Publications

Abstract Computer simulations of salt solubility used a Java applet and Web browser to present an animated illustration of differences in the solubility of salts. The simulation provides an animated, visual interpretation of the different solubilities of related salts based on simple entropy changes associated with dissolution: configurational disorder and thermal disorder. Pairs of compounds chosen for the animation are NaCl/CaCO3 and MgCO3/MgSO4. In the former case, both dissolution are almost athermic, hence the major differences are in configurational disorder and solvation effects. In the latter pair, the major difference lies in thermal disorder, as the dissolution of magnesium sulfate is quite exothermic. Changes in total entropy and in its components are represented qualitatively in bar graphs.

Abstract Two-way data structures were obtained by acquiring UV-vis spectra as function of the time of the alkaline hydrolysis reaction of the antihypertensor Nifedipine in dimethylsulfoxide (DMSO). Sets of three-way data structures were obtained from the analysis of solutions with different concentrations of Nifedipine generated by standard additions to DMSO, Nifedipine standard and a pharmaceutical formulation. PARAFAC and PARAFAC2 methods were used in the analysis of these multi-way data structures and calibration models were developed for Nifedipine quantification in pharmaceutical formulations. For all the three-way data structures a better model fit was found with the PARAFAC2 suggesting that the experimental data sets have deviations from trilinearity. The best concentrations estimations were found with the PARAFAC2 model in the analysis of a [concentration x time (s) x wavelength (nm)] three-way data structure which allows the quantification of Nifedipine in pharmaceutical formulations.

Abstract (Figure Presented) Sharp practice: A water/n-octanol interface can be formed at the tip of a very short and sharp nanopipette, and can be polarized externally (see picture). This interface has been used to investigate various ion-transfer processes and evaluate partition coefficients for ionizable species. © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.

Abstract The coordination chemistry of 2,8-dithia[9],(2,9)-1,10-phenanthrolinophane (L) towards transition and post-transition metal ions such as Cu(II), Cu(I), Cd(II), Co(II), Ni(II), Pb(II) and Zn(II) was studied at the polarized water/1,2-dichloroethane interface by cyclic voltammetry. The dependence of the half-wave transfer potential on the ligand and metal concentrations was used to formulate the stoichiometry, and to evaluate the association constants of the complexes formed between L and Pb(II).

Abstract A catanionic surfactant consists of paired oppositely charged surfactant ions, acting as counterions to each other. In this work, a series of catanionic surfactants with the cationic quaternary ammonium group and anionic sulfate group, with variation of chain length and number of chains, have been synthesized. The results of the Langmuir pressure-area isotherms, for compression and cycles of compression-decompression, have been used to discuss the effect of length and number of hydrophobic chains on the phase behavior and molecular organization of the spread catanionic film. The molecular desorption from the catanionic film, involving either dissolution of the catanionic surfactant or desorption of the organic counterion (short chain ionic moiety), is critically discussed.

Abstract An experimental setup to study the pH dependence of standard ion-transfer potentials at the water/NPOE interface is presented. The system is composed of a microhole generated by laser photoablation in a 12-mu m polyethylene terephthalate film, the aqueous phase consisting of a commercial immobilized pH gradient gel reswelled in electrolyte solution and a droplet of organic solution. Two electrodes are used, an Ag/AgCl aqueous electrode and an Ag/AgTPBCl organic electrode. This setup is applied to the study of two ionizable compounds (pyridine, 2,4-dinitrophenol). Thermodynamic parameters such as the standard transfer potential, the Gibbs energy of transfer, and the partition coefficients of the ionized forms as well as the neutral forms of these drug compounds are evaluated by differential pulse voltammetry. The data obtained are summarized in ionic partition diagrams, which are a useful tool for predicting and interpreting the transfer behavior of ionized drugs at the liquid/liquid interfaces mimicking the biological membranes.

Abstract The pure organic liquids nitrobenzene (NB) and 2-nitrophenyl octyl ether (NPOE) have been studied by means of molecular dynamics simulations. Both solvents are extremely important in various interfacial processes, mainly connected with ion transfer taking place across the interface with water. Thermodynamic (mass density, enthalpy of vaporization, isothermal compressibility, dipole moment) and dynamic (viscosities and self-diffusion coefficients) properties of both liquids have been calculated and are in very good agreement with the experimental data. In the case of NB, several potentials have been tested and the obtained results compared and discussed. In most cases, the OPLS all-atom potential gives results that are in better agreement with available experimental values. Atomic radial distribution functions, dihedral and angle distributions, as well as dipole-orientation correlation functions are used to probe the structure and interactions of the bulk molecules of both organic solvents. These were seen to be very similar in terms of structure and thermodynamics, but quite distinct in terms of dynamic behavior, with NPOE showing a much slower dynamic response than NB. A simulation study of the simple Cl- and K+ ions dissolved in both solvents has been also undertaken, revealing details about the diffusion and solvation mechanisms of these ions. It was found that in both liquids the positive potassium ion is solvated by the negative end of the molecular dipole, whereas the negative chloride ion is solvated by the positive end of the dipole.

Abstract In this paper, results obtained from molecular dynamics (MD) simulations on the water-saturated organic liquids nitrobenzene (NB) and 2-nitrophenyloctyl ether (NPOE) are presented. Both solvents are extremely important in many areas of chemistry, and they are seen as the most promising successors of n-octanol for ion transfer studies at the liquid-liquid interface. Thermodynamic (mass density, enthalpy of vaporization), structural (radial distribution functions) and dynamic (diffusion coefficients) properties of both liquids have been calculated and compared with those obtained from MD simulations of the corresponding pure (water-free) organic liquids. The relatively polar nature of NB allows for an efficient incorporation of water into the organic liquid, minimizing the impact on the solvent structure and dynamics. NPOE exhibits added hydrophobicity due to the presence of along aliphatic chain, and this has an effect on its water solvation properties. Steric effects caused by these chains, together with the much slower dynamic response of the solvent, substantially hinder the motion of water and prevent interactions with other water molecules. The different behaviour of these solvents towards water provides a good means for studying the properties of these liquids at the interface with water.

Abstract Solid-phase microextraction commercial fibers present a few drawbacks such as relatively low recommended operating temperature, instability and swelling in organic solvents, breakage of the fiber, stripping of coatings, and bending of the needle. Some of these problems have been obviated by covalent bonding of the polymer phase to the fused-silica substrate by sol-gel, but the easy breakage of the fiber remains a problem. In the present work, the known occurrence of titanol groups at the surface of titanium wire was exploited to produce sol-gel fibers supported on this unbreakable substrate. Scanning electron microscopy analysis revealed the film formation on titanium wire surface while temperature and solvent stability as well as durability tests showed that the sol-gel film was tightly attached to the substrate, thus suggesting covalent bonding. The use of this type of fiber is currently generalized in our laboratory without any breakage or stripping out incidents up to the moment.

Abstract Synchronous fluorescence spectra (excitation wavelength range between 280 and 510 ran and wavelength interval of 25 nm) of three samples of fulvic acids (FA) were obtained as a function of the pH, in the range from 2.0 to 10.5, and as a function of the FA concentration, in the range from 20 to 180 mg/L. FA were obtained from composted livestock materials (IsFA), composted sewage sludge (csFA), and Laurentian soil (laFA). Three-dimensional spectral matrices were obtained (wavelength, pH and FA concentration) and multivariate curve resolution (MCR) was used to calculate spectra and fluorescence intensity profiles for the detected components. Cluster analysis of the calculated spectra showed the existence of similar and unique fluorescent properties in the three FA samples. Some of the calculated fluorescence intensity profiles have a shape compatible with acid-base species distribution diagrams, which allowed pKa values to be estimated, namely, a well-defined acid-base equilibrium with pKa 5.7 +/- 0.2 (IsFA), 6.9 +/- 0.4 (csFA), and 5.5 +/- 0.2 (laFA); and other acid-base systems not well defined with pKa at about 3.0 and 8.6. Other spectral variations revealed the existence of inner-filter effects or self-quenching as the concentration of FA increases.