La Naturalesa de l'enllaç químic. Contribucions de Linus Pauling

Aquest treball comença amb un breu repàs de l'evolució històrica del concepte d'enllaç químic, seguit d'una introducció als dos models que, en el marc de la mecànica quàntica, es fan servir actualment per explicar aquell concepte (orbitals moleculars i enllaç-valència). Després d'aquesta introducció, es presenten les principals aportacions fetes per Linus Pauling en relació amb l'enllac, la major part de les quals s'enquadren en el context enllaç-valència. El treball de Heitler i London, considerat com l'inici del model enllaç-valència, posa de manifest que, quan els electrons que formen l'enllaç tenen els seus spins antiparal•lels, se situen preferentment entre els nuclis i poden llavors atreure'ls amb intensitat sufficient per vèncer la repulsió internuclear. En paraules de Pauling, la força química és una força d'origen electrostàtic, que pot ser atractiva o repulsiva depenent de l'orientació relativa dels spins electronics. El treball de Pauling relacionat amb la comprensió de l'enllaç és molt ampli. Una de les claus per entendre el seu èxit es deu a la concurrència de dos factors: la seva condició de químic, d'una Banda, i, de l'altra, el seu coneixement de la mecànica quàntica quan tot just s'estava desenvolupant en un restringit cercle de físics teòrics. Aquesta formació li permeté entroncar els conceptes quàntics amb el model de Lewis, ben conegut per la resta de químics. Destaquem l'especial rellevància de les publications dels anys trenta, les quals inclouen les contribucions més importants en relació amb la naturalesa de l'enllaç. Una d'aquestes és el concepte d'hibridació, el qual, malgrat el seu actual desprestigi per part de molts químics quàntics, ha tingut una importància històrica cabdal, en permetre racionalitzar l'estructura de la major part de les molècules i, en particular, dels complexos dels metalls de transició. Una altra contribució rellevant es el concepte de ressonància, desenvolupat per explicar l'estructura de les molècules que, com el benzè, no es poden representar amb una sola estructura de Lewis. Una aplicació menys coneguda d'aquest concepte fou el model introduït per Pawling per explicar les especials propietats dels metalls. El treball conclou amb una breu incursió en les actuals perspectives del mètode enllaç-valència. En els últims anys, aquest mètode ha recuperat interès gràcies als seus avantatges de cara a la interpretació dels resultats de sofisticats càlculs mecanoquàntics utilitzant les idees intuïtives que, de manera quasi constant, fan servir els químics en els seus raonaments. Aquest és, de fet, el punt clau que Pauling ha defensat al llarg de la seva carrera científica i que, malgrat el seu indiscutible interès, queda, de vegades, relegat a un segon terme a causa d'una excessiva preocupació per la precisió dels càlculs

Dissolution and phosphate-induced transformation of ZnO nanoparticles in synthetic saliva probed by AGNES without previous solid-liquid separation. Comparison with UF-ICP-MS

The variation over time of free Zn2+ ion concentration in stirred dispersions of ZnO nanoparticles (ZnO NPs) prepared in synthetic saliva at pH 6.80 and 37 degrees C was followed in situ (without solid liquid separation step) with the electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). Under these conditions, ZnO NPs are chemically unstable due to their reaction with phosphates. The initial stage of transformation (around 5-10 h) involves the formation of a metastable solid (presumably ZnHPO4), which later evolves into the more stable hopeite phase. The overall decay rate of ZnO NPs is significantly reduced in comparison with phosphate-free background solutions of the same ionic strength and pH. The effective equilibrium solubilities of ZnO (0.29-0.47 mg.L-1), as well as conditional excess-ligand stability constants and fractional distributions of soluble Zn species, were determined in the absence and presence of organic components. The results were compared with the conventional ultrafiltration and inductively coupled plasma-mass spectrometry (UF-ICP-MS) methodology. AGNES proves to be advantageous in terms of speed, reproducibility, and access to speciation information. KeywordsThis work was supported by the Spanish Ministry MINECOunder Grant No. CTM2016-78798 and European UnionSeventh Framework Programme FP7-NMP.2012.1.3-3 underGrant No. 310584 (NANoREG). FQ gratefully acknowledgesa grant from AGAUR

Performance of an Expert Sensory Panel and Instrumental Measures for Assessing Eating Fruit Quality Attributes in a Pear Breeding Programme

Breeding programmes count on stable trained panels that support breeding evaluation selections. This work aimed to evaluate the performance of a small expert panel in the join IRTA-PFR breeding programme to validate its use in the sensory assessments of fruit pear genotypes during the selection process. A breeding F1 population of 80 pear seedlings from this programme was used. Descriptors and standard references used for sensory evaluations of pear attributes were previously defined by the four members of the expert panel. A General Procrustes Analysis (GPA) was applied to analyse the relations between instrumental and sensory traits. The results showed a good relationship between sensory attributes such as firmness and crispness with penetrometer measures. A high correlation was also found between sensory sourness and titratable acidity (TA). Panel performance was evaluated in terms of reproducibility, homogeneity, and panel consonance. The results indicated that the experts were very consistent and had a good performance. The work demonstrates, for the first time, that a small expert trained panel could be efficiently used in pear breeding programmes and allows for the selection process in a more economical and available way in contrast to the larger sensory panels conventionally used.info:eu-repo/semantics/publishedVersio

Dynamics of trace metal sorption by an ion-exchange chelating resin described by a mixed intraparticle/film diffusion transport model. The Cd/Chelex case

The time-evolution of Cd2+ ion sorption by Chelex 100 resin was studied in batch experiments as a function of time, pH, ionic strength, stirring rate, mass of resin and initial metal ion concentration. In the experimental conditions, the amount of resin sites are in excess with respect to the amount of metal ion, leading to extensive depletion of metal in bulk solution when equilibrium is reached. The data were described using a mixed control mass transport model in finite volume conditions (MCM) that includes explicitly both intraparticle and film diffusion steps. Exact numerical computations and a new approximate analytical expression of this model are reported here. MCM successfully predicts the influence of pH and ionic strength on the experimental Cd(II)/Chelex kinetic profiles (which cannot be justified by a pure film diffusion controlled mechanism) with a minimum number of fitting parameters. The overall diffusion coefficient inside the resin was modelled in terms of the Donnan factor and the resin/cation binding stability constant. The values of the latter coefficient as a function of pH and ionic strength were estimated from the Gibbs-Donnan model. Even though MCM is numerically more involved than models exclusively restricted to film or intraparticle diffusion control, it proves to be accurate in a wider range of values of the mass transfer Biot number and solution/resin metal ratios.The authors gratefully acknowledge support for this research from the Spanish Ministry MINECO (Projects CTM2013-48967 and CTM2016-78798) and by the “Comissionat d'Universitats i Recerca de la Generalitat de Catalunya” (2014SGR1132). FQ acknowledges a grant from AGAUR

Numerical Simulation of Non-Linear Models of Reaction—Diffusion for a DGT Sensor

In this work, we present a novel strategy for the numerical solution of a coupled system of partial differential equations that describe reaction–diffusion processes of a mixture of metals and ligands that can be absorbed by a sensor or a microorganism, in an aqueous medium. The novelty introduced in this work consisted of an adequate database management in conjunction with a direct iterative schema, which allowed the construction of simple, fast and efficient algorithms. Except in really adverse conditions, the calculation is converging and satisfactory solutions were reached. Computing times showed to be better than those obtained with some commercial programs. Although we concentrate on the solution for a particular system (Diffusive Gradients in Thin Films [DGT] sensors), the proposed algorithm does not require major modifications to consider new theoretical or experimental configurations. Since the quality of numerical simulations of reaction–diffusion problems often faces some drawbacks as the values of reaction rate constants increase, some additional effort has been invested in obtaining proper solutions in those cases.This research was funded by University of Lleida and University of Tolima