Two interacting particles in a disordered chain III: Dynamical aspects of the interplay disorder-interaction

The interplay between the quantum interferences responsible for one particle localization over a length L₁, and the partial dephasing induced by a local interaction of strength U with another particle leading to partial delocalization over a length L ₂ > L ₁ , is illustrated by a study of the motion of two particles put close to each other at the initial time. Localization is reached in two steps. First, before the time t₁ necessary to propagate over L₁, the interaction slows down the ballistic motion. On the contrary, after t₁ the interaction favors a very slow delocalization, characterized by a spreading of the center of mass, until L₂ is reached. This slow motion is related to the absence of quantum chaos in this one dimensional model, the interaction being only able to induce weaker chaos with critical spectral statistics. Under appropriate initial conditions, the motion remains invariant under the duality transformation mapping the behavior at small U onto the behavior at large U. Received 24 August 1998     

Simultaneous spectrophotometric determination of diuretics by using multivariate calibration methods

The wavelength range and number of factors used in partial least-squares (PLS) calibration for the resolution of the dihydralazine (DHZ)-hydrochlorothiazide (HCT) binary mixture and the dihydralazine-hydrochlorothiazide-reserpine ternary mixture were optimized in terms of the relative standard error (R.S.E.) and relative mean standard error (R.M.S.E.). Under the optimum conditions thus established, synthetic mixtures of the analytes can be resolved with errors and relative standard deviations (R.S.D) less than 4.5 and 1.0%, respectively. The ensuing method, which was validated by comparison with high performance liquid chromatography (HPLC), also gives good results with real samples (pharmaceutical preparations).     

Surface modification of activated carbons for CO2 capture

The reduction of anthropogenic CO₂ emissions to address the consequences of climate change is a matter of concern for all developed countries. In the short term, one of the most viable options for reducing carbon emissions is to capture and store CO₂ at large stationary sources. Adsorption with solid sorbents is one of the most promising options. In this work, two series of materials were prepared from two commercial activated carbons, C and R, by heat treatment with gaseous ammonia at temperatures in the 200-800 °C range. The aim was to improve the selectivity and capacity of the sorbents to capture CO₂, by introducing basic nitrogen-functionalities into the carbons. The sorbents were characterised in terms of texture and chemical composition. Their surface chemistry was studied through temperature-programmed desorption tests and X-ray photoelectron spectroscopy. The capture performance of the carbons was evaluated by using a thermogravimetric analyser to record mass uptakes by the samples when exposed to a CO₂ atmosphere.     

Adsorption and desorption of copper and zinc in the surface layer of acid soils

The environmental and health effects of the contamination of soils by heavy metals depend on the ability of the soils to immobilize these contaminants. In this work, the adsorption and desorption of Cu and Zn in the surface layers of 27 acid soils were studied. Adsorption of Cu(II) from 157-3148 mumol L(-1) solutions was much greater than adsorption of Zn(II) from solutions at the same concentration. For both Cu and Zn, the adsorption data were fitted better by the Freundlich equation than by the Langmuir equation. Multiple regression analyses suggest that Cu and Zn adsorption depends to a significant extent on pH and CEC: for both metals these variables accounted for more than 80% of the variance in the Freundlich pre-exponential parameter K(F), and pH also accounted for 57% of the variance in 1/n for Zn and, together with carbon content, for 41% of the variance in 1/n for Cu. The percentage of adsorbed metal susceptible to desorption into 0.01 M NaNO3 was greater for Zn than for Cu, but in both cases depended significantly on pH, decreasing as pH increased. In turn, both pH(H2O) and pH(KCl) are significantly correlated with cation exchange capacity. Desorption of metal adsorbed from solutions at relatively low concentration (787 mumol L(-1)) exhibited power-law dependence on Kd, the quotient expressing distribution between soil and soil solution in the corresponding adsorption experiment, decreasing as increasing Kd reflected increasing affinity of the soil for the metal. The absence of a similarly clear relationship when metal had been adsorbed from solutions at relatively high concentration (2361 mumol L(-1)) is attributed to the scant between-soil variability of Kd at these higher concentrations. In general, adsorption was greater and subsequent desorption less in cultivated soils than in woodland soils.     

Exploring the <Superscript> 6</Superscript>He continuum sea through proton inelastic collisions

The p-⁶He inelastic scattering at 700 MeV u^-1 is studied using Multiple Scattering expansion of the total Transition amplitude (MST) framework with a pseudo-state representation of ⁶He continuum. We show that the differential cross sections calculated with two different pseudo-state bases are in good agreement.     

Ab initio study of screw dislocations in Mo and ta: A new picture of plasticity in bcc transition metals

We report the first ab initio density-functional study of <111> screw dislocation cores in the bcc transition metals Mo and Ta. Our results suggest a new picture of bcc plasticity with symmetric and compact dislocation cores, contrary to the presently accepted picture based on continuum and interatomic potentials. Core energy scales in this new picture are in much better agreement with the Peierls energy barriers to dislocation motion suggested by experiments.     

A kinetic study of the autocatalytic permanganate oxidation of formic acid

The kinetics of the permanganate oxidation of formic acid in aqueous perchloric acid has been studied. The results indicate that this reaction is autocatalyzed by both manganese(II) ion (formed as a reaction product) and colloidal manganese dioxide (formed as an intermediate). The apparent rate constants corresponding to the noncatalytic and autocatalytic reaction pathways are given, respectively, by the following equations The activation energies associated with the true rate constants, ??, ??, ??, ??, ??, and ?? are 37.2, 62.5, 70.9, 52.5, 40.8, and 59.9 kJ mol^?1, respectively. The percentage of the total reaction corresponding to each pathway is given for typical experimental conditions. Mechanisms in agreement with the kinetic data are proposed for the six different reaction pathways observed.     

A comparative study of different extraction systems for the spectrophotometric determination of potassium with 18-crown-6

A comparative study on the extractive-spectrophotometric determination of potassium with 18-crown-6 employing different colored counterions and organic extraction solvents is described.Extraction systems using methyl orange/chloroform, bromcresol purple/chloroform and bromcresol green/benzene demonstrated good preliminar features for such purpose and are studied in detail. Analytical performance characteristics including: sensitivity, selectivity, precision, linearity, etc. are given for each method. The main disadvantage of this type of determinations is the lack of reproducibility. Some explanations to this fact are discussed.A new method for the determination of trace amounts of potassium based on its extraction into chloroform with 18-crown-6 and methyl orange is proposed. The linear working range goes from 0.5 to 7 ppm of potassium, the apparent molar absorptivity being 8.8 × 10³ liters mol^?1 · cm^?1, and the precision, expressed in terms of relative standard deviation, of ±4.6%.     

Poly(alkylcyanoacrylate) colloidal particles as vehicles for antitumour drug delivery: a comparative study

Because of the fundamental importance of new therapeutic routes for cancer treatment, a number of systems based on colloidal particles as vehicles for the delivery of chemotherapeutic agents have been devised. The target is always to provide the proper dose of the antitumour agent only at the desired locus of action, thus reducing the unwanted side effects. The systems studied in this work are nanospheres of the biodegradable polymers poly(ethyl-2-cyanoacrylate), poly(butylcyanoacrylate), poly(hexylcyanoacrylate) and poly(octylcyanoacrylate), all suitable for parenteral administration, as vehicles for 5-fluorouracil, a well studied drug used for the treatment of solid tumours. Two loading methods have been analyzed: the first one is based on drug addition during the process of generation of the particles, by an anionic emulsion/polymerization procedure, and the subsequent drug trapping in the polymeric network. The second method is based on surface adsorption in already formed nanoparticles, after incubation in the drug solution. A detailed investigation of the capabilities of the polymer particles to load this drug is described. The main factors determining the drug incorporation to the polymer network were the type of monomer, the pH and the drug concentration. The release kinetics of 5-fluorouracil is found to be controlled by the pH of the release medium, the type of drug incorporation and the type of polymer.