Saturnino Ibeas

Alkali-metal ion catalysis of the oxidation of L-ascorbic acid by hexacyanoferrate(III) in strongly acidic media

The kinetics of the oxidation of L-ascorbic acid by hexacyanoferrate(III) ions have been investigated in strongly acidic media over a wide range of acidity. Two different mechanisms involving the monoascorbate anion, AH–, the neutral molecule of ascorbic acid AH2, and the protonated species AH+3, as well as the hexacyanoferrate(III) ion Fe(CN)3–6, and the monoprotonated form HFe(CN)2–6 are suggested. Kinetic evidence for a specific catalytic effct by binding of alkali-metal ions to oxidant is reported. The derived rate equations are in good agreement with the experimental observations.

Solvent effects on the thermodynamics and kinetics of coralyne self-aggregation.

The role of solvent effects on the thermodynamics and kinetics of the coralyne self-aggregation process has been investigated in ethanol-water mixtures of different compositions. The changes in the UV/visible spectra of coralyne and FAB/LSIMS mass spectrometry agreed well with the formation of a dimer species. 1D and 2D 1H experiments have allowed one to look into the features of the self-aggregation process and to determine the equilibrium constant and the deltaH0 and deltaS0 values for the aggregate formation in 0-50% ethanol-water mixtures. The kinetics of self-aggregation has been investigated by the T-jump chemical relaxation method, and the results have been interpreted in terms of dimer formation. The dependence of the relative viscosity of coralyne solutions on the dye concentration was studied in different ethanol-water mixtures. Finally, it was found that coralyne behaves as a solvatochromic indicator which is preferentially solvated according to the sequence ethanol > ethanol-water > water. All of the results concur in elucidating the relevant role of the hydrophobic interaction process of coralyne stack formation.

Shear viscosities of binary mixtures of pyrrolidin-2-one with C6–C10n-alkan-1-ols

Excess viscosities and excess Gibbs energies of activation for viscous flow at five temperatures have been calculated for binary liquid mixtures of pyrrolidin-2-one with (C6–C10) alkan-1-ols from the experimental densities and viscosities of the mixed fluids. The excess viscosities and excess Gibbs energies of activation were negative, and increased with increasing temperature. The fitting of viscosities to the one-parameter (d) Nissan–Grunberg equation, and the two-parameter (ν12 and ν21) McAllister equation was extended to the C1–C5 alkanols using earlier results, and examined in terms of interactions between unlike molecules. The ν12 and ν21 parameters were positive and increased with increasing temperature. A minimum in the excess Gibbs energies and d values at equimolar concentrations of the two components has been observed for the pyrrolidin-2-one–pentan-1-ol system.

ZWITTERIONIC PYRIDINECARBOXYLIC ACIDS

A study of the acid-base behaviour of the three isomeric pyridinecarboxylic acids (picolinic, nicotinic and isonicotinic acid) was carried out using spectrophotometric and potentiometric measurements. The cationic form of picolinic acid converts partially into the corresponding zwitterion (pK1) within a borderline acidity range where neither the pH scale nor the acidity functions work satisfactorily. Protonation of the carboxyl groups (pK33) occurred at the highest acidity levels employed. The medium effects observed on the spectral curves were corrected by factor analysis. The potentiometric measurements gave values for pK1 and pK2 only, which were in good agreement with those determined spectrophotometrically.

Forced Solid-State Interactions for the Selective “Turn-On” Fluorescence Sensing of Aluminum Ions in Water Using a Sensory Polymer Substrate

Selective and sensitive solid sensory substrates for detecting Al(III) in pure water are reported. The material is a flexible polymer film that can be handled and exhibits gel behavior and membrane performance. The film features a chemically anchored salicylaldehyde benzoylhydrazone derivative as an aluminum ion fluorescence sensor. A novel procedure for measuring Al(III) at the ppb level using a single solution drop in 20 min was developed. In this procedure, a drop was allowed to enter the hydrophilic material for 15 min before a 5 min drying period. The process forced the Al(III) to interact with the sensory motifs within the membrane before measuring the fluorescence of the system. The limit of detection of Al(III) was 22 ppm. Furthermore, a water-soluble sensory polymer containing the same sensory motifs was developed with a limit of detection of Al(III) of 1.5 ppb, which was significantly lower than the Environmental Protection Agency recommendations for drinking water.

Theoretical and Experimental Study of the Acetohydroxamic Acid Protonation: The Solvent Effect

The mechanism of the protonation of acetohydroxamic acid is investigated comparing experimental results and ab initio calculations. Experimentally, the UV spectral curves were recorded at different temperatures, at constant dioxane/water concentration, and at very high concentrations of strong mineral acids. The process is followed by monitoring the changes in the UV curves with increasing acid concentration. The molecular structures and the solvation energies were calculated with the RHF, B3LYP, and MP2 methods. The solvent is treated as a continuum of uniform dielectric constant. The isolated molecule of acetohydroxamic acid exhibits two protonation sites, the carbonyl oxygen and the nitrogen atom. In dioxane/water mixture, the RHF calculations predict the existence of a third cation of low stability, where the proton is bonded to the OH oxygen. With the MP2 ab initio calculations, the free energies of the formation processes in solution of the two most stable cations, CH3COH-NHOH+ (O3H+) and CH3CO-NH2OH+ have been evaluated to be -160.2 kcalmol(-1) and -157.6 kcal mol(-1). The carbonyl site is the most active center in solution and in the gas phase. The carbonyl site is also the most active center in the UV measurements. Experimentally, the ionization constant was found to be pK(O3H+) = -2.21 at 298.15 K, after the elimination of the medium effects using the Cox-Yates equation for hight acidity levels. Experiments and ab initio calculations indicate that K(O3H+) decreases with the temperature.

An organic/inorganic hybrid membrane as a solid "turn-on" fluorescent chemosensor for coenzyme A (CoA), cysteine (Cys), and glutathione (GSH) in aqueous media.

The preparation of a fluorogenic sensory material for the detection of biomolecules is described. Strategic functionalisation and copolymerisation of a water insoluble organic sensory molecule with hydrophilic comonomers yielded a crosslinked, water-swellable, easy-to-manipulate solid system for water “dip-in” fluorogenic coenzyme A, cysteine, and glutathione detection by means of host-guest interactions. The sensory material was a membrane with gel-like behaviour, which exhibits a change in fluorescence behaviour upon swelling with a water solution of the target molecules. The membrane follows a “turn-on” pattern, which permits the titration of the abovementioned biomolecules. In this way, the water insoluble sensing motif can be exploited in aqueous media. The sensory motif within the membrane is a chemically anchored piperazinedione-derivative with a weakly bound Hg(II). The response is caused by the displacement of the cation from the membrane due to a stronger complexation with the biomolecules, thus releasing the fluorescent sensory moieties within the membrane.

Aromatic polyamides and acrylic polymers as solid sensory materials and smart coated fibres for high acidity colorimetric sensing

Reusable colorimetric acid responsive coated fibres and manageable films or membranes have been successfully designed and prepared herein. The design of the materials rely on the preparation of condensation and addition monomers both having the azobenzene group, which is used as a dye moiety and as a weak basic motif, and a N,N-dimethylamino moiety. The N,N-dimethylamino moiety is also used as a weak, albeit stronger, basic group as well as an electron donor or electron withdrawal group, depending on its protonation state. For the sake of applicability, the coated fibres were cotton commodity fabrics, and high-tech aromatic polyamide yarns and fabrics. The high-performance aromatic polyamides and the versatile acrylic structures, along with the pendant weak basic groups, with pKas in water ranging from 1.78 to −0.5 and in air from −1.5 to −3.9, provide the materials with a colorimetric sensing capability over a wide acidity sensing window. This sensing window ranges from 1 × 10−2 to 3 M for perchloric acid in water and from 4 × 10−7 to 9 × 10−2 atm for vapour pressure of hydrogen chloride in air. The colour change of the sensory materials from yellow/blank to red or purple, which occurs upon contact with acidic media, was easily identified using the naked eye. Washing these materials with pure water recovered their original colour and permitted their reuse.

Heat Capacity Behavior and Structure of Alkan-1-ol/Alkylbenzoate Binary Solvents

Heat capacities for the binary mixtures of methanol with (C(1)-C(4)) alkylbenzoates and methylbenzoate with (C(1)-C(11)) alkan-1-ols have been measured over the whole composition range at 298.15 K under atmospheric pressure. From the experimental measurements, the derived excess molar heat capacities and partial excess molar heat capacities at infinite dilution have been calculated. A Redlich-Kister-type equation was fitted to these data, and the fitting parameters and standard deviations have been evaluated. Likewise, the IR spectra for the same systems have been recorded as a function of composition. The sets of experimental data gathered contribute to shed light onto the solvent structure and the underlying molecular interactions between the mixture constituents. The conclusions drawn have been established in terms of solute-solvent and solvent-solvent interactions and the ensuing structural effects between the solvent constituents.

Methacrylate copolymers with pendant piperazinedione-sensing motifs as fluorescent chemosensory materials for the detection of Cr(VI) in aqueous media

A fluorogenic sensory film, or dense membrane, capable of detecting Cr(VI), Fe(III), and Hg(II) in water was prepared. The film was prepared by a bulk radical polymerization of different comonomers, one of which contained a piperazinedione motif as sensory fluorophore. The film exhibited gel-like behavior and was highly tractable, even after being swollen in water. The sensing conditions were chosen to overcome interference from iron and mercury cations, giving rise to a material with a detection limit of 1 ppb for Cr(VI).