The flux dynamics in a polycrystalline sample of Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O_x with a macroscopic cylindrical hole (CH) drilled was investigated by slow transport relaxation (V-t curves) and magnetovoltage measurements (V-H curves). It was monitored that there are several discontinuities in the time evolution of quenched state in V-t curves, which was attributed to the leaving of quantized flux lines trapped through CH together with surface superconducting effects. We observed that asymmetric V-H curves demonstrate unusual remarkable counter clockwise hysteresis effects upon cycling of field. This interesting result was correlated mainly to the flux trapping inside the CH that acts as a macroscopic attractive pinning center for flux lines. Further, the hysteresis effects in V-H curves for a fixed transport current provide a direct evidence that the number of flux lines, measured dissipation and relative decrease/increase in irreversibilities could be determined by sweeping rate of external magnetic field (dH/dt) which leads also to peculiar time effects.     

Comparison of the ELP and multicanonical methods in simulation of the heptapeptide deltorphin

To investigate the performance of the energy landscape paving (ELP) procedure for peptides, we apply it here to deltorphin, a linear heptapeptide with bulky side chains (H-Tyr¹-D-Met²-Phe³-His⁴-Leu⁵-Met⁶-Asp⁷-NH₂) and compare the results with the Multicanonical method (MUCA) in regard of finding the low-energy structures. Deltorphin is modeled in vacuum by the potential energy function ECEPP. Received 23 July 2002 / Received in final form 18 September 2002 Published online 31 December 2002