Publications

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)).

Abstract In this work we report on the remarkable voltammetric features of graphite electrodes, which were modified with mercaptoundecanoic acid derivatized gold nanoparticles (Au-MUA NPs) by simple adsorption from basic aqueous solution. Atomic force microscopy measurements proved a fairly uniform adsorption of the nanoparticles in the form of clusters, and consecutive island formation. The electrochemical features of the modified electrodes were probed by cyclic voltammetry, while using various redox probes in several different setups. The catalytic effects of the adsorbed clusters on the graphite electrodes proved to be highly reproducible, time dependent, and of nonselective nature. The main advantages of the proposed methodology are seen by the simplicity of the modification procedure, the stability of the self-assembled gold nanoparticle film, their applicability in various voltammetric scenarios, and the potential employment of the Au-MUA NP modified electrodes as sensors for various systems.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of combustion and sublimation of 3,4,5-trimethoxyphenol were measured, respectively, by static bomb combustion calorimetry in oxygen atmosphere and by Calvet microcalorimetry. From these measurements, the standard molar enthalpy of formation in both the crystalline and gaseous phase, at T = 298.15 K, were derived: -(643.4 +/- 1.9) kJ . mol(-1) and -(518.1 +/- 3.6) kJ . mol(-1), respectively. Density functional theory calculations for this compound and respective phenoxyl radical and phenoxide anion were also performed using the B3LYP functional and extended basis sets, which allowed the theoretical estimation of the gaseous phase standard molar enthalpy of formation through the use of isodesmic reactions and the calculation of the homolytic and heterolytic O-H bond dissociation energies. There is good agreement between the calculated and experimental enthalpy of formation. Substituent effects on the homolytic and heterolytic O-H bond dissociation energies have been analysed.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpy of formation P-tetralone was measured, at T = 298.15 K, by static bomb calorimetry and the standard molar enthalpy of vaporization, at T = 298.15 K, was obtained using Calvet microcalorimetry. These values were used to derive the standard molar enthalpy of formation of the compound in the gaseous phase, at T = 298.15 K, -(75.2 +/- 2.5) kJ . mol(-1). Additionally, high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy function with extended basis sets and more accurate correlated computational techniques of the MCCM/3 suite have been performed.

Abstract Citrate gold nanoparticles of 13.5 nm. average diameters have been synthesized by Turkevich method and further functionalized by place exchange reaction using mercaptosuccinic acid (MSA), 1,4-benezenedimethanethiol (1,4BDMT) or mercaptoundecanoic acid (MUA) as the incoming ligands. In the first case, the functionalization of the gold clusters with MSA yielded solution of blue color and large aggregates on carbon grids. Moreover, after filtration and drying, any film formed by hydrogen bonding, could not be removed from the filter, being embedded into the filter texture. The second ligand used for the place exchange reaction, the 1,4BDMT, determined the change of the solution color from red to dark purple, and generation of a purple powder but no film formation. In the third case, the purification and drying of the MUA functionalized gold clusters yielded films of purple color with homogenous composition, robustness, soft topographies and remarkable optical properties which were dependent on their thicknesses. The Au-MUA NP films obtained are new, being generated by hydrogen bonding and having features which makes them very attractive for the fabrication of optoelectronic devices. The long alkyl chained thiol proved to be the adequate ligand for film synthesis.

Abstract A new bioluminescent method for coenzyme A (CoA) quantification is described. It is based on the enzymatic conversion of dehydroluciferyl-adenylate (L-AMP) into dehydroluciferyl-coenzyme A (L-CoA) by firefly luciferase (E.C. 1.13.12.7) (LUC), which causes a flash of light that can be measured in a luminometer. The method was subjected to optimization using experimental design methodologies to obtain optimum values for the concentrations of L-AMP ([L-AMP]), luciferase ([LUC]), ATP ([ATP]) and luciferin ([LH(2)]). This method has a linear response over the range of 0.25-4 mu M of CoA, with a limit of detection (LOD) of 0.24 mu M and a limit of quantification (LOQ) of 0.80 mu M. The assay has a relative standard deviation of about 7%. By coupling this optimized procedure to bioluminescent detection, a sensible and robust method can be obtained for the analysis of CoA.

Abstract An estimate of the digestibility and assimilability of butyltins occurring in contaminated wines (Port, red and white) was obtained by means of in vitro studies of gastrointestinal digestion. The influence of the wine matrix on the intestinal permeability was explored by studying the accumulation of butyltins in Caco-2 monolayers either when these species are dissolved in buffer only or in the dialysates of digested wines. Some important information about the fate of the butyltin compounds ingested from contaminated wines could be achieved. Only a very small fraction of the ingested DBT and TBT, the two most toxic forms, appear to be able to reach the epithelium as judged by the small dialyzable fraction found (<2%). This is probably independent from the food/drink matrix introducing these contaminants, since the influence of the involved enzymes appear to be dominant, especially for DBT and TBT. Additionally, the intestinal permeability of the three butyltins was also very low, the wine matrix possibly having a hindrance effect in a few cases.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpy of formation of 1,2,4,5-benzenetetracarboxylic dianhydride in the gaseous phase, -826.8 +/- 3.1 kJ mol(-1), was derived from the standard molar enthalpy of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry and the standard molar enthalpy of sublimation, at T = 298.15 K, measured by Calvet microcalorimetry. In addition, density functional theory calculations have been performed with the B3LYP, MPW1B95, and B3PW91 density functionals and the cc-pVTZ basis set for 1,2,4,5-benzenetetracarboxylic dianhydride and 1,2,4,5-benzenetetracarboxylic diimide. Nucleus-independent chemical shifts calculations show that the aromaticity is restricted to the benzenic ring in both compounds even though they are formally 10 pi polynuclear species.