Local Concentration Effects Research Articles

Effects of random particle dispersion and size on the indentation behavior of SiC particle reinforced metal matrix composites

This study investigates the effects of particle size, volume fraction, random dispersion and local concentration underneath a spherical indenter on the indentation response of particle reinforced metal matrix Al 1080/SiC composites. The ceramic particles in certain sizes and volume fractions were randomly distributed through the composite structure in order to achieve a similar structure to an actual microstructure as possible. The particle size and volume fraction affected considerably indentation depths and deformed indentation surface profiles. The indentation depth increases with increasing particle size, but decreases with increasing particle volume fraction. The experimental indentation depths were in agreement with numerical indentation depths in case the local particle concentration effect is considered. The local particle concentration plays an important role on the peak indentation depth. For small particle sizes and large volume fractions the random particle distribution affects the deformed surface profiles as well as the indentation depths. However, its effect is minor on residual stress and strain distributions rather than levels in the indentation region.

A comprehensive investigation into “controlled/living” chain growth crosslinking copolymerization including a back to basics modeling

AbstractThe mainstay of this description is a modeling of “controlled/living” chain growth crosslinking copolymerization (C/L CC) that is based on a simple equivalent kinetic scheme, resulting from assumptions consistent with an ideal C/L behavior. Analytical expressions of the double bond concentration according to reaction parameters are derived from the corresponding set of differential equations, and consideration of the respective values of the rate constants enables one to forecast the morphology of the branched polymers derived by C/L CC. The kinetic scheme is then enriched with a necessary distinction between intramolecular (cyclization) and intermolecular crosslinking which contribute to the formation of three‐dimensional structures and to the increase of molar masses, respectively. This consideration leads to a key‐equation governing C/L CC, which can be exploited in two ways. Its implicit integration coupled with a semi‐empiric methodology gives an easy access to all the characteristic magnitudes of the products at complete conversion of the double bonds. An alternative approach implying a supplementary modeling of local concentration effects enables its numerical resolution. Modeled trends and predicted characteristic values are then successfully compared with experimental data which relate to xanthate‐mediated radical CC. Specific reasons of dispersity related to the mechanism of CC are also discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5313–5327, 2009

Biochemical Basis for Enhanced Binding of Peptide Dimers to X-Linked Inhibitor of Apoptosis Protein

XIAP (X-linked inhibitor of apoptosis protein) is involved in the mediation of programmed cell death and, therefore, is a target for the development of cancer therapeutics. Peptide mimetics based upon Smac, the natural binding partner of XIAP, and specifically, dimeric peptides, have shown great promise in drug development. In the present work, the basis for enhanced dimer efficacy has been explored. Comparisons are made between the peptide binding site on the BIR3 domain of XIAP alone (residues 238-358) and a less truncated construct that includes both BIR2 and BIR3 domains (residues 151-350). This contingency differentially enhances the binding of dimeric tetrapeptides, potentially by providing additional hydrophobic binding surface. The effect of BIR2 on the BIR3 binding site is sustained, even if the BIR2 binding site is disrupted by mutagenesis, as shown by both a fluorescent competition assay and a polarity sensitive dye, badan. FRET measurements reveal an observed separation of >or=45 A between the BIR2 and BIR3 peptide binding pockets, thereby precluding a direct simultaneous interaction of the dimer molecules with both binding domains. Furthermore, variations in the linker length between dimeric tetrapeptides did not show a predictable trend in binding affinities, suggesting that local concentration effects were also an unlikely explanation for the enhanced dimeric affinities. Taken together, the results suggest that enhanced binding of dimeric peptides likely reflects the increased hydrophobic surface area on or near the BIR3 site and have significant ramifications for the design of therapeutics that target this class of proteins.

Slag inclusion analyses for studying ferrous alloys employed in French medieval buildings: supply of materials and diffusion of smelting processes

This paper aims to propose a data treatment method in order to efficiently exploit the major element compositional analyses made on slag inclusions (SI) of ferrous archaeological artefacts. This method is employed to study the reinforcements used in medieval monuments. The first part of the paper shows: (i) that SI composition can be very variable in an artefact and that it is necessary to analyse a sufficiently redundant number of inclusions; and (ii) that on the one hand a local concentration effect and, on the other hand additives during forging can cause this variability. Nevertheless, after filtering this data, it is showed that specific compounds (called here Non-Reduced Compounds – NRC) have in most of cases a constant ratio in the SI. This allows to evidence the signature of the system (ore, charcoal and pollutant) that produced the ferrous alloys. Moreover, a weighted average compound can be extracted from the results. It is demonstrated that, considering some specific elements with a different behaviour in the direct and indirect processes, it is possible to differentiate the two processes. A methodology is then proposed and applied to several samples gathered on different French medieval buildings to follow the diffusion of the indirect process in France.

Kinetics of the Complex Reaction of Ascorbic Acid with Hydrogen Peroxide Catalyzed by CTAB

Kinetics of the reversible consecutive reaction of ascorbic acid H2A with hydrogen peroxide was studied in phosphate buffer(μ =0.1 mol· L- 1) at pH 7 and 25 ℃ with conduction calorimeter. The apparent rate constants of this reaction, k1, k- 1 and k2, were measured at different concentrations of cetyltrimethylammonium bromide(CTAB). The results indicated that the apparent rate constants of the reaction, k1, k- 1 and k2, were remarkably affected by the cationic surfactant CTAB. k1 initially increased, up to a maximum, and then fell off with increasing CTAB micellar concentration, while k- 1 was hardly affected by CTAB of low concentrations( 2 mmol· L- 1) and increased when the CTAB concentration was more than 5 mmol· L- 1; with the increasing concentration of CTAB, k2 decreased before cmc, then increased up to a maximum, and at last fell off. CTAB micelles of low concentration had almost no influence on the activity of intermediate , but high concentrated micelles could promote the dismutation of and inhibit its oxidation reaction. Electrostatic effect, hydrophobic effect, and local concentration effect of CTAB premicelles and micelles were the main factors influencing this reversible consecutive reaction.

Aqueous Photo-Dimerization Using 2-Pyridylsilyl Group as a Removable Hydrophilic Group

Abstract The [2+2] photo-dimerization of 2-pyridylsilyl-substituted stilbene derivatives was investigated and exclusive dimer production was observed in aqueous dilute solution. This result is explained in terms of the local concentration effect of substrates in water. The transformation of obtained photo-dimer was also investigated.

Microstructural characterisation of fatigue crack initiation in Al-based plain bearing alloys

The fatigue initiation behaviour of two Al-based bearing lining alloys has been assessed by image analysis tessellation techniques and subsequent adaptive numerical classification approaches. Fatigue initiation occurs by decohesion at Si particles, and in the absence of Si particles decohesion occurs at Sn particles. In both cases, relatively unclustered, large particles initiate fatigue, and the relative position of the nearest neighbour particle seems to have significance. This can be explained qualitatively in terms of strain mismatch arising from differing elastic moduli between the particles and surrounding Al matrix and local stress concentration and superposition effects. The lining alloy containing no Si and high Sn content exhibits more directionality, and a heavier dependence on angular information is found by the classifier.

Studies on the reaction kinetics and the mechanism of hydrolysis of bis(4-nitrophenyl) phosphate (BNPP) catalyzed by oxamido-bridged dinuclear copper(II) complexes in micellar solution

Oxamido-bridged dinuclear copper(II) complexes, used as symmetric two-center catalysts for the cleavage of BNPP, were synthesized and characterized. The reaction kinetics and the mechanism of hydrolysis of BNPP catalyzed by metallomicelles, made from complex (A) or (B) and a surfactant (LSS or CTAB), were investigated. A kinetic mathematical model for BNPP cleavage was also proposed. The results showed that the reaction rate for the catalytic hydrolysis of BNPP, compared with spontaneous hydrolysis of BNPP, increased by a factor of ca. 1 × 106 due to the synergistic effect of two copper ions in the complex and the local concentration effect of the micelle. The study indicates that the metallomicelle-containing oxamido-bridged dinuclear copper(II) complex may be a potential catalyst for the hydrolysis of BNPP.

The fracture behavior of a Ti-6242 alloy deformed in bending fatigue

In this paper, the fracture behavior of the Ti-6242 alloy subjected to bending fatigue deformation is presented and discussed. Samples of the alloy were deformed to failure in bending over a range of maximum load levels. The macroscopic fracture mode was found to be marginally different over the range of load levels examined. Microscopically the fracture surfaces revealed features reminiscent of locally ductile and brittle mechanisms. The kinetics governing crack advance and fracture is rationalized in light of maximum load, local stress concentration effects, and contributions from intrinsic microstructural features.

High local protein concentrations at promoters: strategies in prokaryotic and eukaryotic cells.

The speed of chemical reactions is proportional to the concentration of molecules involved. Since proteins catalyze most of the essential reactions inside a living cell, their concentration should be as high as possible. An economical way to achieve this is through the establishment of small cell compartments. We propose that within these compartments, two types of local concentration effects are at work. (1) With local concentration type I reactions, multimeric proteins bound to a specific DNA sequence have an increased local concentration for a second DNA site sufficiently close-by, or for proteins bound to such a site. (2) For type II effects, DNA can be used as a scaffold to build unique nucleoprotein complexes that would otherwise not exist free in solution. These complexes are proficient in establishing longer-range interactions with similarly unique complexes located far away on the genome. We discuss the consequences of these local concentration effects in the light of the markedly different sizes of prokaryotic and eukaryotic cells and of their genomes.

MICELLAR CATALYSIS OF COMPOSITE REACTIONS I MICELLAR EFFECT ON THE CONSECUTIVE FIRST ORDER REACTION

ABSTRACT The micellar catalytic model (or the consecutive first order reaction has been proposed in this paper. It was applied to the alkaline hydrolysis of dimethyl phthalate in micellar solutions of surfactants (CTAB, SDS and Triton X-100), and the alkaline hydrolysis of bis (2,4-dinitrophenyl) posphate in CTAB micellar solution. Rate constants obtained in micellar phase indicate that the two steps of alkaline hydrolysis of dimethyl phthalate are both inhibited by all of the surfactants investigated. CTAB micelle exhibits a greater catalytic effect on the alkaline hydrolysis of bis (2, 4-dinitrophenyl) phosphate. this may be arised from the local concentration effect of hydroxide ion in CTAB micellar phase. Nevertheless. the second order rate constant of bis-(2, 4-dinitrophenyl) phosphate in the micellar phase is smaller than that in the bulk phase.

THE EFFECT OF MIXED MICELLES OF SURFACTANTS ON THE ALKALINE HYDROLYSIS OF DIMETHYLFORMAMIDE

ABSTRACT The alkaline hydrolysis of dimethylformamide has been studied at 40°C In mixed micelles of surfactants (CTAB—Brij35, SD6 — Brij35, CTAB — BHMI, SDS —BHMI, Brtj35—BHMI) and functional molecule 1—benzyl—2—hydroxymethylimidazole (BHMI) solutions with analog thermogram method of thermokinetics. The kinetic parameters km In micellar pseudophase, rate constant ke in functional molecular solution, and the association constant of reactant with micelle Ki, with functional molecule K, have been calculated from kinetic equations of mixed micellar catalysis in the case of [S]>[M]. The results indicate that all the researched systems exhibit catalytic effect on the reaction. Polarity and local concentration effect were employed to explain these phenomena. A mechanism of catalysis of BHMI and mixed micelles of a surfactant and BHMI was proposed.

THE EFFECT OF SURFACTANT MICELLES ON THE ALKALINE HYDROLYSIS OF DIMETHYLFORMAMIDE

ABSTRACT The alkaline hydrolysis of dimethylformamide has been studied at 40'C in micellar solutions of single surfactant (CTAB. SDS. Brij 35) with the analog thermoanalytical curve method of thermokinetics. A kinetic equation of micellar catalysis under the condition of highter reactant concentration than micellar concentration ([S]>[M]) has been derived from the pseudophase model of micellar catalysis and some relative assumptions, The kinetic parameters. km, k2mand the association constant of reactant with micelle K1, have been calculated in this way. the results indicate that these surfactant micelles exhibit catalytic effect on the reaction. This is attributed to the micropolarity and local concentration effect of micelles.

Keratin cystine reactivity in microemulsion media: influence of cosurfactant chain length

The influence of n-alkanol chain length on keratin cystine reactivity in microemulsion media has been investigated. Microemulsions of the systems water/sodium dodecylsulfate/ n-alkanol/ n-dodecane incorporating a constant concentration of reducing (thioglycolic acid) or oxidising (hydrogen peroxide) agent were chosen as reaction media. The results obtained showed that keratin cystine reactivity was faster the longer the n-alkanol chain length, an effect related to the partition of reagent among the different microemulsion domains as well as to the microstructure of the reaction media. Microemulsions with n-decanol possess a better defined droplet structure and therefore the local concentration effect of reagent in the aqueous droplets is higher than in microemulsions with n-pentanol.

Drug particle size reduction for decreasing gastric irritancy and enhancing absorption of naproxen in rats

Gastric mucosal damage, the most common event following oral administration of NSAIDs, is due to a combination of a systemic effect and a high local drug concentration effect. It has been demonstrated in rats that the gastric irritation induced following oral administration of naproxen was decreased by reducing drug particle size from 20–30 μm to 270 nm and stabilizing the particles in suspension with pluronic F-68. The reduction in irritation is attributed to a decrease in the local high and prolonged concentration of naproxen attributable to reduced crystal size. Further the amount of irritation observed with the orally administered nanoparticle formulation is similar to that following intravenous administration of naproxen. The size reduction of naproxen was also associated with an apparent increase in the rate of absorption by approx. 4-fold. The increase in the rate of absorption is attributed to an increase in surface area for dissolution for the NanoCrystal formulation.

Oxidation of ketene‐S,S‐acetals, thioacetals, and thioketals by dimethyldioxirane: A convenient method for the preparation of bis‐sulfones

AbstractThe oxidation of a series of ketene‐S,S‐acetals, 2a–f, and a series of thioacetals and thioketals, 4a–g, with a fourfold or greater excess of dimethyldioxirane, 1, produced the corresponding bis‐sulfones 3a–f and 5a–g in excellent yields. The reaction of the bis‐sulfides with one, two, and three equivalents of 1 yielded monosulfoxides, bis‐sulfoxides, and sulfoxide‐sulfones, respectively, as the major products. The order of addition of the reactants as well as the temperature of the reaction mixture changed the product distributions. The use of low‐temperature, rapid stirring and the addition of the dioxirane solution to the bis‐sulfide maximized the yield of the major product. The results are consistent with an electrophilic oxygen‐atom transfer mechanism in which the rate of sulfide oxidation is much faster than oxidation of sulfoxide to sulfone. However, sulfoxide oxidation is of a sufficient rate such that local concentration effects affect the results.

Photoinduced Electron Transfer between C60 and Polysilane Studied by Laser Flash Photolysis in the Near-IR Region

The photoinduced electron transfer between C[sub 60] and a [sigma]-conjugated polysilane was investigated by laser flash photolysis in the near-IR region. The transient absorption spectra of C[sub 60] triplet ([sup 3]C[sub 60]*), C[sub 60] radical anion, and the poly(methylphenylsilylene) radical cation were observed in the region 600-1600 nm in a polar solvent. The quenching rate constant k[sub q] of [sup 3]C[sub 60]* corresponded well with the formation rate constant k[sub f] of the poly(methylphenylsilylene) radical cation. Direct electron transfer from poly(methylphenylsilylene) to [sup 3]C[sub 60]* is suggested. The electron-transfer rate constants between C[sub 60] and poly(methylphenylsilylene) showed the polar solvent effect and the local concentration effect of the polymer chain. 33 refs., 9 figs.

Damage mechanisms and failure modes in cross-ply laminates under monotonic tensile loading: The influence of fiber sizing

In this study, the effect of fiber-matrix interphase on the damage modes and failure mechanisms in (0, 903), cross-ply graphite-toughened epoxy laminates is investigated. Two material systems (designated as 810 A and 810 O) with the same fiber and same matrix, but with different fiber sizings, were used to study the effect of the interphase. The system designated as 810 A contained an unreacted Bisphenol-A (epoxy) sizing, while a thermoplastic polyvinylpyrrolidone (PVP) sizing was used in the 810 O system. Damage accumulation in the cross-ply laminates under monotonic tensile loading was monitored using edge replication, x-ray radiography, acoustic emission, optical and scanning electron microscopy. Results indicate that the fiber-matrix bond strength is lower in the 810 O system compared to the 810 A system. Transverse matrix cracking initiates at a significantly lower stress level in the 810 O laminate. The 810 O laminates also exhibit longitudinal splitting, while the stronger bonding suppress this damage mode in the 810 A laminates. Numerous local delamination occur on the 0/90 interface at the intersection of 0 and 90 degree ply cracks, in the 810 O laminates. These are absent in the 810 A laminates. The failure modes are also different in the two material systems used in this study. The 810 A laminate exhibits a brittle failure, controlled by the local stress concentration effects near broken fibers. In the 810 O laminates, the presence of longitudinal splits result in the reduction of stress concentration effects near fibe fractures. This results in a global strain controlled failure in the 810 O system. It is concluded that the presence of different fiber sizings result in different damage modes and failure mechanisms in the cross-ply laminates used in this study.

Acceleration of ionic reactions by naturally occurring glycosaminoglycans. II.

The reactions of halopentammine cobalt(III) complexes with both Hg(II) (aquation reaction) and Fe(II) (electron transfer reaction) have been studied in the presence of chondroitin-4-sulphate and heparin. The rates of both reactions are increased in the presence of the glycosaminoglycan polyeletrolytes, but by different mechanisms. Inhibition of the Hg(II) reaction at lower polymerto-reagent concentration ratios is consistent with specific polymer–Hg interactions which render a population of ‘bound’ ions unreactive. However, the activation energy of the Hg-induced aquation is lowered in the presence of the polyanion, consistent with stabilization of the expected trivalent cationic intermediate by electrostatic effects. Heparin, with the higher charge density, is especially effective in this regard. In contrast, the reduction of the Co(III) complexes by Fe(II) experiences an increased activation energy in the presence of polymer. The observed acceleration of this reaction may then be due largely to local concentration effects in the polymer domain, as implied by two-phase model. Investigation of the mechanisms of these rate accelerations will therefore provide more detailed information on the electrostatic fields and specific metal ion interactions of these important biological polymers.

Polyarylates (I): Investigation of the interfacial polycondensation reaction by UV

AbstractThe interfacial polycondensation of bisphenol A with isophthaloyl chloride and terephthaloyl chloride in dichloromethane with triethylbenzylammonium chloride as the catalyst was described. A well‐defined two‐phase system had been observed, so the concentration of bisphenolate in the aqueous phase could be determined by UV spectrometer. The conversion was found to increase rapidly with reaction time, but the rate of increase in molecular weight was slow. At the initial stage with conversion up to 95%, the reaction proceeded similarly to that of solution polycon‐densation because the synthesized polyarylate could be dissolved in dichloromethane, and a polymer with a low molecular weight resulted. At the final stage of the reaction, the molecular weight was increased to a very high value due to the local concentration effect. The mole ratio of bisphenol A to diacid chlorides was found to affect the molecular weight very much. In the case of a mole ratio less than 1, the reaction remained in the initial stage mostly, and the molecular weight was low. However, for a mole ratio greater than 1, polyarylate with a very high molecular weight could be obtained because the local concentration effect was much more significant owing to the high concentration of bisphenolate that remained in the aqueous phase at the final stage.