Publications

Abstract The association of Al(III) with a soil fulvic acid (fua) was monitored in the pH range of 2.5-5.0 by synchronous fluorescence (SyF) spectroscopy. Sets of spectra of 100 mg/l fua solutions obtained at increasing Al(III) concentration (up to about 0.7 mM) at each pH (2.5, 3.0, 3.5, 4.0, 4.5 and 5.0) were preprocessed by the self-modeling mixture analysis method SIMPLISMA, with inverted second derivative spectra in the pure variable selection step, to determine the number and to calculate the spectra and concentration profiles of the components. This procedure detected two major components at all pH values: (i) one shows an increase of fluorescence which corresponds to a fluorescent complex formed between Al(III) and one of the binding sites of the fua; (ii) the other shows a decrease of fluorescence probably due to the decrease of the free concentration of a fluorescent binding site that complexes Al(III) forming a complex that does not show fluorescence. Considering the component that shows increase of fluorescence, conditional stability constants [log(K) with standard deviation in parenthesis] were estimated: pH=2.5, 4.5(3); pH=3.0, 4.6(3); pH=3.5, 4.8(2); pH=4.0, 5.2(1); pH=4.5, 6.1(2); and pH=5.0, 6.2(3). These results show that the stability of the complexes between fua and Al(III) decreases markedly for pH<4.5. This behavior may have an important influence on the changes of the environmental speciation of Al(III) upon acidification.

Abstract The following standard molar enthalpies of formation in the gaseous state at 298.15 K were determined from the enthalpies of combustion of the crystalline solids, and their enthalpies of sublimation and the mean (N-O) bond dissociation enthalpies were derived. Delta(f)H(m)(0)(g); [D(N-O)] (kJ mol(-1)): quinoxaline 1,4-dioxide, 227.1 +/- 2.4; 255.8 +/- 2.0; 2-methylquinoxaline 1,4-dioxide, 169.9 +/- 7.2; 268.3 +/- 4.9; 2-methyl-3-acetylquinoxaline 1,4-dioxide, 33.1 +/- 5.0; 251.6 +/- 4.2; 2-phenyl-3-benzoylquinoxaline 1,4-dioxide, 355.2 +/- 7.1; 227.3 +/- 5.4; 2-methyl-3-carbomethoxyquinoxaline 1,4-dioxide, -148.7 +/- 3.2; 242.3 +/- 3.9; pyrazine 1,4-dioxide, 186.5 +/- 1.9; 254.0 +/- 2.3; 2-methyl-5-pyrazinecarboxylic acid, -213.6 +/- 1.7. Unconstrained geometry optimizations by ab initio calculations showed the effect of steric hindrance on changes in extended delocalizations and were in accord with the trends in the mean bond dissociation enthalpies.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of formation for crystalline 2-phenylquinoline and 2,2'-biquinoline were derived from the standard molar enthalpies of combustion, in oxygen, at the temperature T = 298.15 K, measured by static bomb combustion calorimetry; the Knudsen mass-loss effusion technique was used to measure the vapour pressures of the crystals as functions of temperature, and the standard molar enthalpies of sublimation, at T = 298.15 K, were derived using the Clausius-Clapeyron equation. [GRAPHICS] (C) 1997 Academic Press Limited.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpy of formation for liquid 2,4,6-trimethylpyridine was derived from the standard molar enthalpy of combustion, measured by static bomb combustion calorimetry in oxygen, at the temperature T=298.15 K: Delta(f)H(m) degrees = -(31.0 +/- 2.3) kJ.mol(-1). (C) 1997 Academic Press Limited.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation Delta(f)H(m) degrees for crystalline 5-, 6-, 8-nitroquinoline and 8-nitroquinaldine were derived from the standard molar enthalpies of combustion in oxygen at T = 298.15 K Delta(c)H(m) degrees, measured by static bomb combustion calorimetry; the Knudsen mass-loss effusion technique was used to measure the vapour pressures of the crystals as functions of temperature, and the standard molar enthalpies of sublimation, at T = 298.15 K, Delta(cr)(g)H(m) degrees were derived by the Clausius-Clapeyron equation. Direct measurements of the standard molar enthalpies of sublimation, using microcalorimetry, for 5-, 6-, 8-nitroquinoline, and 8-nitroquinaldine confirmed the values from the Knudsen technique. [GRAPHICS] (C) 1997 Academic Press Limited.

Abstract The standard (p° = 0.1 M Pa) molar enthalpies of formation for 4-methylbiphenyl, 4,4'-dimethylbiphenyl and 4,4'-dimethyl-2,2'bipyridine were derived from the standard molar enthalpies of combustion, in oxygen, at T= 298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T= 298.15 K, were measured by Calvet microcalorimetry. Theoretical calculations at the ab initia restricted Hartree-Fock (RHF) level with second-order Moller-Plesset (MP2) perturbation theory correlation corrections and density functional theory (DFT) have been carried out for all these molecules in order to access their stabilitites relative to the non-substituted ones. The theoretical results are in general good agreement with the experimental results.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation for 4-methylbiphenyl, 4,4'-dimethylbiphenyl and 4,4'-dimethyl-2,2-'bipyridine were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, were measured by Calvet microcalorimetry. Theoretical calculations at the ab initio restricted Hartree-Fock (RHF) level with second-order Moller-Plesset (MP2) perturbation theory correlation corrections and density functional theory (DFT) have been carried out for all these molecules in order to access their stabilitites relative to the non-substituted ones. The theoretical results are in general good agreement with the experimental results.

Abstract The differential capacity of a large number of liquid-liquid interfaces has been determined by impedance spectroscopy. Even in the absence of ion pairing, the interfacial capacity was always larger than predicted by Gouy-Chapman theory. These findings are explained within a simple model, in which an extended mixed-boundary layer exists at the interface, whose thickness extends over several solvent diameters.