Neutron capture cross section of unstable 63Ni: implications for stellar nucleosynthesis

The 63Ni(n,γ) cross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from   kT=5-100  keV with uncertainties around 20%. Stellar model calculations for a 25M⊙ star show that the new data have a significant effect on the s-process production of 63Cu, 64Ni, and 64Zn in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova.

Effects of dietary cadmium exposure on osmoregulation and urine concentration mechanisms of the semi-desert rodent Meriones shawi

Contamination by cadmium in the environment is of great concern because of its toxicity and threats to human and animal health. The current study was conducted to investigate the effects of a cadmium contaminated diet on the osmoregulation and urine concentration mechanisms of the semi-desert rodent Meriones shawi and eventual accumulation of this metal in vital organs such as the kidneys, which are directly implicated in water regulation. Originally, we used Differential Pulse Anodic Stripping Voltammetry (DPASV) to avoid the matrix interference due to the highly organic content in the biological samples. Our results show that Meriones shawi successfully maintained a homeostasis state and presented a special adaptation to regulate urine volume during cadmium exposure by decreasing diuresis and increasing urinary osmolality. The plasma osmolality and hematocrit remained constant throughout the experiment. The stripping signals of cadmium are linear up to 0.3-100 μg L(-1) range, with a detection limit of 0.28 μg L(-1). The DPASV technique was useful for easy, fast, selective and sensitive determination of Cd, which permits working at cellular concentration. This gives us more information about the chemical form in which Cd is introduced into the organ, as well as the intracellular Cd quantities. This study has potential importance if this valuable novel animal model, imitating human and animal environmental chronic exposure to Cd, could serve as an appropriate terrestrial biomonitor for Cd contaminated sites. These results are encouraging in the context of developing a low-cost and fast technology for the detection of pollutants and for studying the impairment caused by their effects.

Determining alpha-tocopherol distributions between the oil, water, and interfacial regions of macroemulsions: novel applications of electroanalytical chemistry and the pseudophase kinetic model

The assumptions of the pseudophase model for chemical reactivity in homogeneous microemulsions are used to determine the distribution of alpha-tocopherol (TOC) in macroemulsions from changes in the observed rate constant (k(obs)) for reaction between 4-hexadecylarenediazonium ion (16-ArN2+) probe and TOC with increasing surfactant concentration. Two partition constants are needed to describe the distribution of TOC or other antioxidant (AO) or polar uncharged molecule between the oil and interfacial (P(O)(I)) and the water and interfacial (P(W)(I)) regions of stirred fluid emulsions. The observed rate constants are measured electrochemically. Here we report values of P(O)(I) and P(W)(I) for the distribution of TOC in octane/acidic water/C12E6 (hexaethylene glycol monododecyl ether) and octane/acidic water/C12E4 (Brij 30, tetraethylene glycol dodecyl ether) emulsions obtained by fitting two kinetic data sets with an equation based on the pseudophase model and solving two equations in two unknowns. The partition constants were used to estimate the %TOC in each region of the emulsions. In 1:1 oil:water C12E6 emulsions, at 2% volume fraction of C12E6, 73% of TOC is in the interfacial region, 26% in the octane and about 1% in the water. The distributions of TOC in C12E4 emulsions are similar. The combined electrochemical-pseudophase model approach is applicable to any AO or other compound that reacts with 16-ArN2+. The second-order rate constant, k(I), for reaction in the interfacial region of the emulsions is also estimated from the kinetic data and is about the same for both surfactants (k(I) approximately 0.1-0.2 M(-1)s(-1)) showing that the medium properties of the interfacial regions of C12E6 and C12E4 emulsions are similar. Comparison of these rate constants for a variety of AOs may provide a scale of AO efficiency that is independent of AO distribution between the oil, interfacial and aqueous regions of emulsions.

Determining partition constants of polar organic molecules between the oil/interfacial and water/interfacial regions in emulsions: a combined electrochemical and spectrometric method

We have developed a new approach for estimating distributions of polar additives in opaque, surfactant based, macroemulsions based on the pseudophase model for homogeneous micellar and microemulsion solutions. The distribution of a polar additive, such as an antioxidant, AO, within emulsions is expressed in terms of two partition constants, one between the oil and interfacial regions, P(O)I, and the other between the water and interfacial regions, P(W)I. To estimate values for P(O)I and P(W)I requires fitting two independent data sets with two independent mathematical relations and solving equations simultaneously for the two parameters. The experimental protocols were developed for determining the partition constants of tertbutylhydroquinone, TBHQ, in a stirred emulsion composed of octane, dilute aqueous acid, and hexaethyleneglycol monododecyl ether, C12E6. One data set was obtained by electrochemical determination of the observed rate constant, k(obs), for reaction of TBHQ with an arenediazonium ion probe as a function of C12E6 volume fraction. The second data set was obtained by determining the partition constant, P(O)W, of TBHQ between octane and water in the absence of surfactant by UV-visible spectrometry. TBHQ is almost 30 times more soluble in water than octane: P(O)W = 27.5. The values of the partition constants in the emulsion are P(O)I = 1.84 x 10(4) and P(W)I = 6.73 x 10(2). The partition constants were used to estimate the fraction of TBHQ in each region; for example, 96% of the TBHQ is located in the interfacial region at 0.02 volume fraction of C12E6. Our approach is quite general and should be applicable to any polar organic compound that reacts with the arenediazonium ion probe in emulsions composed of virtually any type of oil and surfactant. Comparisons of the rate constants for reaction of the antioxidant in the interfacial region of the emulsion, which can be obtained from the electrochemical results, may lead to a scale of antioxidant efficiency that is independent of the distribution of the antioxidant in the emulsion.

Monitoring dediazoniation product formation by high-performance liquid chromatography after derivatization

A derivatization protocol that exploits the rapid reaction between arenediazonium ions and a suitable coupling agent followed by high-performance liquid chromatography analyses of the reaction mixture was employed to determine the product distribution, the rate constants for product formation and the association constant of 4-nitrobenzenediazonium, PNBD, ion with beta-cyclodextrin, beta-CD. The derivatization of PNBD with the coupling agent leads to the formation of a stable azo dye that prevents by-side reactions of PNBD with the solvents of the mobile phase, including water, or the metallic parts of the chromatographic system that would eventually lead to erroneous identification and quantification of dediazoniation products. The results show that in the presence of beta-CD, nitrobenzene is formed at the expense of 4-nitrophenol, which is the major product in its absence. The observed rate constants for the interaction between PNBD and beta-CD increase upon increasing [beta-CD] showing a saturation profile indicative of the formation of an inclusion complex between PNBD and beta-CD. By fitting the experimental data to a simplified Lineaweaver-Burk equation, the corresponding association constant and the maximum acceleration rate of beta-CD towards PNBD were estimated. The protocol is applicable under a variety of experimental conditions provided that the rate of the coupling reaction is much faster than that of dediazoniation.

Micellar Effects on Dediazoniation and on Azo Coupling Reactions

The effects of a sodium dodecyl sulfate, SDS, micellar solution on the coupling rates of two arenediazonium ions, ArN(2)(+), with the hydrophobic 1-naphthylamine, 1NA and N-(1-naphthyl) ethylenediamine, NED, coupling agents and with the hydrophilic Na salt of 2-naphthol-6-sulfonic acid, 2N6S, have been studied. First, we explored the micellar effects on the thermal decomposition of the arenediazonium ions. The observed rate constants are slightly depressed or increased, depending on the nature of ArN(2)(+), compared to those in pure water upon increasing [SDS]. Estimations of the corresponding association constant to the micelle indicate that a significant fraction of the arenediazonium ions are incorporated into the micelles even at low surfactant concentrations. The sulfonate group in 2N6S prevents its incorporation into the micellar aggregate due to the electrostatic barrier imposed by the micelles and, in consequence, the coupling reaction is inhibited. In contrast, when employing the naphthylamine derivatives, the observed rate constant increase rapidly up to a maximum at [SDS]</=0.01 M and then decreases smoothly due to the dilution effect exerted by the micelles. The results are discussed in terms of the distribution of reactants between the micellar and aqueous pseudophases, the micellar-induced shift in the pK(a) of the amines, and the concentration/dilution effect exerted by the micelles.

Electrochemical monitoring of the thermal decomposition of arenediazonium ions with time in acidic MeOH/H 2 O mixtures: dediazotization rate constant

The rate of the spontaneous decomposition of 4-methylbenzenediazonium tetrafluoroborate, 4MBD, was determined. The effects of solvent composition (MeOH/H2O) on its electrochemical processes and on those of the derivatized 2-naphthol-6-sulfonate-4-toluene (6S2NPT) azo dye were investigated by Differential Pulse Polarography (DPP). This electrochemical technique shows an effective sensitivity for detecting arenediazonium ions and derivatized azo dyes.