Hydrodynamic and interparticle potential effects on aggregation of colloidal particles

The effects of both hydrodynamic interaction and the form of the interparticle potential on the aggregation process for dispersed spherical particles are investigated by computational simulation. The simulation methods of Brownian Dynamics (BD) and Stokesian Dynamics (SD) are applied, over a range of solid volume fraction of $0.04???0.12$. The interparticle potential is a combination of a generalized Lennard-Jones form and a Yukawa potential, the latter of which describes a screened electrostatic repulsion at longer range. The combined potential is parameterized to include a roughly constant primary minimum near contact, along with a variable repulsive barrier at slightly larger separation. The microstructure is characterized through the pair distribution function, g(r), and the static structure factor. The repulsive barrier reduces the rate of aggregation and is also seen to affect the structure, with a large repulsion associated with a more tenuous structure. This is reflected in the potential having a strong effect on the evolution of ‘bonds’ per particle. Hydrodynamic interactions were found to reduce the solid fraction required for percolation, with the influence depending upon the form of the potential; the difference in percolation threshold was significant, with ${?}_{c\text{,}\mathit{SD}}?0.06$ and ${?}_{c\text{,}\mathit{BD}}?0.08$ a typical difference for moderate repulsion barriers. These results are for 864 particles in a cubic unit cell. To address the mechanism for this influence of hydrodynamic interactions, a complementary analysis of the evolution of numerous independent three-particle aggregates was performed. The analysis shows that hydrodynamic interaction slows the evolution toward a condensed aggregate of lowest potential energy in a way which cannot be explained by a simple rescaling of the drag due to uncorrelated particle motions.     

A statistical measure of association and a series expansion of chain conformations

A simple, easily calculated, nonparametric statistic is described that can detect the presence of a functional relationship in bivariate data. Given a sample of data points (x,y), the statistic's value is nearly 1 if y is a linear function of x with little noise; it is greater than 1 if y is a nonlinear function of x; and it is close to 2 if x and y are uniformly and independently distributed. The statistic can be used to rapidly screen through large data sets to identify the most functionally related variable pairs. As an illustration, the statistic is used to detect relations between polypeptide conformational energy and functions of a series expansion for chain conformations.     

Thermodynamic theory of viscoelasticity

The relaxation spectra in polymers arise from the existence of many possible modes for dissipating the strain energy raised by the imposed force. These modes are made up by coupling the simplest and fastest mode of relaxation involving the rotation of a conformer, typically represented by the picosecond rotation of the carbon to carbon bond. This fast relaxation process cannot take place easily in the condensed state crowded by the densely packed conformers, necessitating cooperativity among them. The domain of cooperativity grows at lower temperatures, toward the infinite size at the Kauzman zero entropy temperature. From the temperature dependence of the domain size, the well-known Vogel equation is derived, which is numerically equivalent to the empirical WLF and free volume equations. The molar volume is a crucial factor in determining the molar free volume and, therefore, in determining theT _g of a material. The molar ΔC _P is proportional to the logarithmic molar volume, and is greater for a polymer with a higherT _g, but ΔC _P per gram for it is smaller, as it is proportional to (logM) divided byM, whereM is the molecular weight of the conformer. From this theory, it is possible to predict the dependence of the characteristic relaxation time on temperature if eitherT _g or the conformer size is known, since one can be derived from the other. From the Vogel equation with all parameters thus derived, it is possible to obtain a master relaxation curve and the spectrum from one set of dynamic mechanical data taken at one frequency over a range of temperatures. Whereas the linear viscoelastic principle is limited to small strains only, a real polymer is often deformed well beyond such a limit. Above a certain limit of strain energy level, linear viscoelastic deformation is no longer possible and the plastic deformation takes over. However, because a polymer typically manifests a spectrum of relaxation times, its behavior is a combination of viscoelastic and plastic behaviors. The ratio between the two behaviors depend on the rate of deformation, and can be precisely predicted from the linear viscoelastic relaxation spectrum. The combined behavior is termed viscoplasticity, and it applies to a wide range of practically important mechanical behaviors from the flow of the melt to the yield and fracture of glassy and crystalline solids.     

Metabolites of microorganisms. Aranciamycin

Aranciamycin is a new antibiotic isolated from cultures of a strain of Streptomyces echinatus. The orange-yellow acidic compound, C₂₇H₂₈O₁₂, is a glycoside of an aglycone, aranciamycinone, and a 6-deoxyhexose monomethyl ether. The aglycone, C₂₀H₁₆O₈, is composed of a 1, 8-dihydroxyanthraquinone nucleus, to which a fourth carbocyclic six-membered ring is condensed, bearing a tertiary C-methyl group, a tertiary and a secondary hydroxyl group, a secondary methoxyl group, and a carbonyl group. A partial formula of the aglycone is given. Aranciamycin as well as its aglycone are strongly inhibitory against gram-positive bacteria on synthetic media. The antibiotic activity is considerably decreased by the addition of amino-acid combinations and pyruvate to the test medium.     

Preparation of ultramicro-, micro-, and supermicroporous carbon adsorbents by template procedure

The new template procedure for preparing ultramicro-, micro-, and supermicroporous carbon adsorbents is proposed on the basis of the use of two polymers with different thermal stabilities as a template matrix and a carbonizable carbon source. The polymeric template matrix is removed by thermal decomposition in the low-temperature pyrolysis of the polymer mixture, whereas the second polymer component of a mixture is transformed into a carbonizate. The resultant monolithic carbonizate is the polymeric matrix replica and represents a molecular-sieve ultramicroporous carbon adsorbent, which adsorbs water vapor at 293 K and does not adsorb nitrogen vapor at 77 K. The activation of this carbonizate with water vapor leads to a series of micro-and supermicroporous carbon adsorbents with a broad range of the parameters of a pore structure: BET specific surface area S _BET is 610–2130 m²/g, micropore volume W ₀ is 0.20–0.69 cm³/g, micropore half-width x ₀ (slit model) is 0.20–0.70 nm, mesopore specific area S _me is 20–1000 m²/g, and characteristic adsorption energy E ₀ is 15.6–27.4 kJ/mol. Original Russian Text ? O.K. Krasil’nikova, A.M. Voloshchuk, A.E. Evsyukhin, N.Y. Lomovsakaya, 2006, published in Kolloidnyi Zhurnal, 2006, Vol. 68, No. 2, pp. 207–213.     

Dynamics of a polymer attached to a surface: Bead and spring model

The Smoluchowski formalism is used to solve the problem of a bead of frictional resistance β attached to a surface with a spring of force constant _k over which a linear shear field of strenght α flows. The power dissipation is given by βα²kT/_k. k and T have their usual meanings. The result is generalized to an n-bead polymer. It is found that the power dissipation of a Rouse model polymer attached to a surface at one end is twice that of an identical polymer flowing freely in solution. If the force constant _k arises from an entropy force, then, because of the effect of the surface on the number of polymer configurations, there is an additional factor of two. The same relationship is expected to also hold for the frequency-dependent power dissipation. It is argued that a net circulation exists in the beads above the surface and that the magnitude of the circulation is roughly comparable to that which exists in a polymer freely rotating in solution under a shear field of the same magnitude.     

Spin-free quantum chemistry. XXVI. The Ising,small-bipolaron theory of cuprate superconductivity

The Ising, small-bipolaron (ISB ) theory is a strong-coupling theory of cuprate superconductivity which is based on the negative-U, Hubbard Hamiltonian. Its ground state is composed of (small) bipolarons and (small-bipolaron) holes with a vibronically induced, bipolaron-hole exchange interaction, J_BH, between them. The energy gap, Δ(0), is taken to be equal to the dissociation energy of a small bipolaron and which, since it is defined spectroscopically, is not an order parameter. The application of the Ising mean-field theory to the highly degenerate ground-state yields a second-order phase change with kT_C/2 = J_BH and a real order parameter, Ω(T), which is valid over the entire temperature range from zero to T_C. Near T_C, the Ising free-energy functional takes the same form as does the Landau. In the presence of an electromagnetic field, the Ising functional is a generalization of the Ginzburg-Landau functional which employs a complex order parameter and which is invariant under the electromagnetic gauge transformation. The breaking of the gauge invariance yields the London theory of superconductivity. © 1996 John Wiley & Sons, Inc.     

To a Theory of Electrocatalysis for the Hydrogen Evolution Reaction: The Hydrogen Chemisorption Energy on the Transition Metal Alloys within the Anderson–Newns Model

Estimates of the energy of chemisorption of hydrogen (ECH) on a number of alloys of transition metals that are of interest from the viewpoint of a theory of electrocatalysis are made within the Anderson–Newns model. As a simplest example, the behavior of ECH as a function of the position of the Fermi level of a system for nickel containing admixtures of simple metals is studied in a fixed-band approximation. As alternative examples, where this approximation is not fulfilled, a calculation of ECH on tungsten carbide and a Pd_{1 – x} Ag _x alloy is performed. It is shown that agreement between calculated ECH and observed ECH is reached in the case of an Ni_{1 – x} Cu _x alloy after taking into account mutual influence of processes of adsorption and segregation on the alloy surface. Dependence of rates of a number of electrochemical reactions on the composition and electronic structure of alloys is discussed for all the alloys considered and a comparison with experiment is performed.     

Precipitate coating on cellulose fibre as sorption medium for selenium preconcentration and speciation with hydride generation atomic fluorescence spectrometry

Lanthanum hydroxide precipitate is for the first time coated onto cellulose fibre and serves as a novel sorption medium for separation and speciation of inorganic selenium. A micro-column packed with precipitate-layer-coated cellulose fibre is incorporated into a sequential injection system for selenite retention from a neutral aqueous solution, which is afterwards stripped with a NaBH₄-NaOH solution as eluent. The hydride generation is actuated by merging the eluate and hydrochloric acid downstream, followed by the detection with atomic fluorescence spectrometry. Total inorganic selenium is derived by pre-reduction of selenate and speciation is estimated by difference. The coated precipitate layer can be used for 150 runs for selenium sorption, offering a clear advantage over the conventional precipitation protocols where a large amount of precipitate is dissolved into a small volume of eluent which might interfere with the detection. With a sample volume of 1.0 mL, an enrichment factor of 9.7 and a detection limit of 9 ng L⁻¹ are obtained in a linear range of 0.05-2.5 μg L⁻¹. A sampling frequency of 24 h⁻¹ is achieved along with a R.S.D. of 1.7% at 0.5 μg L⁻¹ Se(IV). The procedure is validated by analyzing selenium in a reference material GBW 10010 (rice) and a human hair sample. It is further demonstrated by speciation of inorganic selenium in surface water samples by pre-reduction of selenate.     

A review and tutorial discussion of noise and signal-to-noise ratios in analytical spectrometry—III. Multiplicative noises

In this review, signal-to-noise ratios are discussed in a tutorial fashion for the case of multiplicative noise. Multiplicative noise is introduced simultaneously with the analyte signal and is therefore much more difficult to reduce than additive noise. The sources of noise, the mathematical representation of noise, and the major types of noises in emission and luminescence spectrometry are discussed. If the limiting source of noise is multiplicative white noise, the signal-to-noise ratio for optimal sampling time τ_s increases as the square root of the response or integration time of the readout and is independent of the rate at which sample and reference are measured. The variation of multiplicative flicker noise with variation in sampling time, τ_s, time interval between sample and reference measurements, T, and response (τ_r) or integration (τ_i) time is discussed in some detail. The optimal system for the case of multiplicative noise is a dual channel approach in which the sample and reference are measured simultaneously and a ratio of signals is taken. Although the best reference in most cases of interest to analytical chemists is a calibration standard, it is often impossible to measure a sample and a calibration standard simultaneously and so an internal standard, a detector monitoring the source intensity, etc., may be useful.     

Theoretical study of photochemical reactions: Electron assignment and the state correlation diagram

A general way for drawing the state correlation diagram and seeking the reaction path is presented. If a high-symmetry reacting system is given, its least-motion path that maintains the symmetry is primarily examined. For a given state, it is judged whether the least-motion path is symmetry allowed or forbidden. If allowed, it is called the direct process. If forbidden, the symmetry imposed on the system should be relaxed, resulting in the mixing of MO 's. Then, the energy barrier of the avoided crossing for some excited states is removed and the possible reaction path is found. After this procedure, the symmetry-allowed paths may be sought by the geometry optimization with a suitable wave function. By the use of such a procedure, the dissociation of diazomethane and (3H-)diazirine is found to proceed via the C_s and C₂ symmetries.     

Plutonium-244 fission xenon and primordial xenon in lunar samples and meteorites

Xenon found in lunar samples is a binary mixture of²⁴⁴Pu fission xenon and a trapped xenon, whose isotopic composition often shows a striking resemblance to that ofTakaoka's¹ primitive xenon. The decay product of¹²⁹I is conspicuously absent in lunar samples and this may be attributed to the facts that (a) the half-life of¹²⁹I is much shorter than that of²⁴⁴Pu, and (b) the separation of xenon from plutonium may take place easily, since the former is a gaseous element, while the latter is a refractory element. The separation of xenon from iodine may not take place easily, however, since the former is a gaseous element, while the latter is a volatile element. The isotopic compositions of the trapped xenon released from ordinary chondrites and achondrites resemble that ofTakaoka's primitive xenon, which has been mass-fractionated in such a manner that the heavier isotopes are systematically enriched relative to the lighter isotopes.     

Monitoring industrial wastewater by online GC-MS with direct aqueous injection

An online GC–MS-system for automated monitoring of crude wastewater at a complex chemical production site is presented. The modular system is, in principal, based on commercial equipment, but utilizes a special, two-stage injector, which consists of a splitless vaporization chamber on top of a PTV injector filled with Tenax. This set-up enables direct injection of wastewater. Almost 140 volatile and semi-volatile compounds are calibrated down to 1 mg L⁻¹, which is sufficient for analysis of the influent of the wastewater-treatment plant. Two instruments analyze alternately, every 20 min, and the instrument cycle time is 40 min. The quantitative results are transferred to a database which is connected to a process-control system. Depending on the nature and concentration of a compound, an alarm can be generated and the wastewater stream can be diverted into an “off spec tank” if necessary. The GC–MS-system operates quasi-continuously with a system availability >98%. Data quality is automatically controlled in each run and by daily analysis of a quality-control sample. The development of a novel stacked PTV–PTV injector design to expand the range of analytes to selected basic compounds is described.     

Irreversible and Reversible Deactivation of Bilirubin Oxidase by Urate

Oxygen is electroreduced to water on a carbon cathode coated with wired bilirubin oxidase in a pH 7.4 0.15 M NaCl phosphate buffer solution at 37 °C at much lesser polarization than it is on a pure platinum cathode in 0.5 M H₂SO₄. While the wired bilirubin oxidase cathode operates for over a week in the aerated or oxygenated buffer solution, it is degraded rapidly when in serum. We reported earlier that in the presence of O₂ an intermediate product of the electrooxidation of urate, which is a normal serum component, irreversibly damages the wired bilirubin oxidase and also reported that the electrocatalyst is irreversibly damaged, in the absence of urate, when it is brought, by disconnecting the electrode, to the O₂/H₂O half cell potential at pH 7.4. Here we report that a) dissolved bilirubin oxidase is irreversibly and rapidly damaged by urate in the presence of O₂; and b) that the immobilized wired bilirubin oxidase electrocatalyst is not only irreversibly deactivated by urate in the presence of O₂ in a few hours, but is initially reversibly deactivated, in 1 min or less, by the urate in the presence of O₂.     

Is chrysocolla (Cu,Al)2H2Si2O5(OH)4·nH2O related to spertiniite Cu(OH)2?—A vibrational spectroscopic study

Chrysocolla (Cu, Al)₂H₂Si₂O₅(OH)₄·nH₂O is a hydrated copper hydroxy silicate and is commonly known as a semi-precious jewel. The mineral has an ill defined structure but is said to be orthorhombic, although this remains unproven. Thus, one of the few methods of studying the molecular structure of chrysocolla is to use vibrational spectroscopy. Chrysocolla may be defined as a colloidal mineral. The question arises as to whether chrysocolla is a colloidal system of spertiniite and amorphous silica. The main question addressed by this study is whether chrysocolla is (1) a mesoscopic assemblage of spertiniite, Cu(OH)₂, silica, and water, (2) represents a colloidal gel or (3) is composed of microcrystals with a distinct structure.Considerable variation in the vibrational spectra is observed between chrysocolla samples. The Raman spectrum of chrysocolla is characterised by an intense band at 3624 cm⁻¹ assigned to the OH stretching vibrations. Intense Raman bands found at 674, 931 and 1058 cm⁻¹ are assigned to SiO₃ vibrations. The Raman spectrum of spertiniite does not correspond to the spectrum of chrysocolla and it is concluded that the two minerals are not related. The spectra of chrysocolla correspond to a copper silicate colloidal gel.     

4,4′‐Bipyridine as a Unidirectional Switching Unit for a Molecular Pushing Motor

A chirality switch in which the intrinsic chirality of a 4,4′‐bipyridine is combined with a metal‐ion‐induced switching principle is described. In the uncomplexed state the 4,4′‐bipyridine unit, which is linked to an S,S,S,S‐configured cyclic imidazole peptide, is P‐configured. The addition of zinc ions leads to a rotation around the C?C bond axis of the 4,4′‐bipyridine and the M isomer of the metal complex is formed. By addition of a stronger complexing agent the metal ions are removed and the switch returns to its initial position. The combination of the chirality switch with a second switching unit allows the construction of a molecular pushing motor, which is driven chemically and by light.     

Low-temperature synthesis of a green material for lithium-ion batteries cathode

Abstract

Battery technology is an important anthropogenic source of the heavy metals which are highly threatening to human health. A category of rechargeable lithium batteries that is of great interest is the set of batteries where the cathode material is a lithium iron phosphate (LiFePO₄). LiFePO₄ is an environmentally friendly and safe lithium-ion battery cathode material, but it has a key limitation, and that is its extremely low-electronic conductivity, a problem that can be greatly overcome by zinc-doping LiFePO₄. For the first time to our knowledge, a low-temperature method, that is advantageous both economically and technologically, for the synthesis of a zinc-doped LiFePO₄ is presented. Since the method appears to be applicable for synthesizing various zinc-doped LiFePO₄ compounds with the general formula LiFe_1?x Zn _x PO₄ (0<x<1), it is very promising for the production of a green cathode material for lithium-ion batteries.     

A new general-purpose isothermal microcalorimeter for use at temperatures up to 200 °C

The design and some properties of a new general-purpose isothermal microcalorimeter are reported. The instrument is a twin thermopile heat conduction calorimeter, which is designed for use up to 200 °C. The calorimetric units and surrounding heat sink are suspended inside a hollow aluminium construction, which is thermostated. Above that unit a second thermostated block is positioned and the whole assembly is suspended inside a Dewar vessel. When the instrument is used at room temperature and below, the thermostated units are cooled by use of an insertion Peltier effect cooler. The instrument can be used with a wide range of different reaction vessels (diameter 14 mm). Baseline experiments have been conducted in the temperature range 15-200 °C. Typical values obtained during 10 h periods at 200 °C are ±3 and ±10 nW for the baseline drift and baseline fluctuations, respectively. The heat detection limit, determined by release of electrical energy, is about 2 μJ. Preliminary stability measurements have been conducted at 100 °C on samples of stabilised and non-stabilised polyamide film.     

Indirect determination of iodide by tungsten coil atomic emission spectrometry

The spectroscopic determination of iodide is a difficult challenge, especially in small sample volumes. The strongest transition lines for this element lie in the vacuum ultraviolet region of the spectrum, so most conventional instruments produce very weak signals. This work describes a tungsten coil atomic emission procedure for the indirect determination of iodide. A 25 μl aliquot of a solution containing a known amount of indium is deposited on the tungsten coil and dried with a simple heating program. Once the coil is dry, 25 μl of an iodide solution is added to the coil. The solution is dried and vaporized at high current. The atomic emission signal for In at 451.1 nm is monitored. In the presence of iodide, InI is formed and the In emission signal is attenuated. This attenuation is proportional to iodide concentration with a method detection limit of 0.6 mg l^{− 1} iodide using an In concentration of 10 mg l^{− 1}, and 3 mg l^{− 1} iodide using an In concentration of 50 mg l^{− 1}. Linear calibration curves span a range of two orders of magnitude. Analysis of a deionized water sample spiked with 50 mg l^{− 1} iodide gives a recovery of 100% and a precision of 5.5% relative standard deviation. Analysis of a tap water sample spiked with 50 mg l^{− 1} iodide gives a recovery of 140% and a precision of 7.1% relative standard deviation. The poor accuracy for the tap water analysis may arise from the reaction of In with other halides in the sample. This is the first report of determination of a halogen using the tungsten coil atomizer.