Publications

Abstract A thermophysical and thermochemical study has been carried out for crystalline imidazolidin-2-one and N,N'-trimethyleneurea [tetrahydropyrimidin-2(1H)-one]. The thermophysical study was made by differential scanning calorimetry, d.s.c., in the temperature intervals between T = 268 K and their respective melting temperatures. Several solid-solid transitions have been detected in imidazolidin-2-one. The standard (p degrees = 0.1 M Pa) molar enthalpies of formation, at T = 298.15 K, for crystalline imidazolidin-2-one and N,N'-trimethyleneurea [tetrahydropyrimidin-2(l H)-one], were deter-mined using static-bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, for the two compounds were derived from the variation of their vapour pressures, measured by the Knudsen effusion method, with the temperature. These two thermochemical parameters yielded the standard molar enthalpies of formation of the two cyclic urea compounds studied in the gaseous phase at T = 298.15 K. These values are discussed in terms of molecular structural contributions and interpreted on the bases of the "benzo-condensed effect" and of the ring strain of imidazolidin-2-one.

Abstract A novel procedure for solid-phase microextraction fiber preparation is presented, which combines the use of a rigid titanium alloy wire as a substrate with a blend of PDMS sol-gel mixture/silica particles, as a way of increasing both the mechanical robustness and the extracting capability of the sol-gel fibers. The similar to 30 mu m average thick fibers displayed an improvement in the extraction capacity as compared to the previous sol-gel PDMS fibers, due to a greater load of stable covalently bonded sol-gel PDMS. The observed extraction capacity was comparable to that of 100 mu m non-bonded PDMS fiber, having in this case the advantages of the superior robustness and stability conferred, respectively, by the unbreakable substrate and the sol-gel intrinsic characteristics. Repeatability (n = 3) ranged 1-8% while fiber production reproducibility (n = 3) ranged 15-25%. The presence of the silica particles was found to have no direct influence on the kinetics and mechanism of the extraction process, thus being possible to consider the new procedure as a refinement of the previous ones. The applicability potential of the devised fiber was illustrated with the analysis of gasoline under the context of arson samples.

Abstract The interaction between polyelectrolytes based on dextran with pendant N-alkyl-N,N-dimethyl-N-(2-hydroxypropyl) ammonium chloride groups, where n = 2, 4, 8, 12, or 16, and sodium alkyl sulfates, SC(n)S, with n = 8, 10, 12, 14, and 16, has been studied by conductometry and fluorescence techniques. Comparison of cumulative specific conductivities of the mixtures of polymer - surfactant over a large surfactant concentration range, with those of pure surfactant and NaCl, has clearly shown that the surfactants start to bind to polymer at very low concentrations (10(-6) M), forming mixed aggregates. The steady-state emission fluorescence measured in the presence of pyrene, 1,3,6-diphenylhexatriene (DPH), and 1-pyrenylbutyric acid sodium salt demonstrated the existence of a critical surfactant concentration (CAC(S)) at which the previously formed mixed aggregates are interconnected due to self-association of surfactant molecules included in different mixed polymer/surfactant aggregates. Above CAC(S), the mixed aggregates change dramatically their properties (hydrophobicity, size, DPH solubilization) which depend on both polymer and surfactant hydrophobicities and concentrations. The characterization of the new formed aggregates at different surfactant concentration ranges is derived mainly from their ability to solubilize hydrophobic compounds. The variety of fluorescence techniques used, combined with conductometric measurements and previous calorimetric information allowed us to provide here a comprehensive study and new interpretation of the solution behavior of these polymer-surfactant systems.

Abstract Alkylimidazolium salts are a very important class of compounds. So far, calorimetry has hardly been used to characterize their solution behaviour. The enthalpies obtained from indirect methods have an intrinsic large uncertainty, and nowadays it is clear that calorimetry is the most sensitive technique for directly measuring the thermodynamic properties of aggregation. In this work, isothermal titration calorimetry (ITC) was used along with conductivity to determine the thermodynamics of aggregation of 1-alkyl-3-methylimidazolium chlorides ([C(n)mim]Cl, n=8, 10, 12, and 14) in aqueous solution. The critical micelle concentrations, cmc, were obtained from conductivity and calorimetry, and the enthalpies of micelle formation, Delta H-mic. were derived from the calorimetric titrations. From conductivity, we could also derive the values for the degree of ionisation of the micelles (a), the molar conductivity (Delta(M)) of the [C(n)mim]Cl micellar species and the molar conductivity at infinite dilution (Delta(infinity)) for the [C(n)mim](+) cations. Values are therefore reported for the enthalpy (Delta H-mic), the Gibbs free energy (Delta G(mic)) and entropy (Delta S-mic) changes for micelle formation, Further, the aggregate sizes and aggregation numbers were obtained by light scattering (LS) measurements. The observed variation of the thermodynamic properties with the alkyl chain length is discussed in detail and compared with the traditional cationic surfactants 1-alkyl-trimethylammonium chlorides, [C(n)TA]CI. The difference in the values of the thermodynamic parameters for both types of surfactants is here related to the structure of their head groups.

Abstract The effect of the pH (from 3 to 10) on the excitation emission matrices (EEMs) of fluorescence of CdTe quantum dots (QDs), capped with mercaptopropionic acid (MPA), were analyzed by multiway decomposition methods of parallel factor analysis (PARAFAC), a variant of the parallel factor analysis method (PARAFAC2) and multivariate curve resolution alternating least squares (MCR-ALS). Three different sized CdTe QDs with emission maximum at 555nm (QDa), 594nm (QDb) and 628nm (QDc) were selected for analysis. The three-way data structures composed of sets of EEMs obtained as function of the pH (EEMs, pH) do not have a trilinear structure. A marked deviation to the trilinearity is observed in the emission wavelength order-the emission spectra suffers wavelength shift as the pH is varied. The pH-induced variation of the fluorescence properties of QDs is described with only one-component PARAFAC2 or MCR-ALS models-other components are necessary to model scattering and/or other background signals in (EEMs, pH) data structures. Bigger sized QDs are more suitable tools for analytical methodologies because they show higher Stokes shifts (resulting in simpler models) and higher pH range sensitivity The pH dependence of the maximum wavelength of the emission spectra is particularly suitable for the development of QDs/EEMs wavelength-encoded pH sensor bioimaging or biological label methodologies when coupled to multiway chemometric decomposition.

Abstract Excitation emission fluorescence matrices (EEMs) of Verapamil drug were obtained by direct and by derivatization fluorescence spectroscopy. The fluorescence excitation and emission wavelengths were displaced to longer wavelengths and the fluorescence intensity was enhanced upon derivation with respect to the native fluorescence of the drug. The complete EEM of the native fluorescence of the drug and of the derivatization product were rapidly acquired by using a charged-coupled device detector (CCD), which is advantageous in terms of speed in the analysis, with respect to the use of a conventional photomultiplier detector. The EEMs were analyzed by several second-order multivariate calibration methods exploiting the second order advantage. The three-dimensional decomposition methods used, based in different assumptions about the trilinearity of the three way data structure under analysis, were parallel factor analysis (PARAFAC), bilinear least squares (BLLS), parallel factor analysis 2 (PARAFAC2) and multivariate curve resolution-alternating least squares (MCR-ALS). The determination was performed by using the standard addition approach. The figures of merit of the PARAFAC and BLLS methods were calculated, obtaining a lower limit of detection with the derivatization procedure, when compared with the direct measurement of the fluorescence of the drug. In Verapamil drug the best estimations were found with the BLLS and the MCR-ALS models. In the quantification of Verapamil in a pharmaceutical formulation the best estimation, when compared with the result obtained by the US Pharmacopeia high performance liquid chromatography approach, was obtained by direct fluorescence spectroscopy with MCR-ALS and by derivatization fluorescence spectroscopy with the PARAFAC2 model.

Abstract A Diltiazem kinetic spectrophotometric method was optimized by factorial analysis. The experimental method is based on a two-stage reaction of Diltiazem with hydroxylamine and a ferric salt: in the first stage there is a hydroxamic acid formation; and, in the second stage there is a red colour complex ferric hydroxamate formation. The variables under investigation were: solvent; hydroxylamine, sodium hydroxide and ammonium ferric sulphate concentrations; volume of perchloric acid; and, temperature. The responses of the reactional system were the maximum absorbance, the wavelength and the reaction time at maximum absorbance. Experimental design methodologies were used in the optimization. Fractional and full factorial designs followed by optimization Box-Behnken and central composite experimental designs were used. The observed optimum conditions were: methanol as reaction solvent; hydroxylamine concentration of 9.375%; sodium hydroxide concentration of 18.750%; ferric reagent concentration of 2.000%; minimum volume of perchloric acid to neutralize the sodium hydroxide; and, room temperature as reaction temperature. With this set of experimental conditions a reaction time of 10.5 s with maximum colour development at 512 nm wavelength was achieved.

Abstract Monolayers of cationic gemini surfactants and their catanionic mixtures with sodium dodecyl sulfate (SDS), have been investigated with the Langmuir trough technique, at the air-water interface. The gemini surfactants are of the alkanediyl-alpha,omega-bis(alkyldimethylammonium) type, here designated as m-2-m, where m and 2 are the alkyl chain and spacer lengths, respectively. For the neat geminis, the stability of the monolayer increases as the chain length increases, starting from soluble films of 12-2-12 to stable films of 18-2-18. For the equicharged m-2-m/SDS mixtures (with m=12, 14, 16 and 18), stable monolayers are obtained. The effect of the gemini chain length on the phase behavior and molecular organization of the films is discussed on the basis of pressure-area isotherms for compression or compression-expansion cycles. The pressure-temperature plots, at constant trough area, yield the desorption temperature and suggest the desorption mechanism of the film molecules. Furthermore, the effect of the mixing molar ratio between m-2-m and SDS on the isotherms, for m = 12 and 14, has also been investigated. It is concluded from the mean area per molecule that the gemini molecules when in excess with respect to equicharged composition desorb from the film, so that the electroneutral composition is maintained.

Abstract Mathematical models under conditions of cyclic staircase voltammetry and electrochemical impedance spectroscopy (EIS), which,consider the kinetic effects due to the complexation reaction by the facilitated transfer of metal ions at polarized interfaces, are presented. Criteria for qualitative recognition of these kinetic effects from the features of simulated cyclic voltammograms are given. In case of the existence of these effects, only the EIS can bring access to the thermodynamics and kinetics of the complexation chemical reaction. Analytical equations for estimating the thermodynamic parameters by such systems under EIS conditions are evaluated. The theoretical results are compared with the experimental results of the facilitated Cu2+ transfer at the polarized water(-)1,2-dichlorethane interface, assisted by two phenanthroline-containing macrocycles. In the experimental case where kinetic effects due to the complexation step exist, we show how elegantly EIS can be used as a too] for estimation of the complexation constant of Cu2+ and 5-oxo-2,8-dithia [9],(2,9)-1,10-phenanthrolinophane (PhenOS(2)).