Publications

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation of crystalline 1,1,1-trifluoro-4-(2-thienyl)-4-hydroxy-3-buten-2-one, HTTFA, and 1,1,1-trifluoro-4-(2-thienyl)-4-mercapto-3-buten-2-one, HTTFAS, at T = 298.15 K were measured by rotating-bomb calorimetry and the standard molar enthalpies of sublimation at T = 298.15 K were measured by microcalorimetry, with the following results: [GRAPHICS] The results demonstrate an increase in stabilization energy of approximate to (38.7 +/- 5.0) kJ.mol(-1) in HTTFA compared with thiophene and that in these molecules, the intramolecular hydrogen bond energy (O-H...O), appears approximate to 20 kJ.mol(-1) larger than for (S-H...O). (C) 1997 Academic Press Limited.

Abstract Microhole arrays created by laser photoablation of thin polymer films were used to support an organic gel phase prepared by addition of 1,3:2,4-dibenzylidene sorbitol (DBS) to a solution of o-nitrophenyloctyl ether (NPOE) and an appropriate supporting electrolyte. The preparation and properties of the organic gel phase are described and the NPOE-DBS gel is assessed. The polarized microhole supported organic gel interface is shown to be capable of use as an amperometric detector for ions. (C) 1997 Published by Elsevier Science Ltd.

Abstract The fulvic acid (fua) fractions of two samples of composted solid wastes [urban (urfua) and livestock (lsfua) wastes], commercialized to be used in agriculture as organic correctives or fertilizers, were analyzed for their affinity towards Cu(II) at pH=6. Molecular fluorescence spectroscopy (synchronous mode) was used to monitor the quenching caused by the complexation upon addition of Cu(II) to fua. Spectral data were preprocessed by a chemometric self-modeling mixture analysis method (SIMPLISMA) to detect the number of different types of fluorescent binding sites that exist in each fua, their spectra and the corresponding quenching profiles [fluorescence intensity as function of the total Cu(II) concentration]. From the analysis of the quenching profiles, the amount of binding sites (C-L) and the corresponding conditional stability constants (K') were calculated. Both fua samples have approximately C-L = 0.21 mmol/g and the logarithms of K' are 4.21(3) and 4.51(8), respectively for urfua and lsfua. The differences detected between these fua samples and those extracted from natural soils can be attributed mainly to the relatively small humification extent of the present anthropogenic fua samples.

Abstract A method for the classification of fulvic acids (FA) and similar substances Is presented, It consists of three steps: (i) for each sample, sets of synchronous molecular fluorescence spect ra as the pH is changed are acquired; (ii) the spectral set is reduced to a small number of spectra and concentration profiles of the detected components by evolving factor analysis with a gradient concentration window (EFA-GCW) are obtained; (iii) the spectra of the components of the samples are subjected to pattern recognition analysis by cluster analysis and multi-dimensional scaling. The procedure was tested with 13 samples of FA of natural and anthropogenic origin. Classification rules as a function of the origin and acid-base properties of the samples, in the form of figures with spectra and tables with spectral characteristics, were obtained. The classification according to the origin of the FA samples is useful for the evaluation of the 'extent of humification of FA, The following information on the acid-base properties of the fluorescent fractions of FA was obtained: only FA extracted from soil have fluorescent fractions with relatively strong acid character (pK(a) approximate to 3.2); all FA extracted from different origins have similar fluorescent structures with relatively weak acid character (pK(a) approximate to 6.2); in addition, FA extracted from low humified materials (leaf litter and composted wastes) have another set of similar fluorescent structures with weak acid character (pK(a) = 9.2).

Abstract The complexation of Cu(II) by two samples of fulvic acids (FA) extracted from the raw sewage sludge (RsFA) of a waste water processing plant and from the composted sludge (CsFA) obtained upon aerobical digestion was studied at pH 6.0. Synchronous molecular fluorescence spectroscopy was used to monitor the association of FA with Cu(II) and a self-modelling mixture analysis method (SIMPLISMA) was used to preprocess the spectral data to calculate the number of components with different quenching profiles as function of the Cu(II) concentration and their spectra. The stability constants and a rough estimation of the binding site concentrations were obtained by a Stern-Volmer analysis, by a non-linear least-square method and by a linear procedure. The analysis of the SyF spectra allowed the identification of two binding sites for both samples. The logarithm of the conditional stability constant corresponding to the 1:1 complex formed between the stronger binding site and Cu(II) is about 4 for both samples.

Abstract <jats:title>Abstract</jats:title><jats:p>Bacterial cells or chitin were exposed to solutions with 100 μM total but only 5 μM free copper, due to the presence of a proper concentration of proline, lysine, cysteine, or ethylenediamine tetraacetate (EDTA). The influence of the nature and concentration of the particles and soluble ligands, on the sorption and on the desorption of the copper, at pH 6.50 and 25.0°C, was investigated. The metal sorbed by the particles and that left in the solution were measured by atomic absorption spectrometry, after different periods of contact between particles and solution. The interpretation of the results was based on the copper(II) speciation calculated through equilibrium approaches applied to homogenous or heterogeneous systems. A significant fraction of copper bound to the organic ligands was displaced to the bacteria or chitin, and the extent of chemical reaction depended on the nature of both the soluble (or leaving) ligands and sites on the particle surface (or entering ligands), as expected by the equilibrium theory. But with chitin, the uptake of copper in the presence of cysteine or EDTA was higher than expected, which may be due to the adsorption of the soluble copper complexes on the particle surface. In consequence of a competition between soluble and particulate ligands (cells or chitin), the free copper(II) concentration decreased in the solution, even in the presence of very strong chelators. The results indicate that copper availability is not a simple function of the initial free copper concentration in the solution. Desorption of the previously fixed copper, originated by free soluble ligands indicated that the sorption of copper was “quasireversible” for both particles, though a larger dismissal of the equilibrium position occurred for the cells, probably due to their biological activity. Both the bacteria and chitin were able to fix metal initially bound to an organic ligand continuously for periods longer than 30 min, the kinetics of uptake varying with the nature of both the leaving and entering ligands. Therefore, long time‐scale techniques, namely batch processes, warrant studies of metal availability in natural systems.</jats:p>

Abstract The association between alpha-cyclodextrin and glycerol has been studied using experimental and theoretical methods. The experimental technique used was microcalorimetric stepwise titration at 288.15, 298.15 and 313.15 K. The formation of a 1:1 complex between alpha-cyclodextrin and glycerol, in aqueous solutions, is indicated. There was no temperature dependence of the observed parameters. The thermodynamic parameters were K = 1.34 +/- 0.07, Delta H degrees = (-3.1 +/- 0.1) kJ mol(-1), Delta G degrees = (-0.7 +/- 0.1) kJ mol(-1) and Delta S degrees = (-8 +/- 4) J K-1 mol(-1) (Gibbs energy and entropy values calculated for 298.15 K). Molecular dynamics simulations have also been carried out. The average structures, obtained from 100 ps of dynamics, are compared with the solvated 'empty' alpha-cyclodextrin average structure. The dynamics calculations show that there is a structurally and energetically favourable association between the molecules, which is in good agreement with the experimental findings.

Abstract The enthalpies of solution of I-pentanol, 1-hexanol and 1-heptanol and water were determined in formamide and in ethylene glycol, at 25 degrees C, by macrosolution calorimetry. The observed enthalpies of solution far the n-alcohols are positive in the two solvents, and of similar magnitude. The enthalpy of solution of water is positive in formamide, and negative in ethylene glycol. From the enthalpies of solution, the enthalpies of salvation and the enthalpies of transfer for organic solvent-->water were calculated Using our values and literature data for alkanols, it was possible to see that both the enthalpies of solution and the enthalpies of salvation presented a constant CH2 increment for the entire series, in contrast with their behavior in water. The methylene increments for these properties in different solvents are compared with parameters considered to reflect the cohesive energy of the solvent.