Publications

Abstract The acid properties of a soil fulvic acid (sfua) were characterized by potentiometric titration with tetrabutylammonium hydroxide in two non-aqueous solvents with high acid-base resolution power, N,N-dimethylformamide (DMF) and acetonitrile. Synchronous fluorescence spectroscopy (SyF) was also used to monitor directly the sfua status during the potentiometric titration in DMF. The potentiometric titration curves showed no clear end-point and the analysis of the sets of spectra obtained at increasing neutralization degree, with a self-modeling curve resolution method (SIMPLISMA), revealed the existence of two components with featureless concentration profiles. Internal standards (maleic, salicylic and p-hydroxylbenzoic acids) were used to determine the amounts of acid groups with different acid strengths in the two non-aqueous solvents. It was shown that the variations observed in the SyF spectra sets of the internal standards are not correlated with those observed in the sfua data. The splitting of the sfua groups in the non-aqueous titration curves seems to be forced artificially depending on the standards used.

Abstract The variations observed in synchronous fluorescence spectra of aqueous solutions (pH = 4) of three fulvic acids (fua) upon variation of the concentration of Al3+ were analyzed by evolving factor analysis (EFA). 'rhe three fua samples were extracted from materials present in a pinewood soil: dry pine needles and the upper and lower soil horizons. The presence of Al3+ induces quenching and enhancement of bands in the synchronous fluorescence spectra, and concentration constraints are proposed for the EFA analysis that allow the calculation, on a self-modeling curve resolution basis, of the number of components that show linearly independent variations, their spectra, and corresponding concentration profiles. Simulated and experimental data sets of simple known compounds (catechol, caffeic acid, salicylic acid, and morin) were also studied as the first validation of the experimental and data treatment procedures.

Abstract The adsorption of mannitol and sorbitol on Au(111) has been followed by differential capacity measurements. Results obtained for the adsorption of these polyalcohols in dilute solutions of HClO4 and NaClO4 showed that the adsorption process could be described by a Frumkin isotherm, the parameters of which were determined and discussed. The values of the standard Gibbs energy of adsorption suggest physical adsorption of mannitol and sorbitol, without the existence of specific interactions of the adsorbed molecules with gold. The values of the interaction parameters are negative in both electrolytes, indicating a net attractive interaction in the adsorbed layer. There are noticeable differences between the values of the standard Gibbs energy of adsorption and of the interaction parameter for the two polyols, which have been interpreted in terms of the conformational properties of the two isomers in the adsorbed layer.

Abstract The behaviour of Au(210) in HNO3 and NaNO3 solutions was studied with cyclic voltammetry and differential capacity measurements as a function of concentration and temperature. The effect of concentration on both j-E profiles and C(E) curves suggests that for low concentrations and around zero charge potential, NO3- adsorption is similar to ClO4- adsorption on Au(210). For higher concentrations, the adsorption of NO3- ions at positive charges seems to occur to a greater extent than ClO4- ions. The entropy of formation of the electrochemical interface was derived from the effect of temperature in HNO3 solution. It was found to be qualitatively similar to what was observed in HClO4, solution.

Abstract The Knudsen mass loss effusion technique was used to measure the vapour pressure of crystalline tris(pentane-2,4-dionato)ruthenium(III) as a function of temperature between 398.18 and 413.15 K using three different effusion holes. From the temperature dependence of the vapour pressure, the standard molar enthalpy of sublimation at the mean temperature of the experimental temperature range, 405.7 K, was derived as 139.7 +/- 2.5 kJ mol-1. Using an estimated value of DELTA(cr)gC(p)o = -50 J K-1 mol-1, the standard molar enthalpy of sublimation at 298.15 K was calculated as 145.1 +/- 2.5 kJ mol-1.

Abstract The standard (po = 0.1 MPa) molar enthalpies of formation for crystallin 3-, 5-, 6-, and 8-aminoquinoline 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 the temperature T = 298.15 K, were derived by the Clausius-Clapeyron equation. Direct measurements of the standard molar enthalpies of sublimation, using microcalorimetry, for 3- and 5-aminoquinoline, confirmed the values from the Knudsen technique. [[formula]].

Abstract The phase equilibria of the sodium dodecyl sulfate (SDS)-didodecyldimethylammonium bromide (DDAB)-water system have been studied by water deuteron NMR and polarizing microscopy methods at 40-degrees-C. H-1 NMR relaxation, PGSE FT-NMR self-diffusion, and dynamic light scattering are used to study aggregate structures of isotropic phases. A pseudoternary representation of the four-component phase diagram contains a large number of regions of homogeneous solutions and liquid crystalline phases, as well as multiphase regions. Isotropic solution regions contain SDS-rich micellar aggregates or (spontaneously forming) vesicles, rich in either surfactant; the vesicles form at very high dilution (<0.1% surfactant). Addition of DDAB to solutions of spherical SDS micelles may induce a micellar growth to rodlike aggregates. There is a large region of bicontinuous cubic liquid crystalline phase, with high concentrations of both surfactants. The phase diagram contains several regions of lamellar liquid crystalline phase, and under certain conditions two lamellar phases may coexist. The two lamellar phases of the binary DDAB-water system can incorporate very different amounts of SDS, the one dilute in DDAB only small amounts, but the concentrated one large amounts. In addition, the lamellar phase may form for concentrated roughly equimolar mixtures of the two surfactants and for mixtures containing high concentrations of SDS. The possible connection between different lamellar regions in an appropriate three (or higher) dimensional phase diagram is discussed as well as the interactions, in terms of electrostatic and molecular packing effects, giving rise to the observed phase behavior and solution microstructure.

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 the enthalpies of sublimation: 1,3-cyclohexanedione, -335.6 +/- 1.6 kJ mol-1; 1,4-cyclohexanedione, -332.6 +/- 1.2 kJ mol-1; 4,4-dimethyl-1,3-cyclohexanedione, -400.4 +/- 2.7 kJ mol-1; and 5,5-dimethyl-1,3-cyclohexanedione, -383.6 +/- 1.9 kJ mol-1. Ab initio calculations were made of the enthalpies of isomerization of the three cyclohexanediones and of the isomerization of 1,3-cyclohexanedione to its enol forms: the keto form of 1,3-cyclohexanedione in the gaseous state was predicted to be energetically more stable than the enol form by 18.0 kJ mol-1. NMR spectra, however, showed the compound in the crystalline state to exist in the enol form with the molecules held together by intermolecular hydrogen bonds. The enthalpy of formation of 1,2-cyclohexanedione in the gaseous state, not amenable to experimental measurement, was predicted to be -300 +/- 5 kJ mol-1, and the conventional strain energies in the three cyclohexanedione isomers were estimated to be of similar magnitude, ca. 13 kJ mol-1.