Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Study of Ligand Substitution Reactions at Pentacyanoferrates(II) in Aqueous Salt and Micellar Solutions

The ligand substitution reactions Fe(CN)(5)(4-(t)bupy)(3-) + 4-CNpy and Fe(CN)(5)(4-(t)bupy)(3-) + pzCO(2)(-) (4-(t)Bupy = 4-tert-butylpyridine; 4-CNpy = 4-cyanopyridine; pzCO(2)(-) = pyrazinecarboxylate) were studied in several aqueous salt and micellar solutions. Kinetic data in aqueous solutions showed that the two processes follow a dissociative mechanism, D, and the dependence of the first-order rate constants on [salt] on electrolyte aqueous solutions allow the estimation of the activation volumes corresponding to both reactions. Under true first-order conditions no kinetic micellar effects were found in anionic (SDS) and nonionic (Triton X-100) aqueous micellar solutions. In cationic micellar solutions (CTAB, CTAC, and TTAB) small kinetic micellar effects were found. These were related to the different ionic concentrations and the different polarity and structure of the Stern layer surrounding the cationic micellar aggregates, where the reactions take place, with respect to pure water. Copyright 2000 Academic Press.     

Primary Transformation Rate Measurements Through Differential Scanning Calorimetry

The primary crystallization of molten alloy systems at high undercooling is studied by a precise quantitative analysis of the calorimetric signal obtained during the transformation in terms of the reaction rate under isothermal and continuous heating regimes. It is shown that, under specific conditions, namely, stoechiometric primary precipitates, generalized relationships for the crystallization enthalpy and the reaction rate may be obtained.     

Adsorptive stripping voltammetry of Tartrazine at the hanging mercury drop electrode in soft drinks

Adsorptive stripping voltammetry was used for the determination of trace amounts of the dye Tartrazine (E-102) by square-wave (SWS) and differential pulse techniques (DPS). Its adsorptive voltammetric behaviour was investigated at different pH media. NH₄Cl/NH₃ buffer solution was chosen as the most suitable, taking into account the sensitivity and definition of the reduction peaks obtained. The effects of the experimental parameters on the determination are discussed. Standard deviations of 3.3% and 2.6% were obtained by SWS and DPS for 100 and 50 μg/L Tartrazine solutions, respectively (n = 10). Both methods were applied to determine the dye in several commercial soft drinks, containing very small amounts of it. Measurements were made directly in the commercial samples. A comparison of the results obtained by the proposed voltammetric methods with those of an HPLC method was also made. Good correlations between the voltammetric results and the values supplied by the manufacturer were found, whereas recoveries of the same order of magnitude were obtained by the HPLC method. Received: 23 May 1996 / Revised: 5 July 1996 / Accepted: 10 July 1996     

In situ studies of methanol decomposition and oxidation on Pd(111) by PM-IRAS and XPS spectroscopy

Methanol decomposition and oxidation on Pd(111) at millibar pressure were studied by in situ polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), on-line gas chromatography and pre- and postreaction X-ray photoelectron spectroscopy (XPS). Various dehydrogenation products such as methoxy CH3O, formaldehyde CH2O, formyl CHO, and CO could be spectroscopically identified. Methanol oxidation proceeds via dehydrogenation to formaldehyde CH2O, which either desorbs or is further dehydrogenated to CO, which is subsequently oxidized to CO2. Carbonaceous overlayers that are present during the reaction may favorably affect the selectivity toward CH2O. The reaction takes place on metallic Pd, and no indications of an involvement of Pd surface oxide were observed.     

Reaction of methyl 4-nitrobenzenesulfonate with Br- in water-formamide tetradecyltrimethylammonium bromide micellar solutions

Summary The reaction methyl 4-nitrobenzenesulfonate + Br^- was studied in water-formamide tetradecyltrimethylammonium bromide micellar solutions. A pseudophase kinetic model was used to quantitatively rationalize the kinetic micellar effects observed     

Density and Sound Velocity Studies of Aqueous Solutions of Tetradecyltrimethylammonium Nitrate at Different Temperatures

The density and ultrasound velocity of aqueous solutions of tetradecyltrimethylammonium nitrate were measured in the temperature range of 15 to 35°C in 5°C intervals. The concentration range covered the premicellar and micellar regimes. By assuming a pseudophase separation model for the micellar system, we applied the densitometric data to estimate the apparent molar volumes and the apparent thermal expansibility coefficients of the surfactant in monomeric and micellar forms. Ultrasound velocity and density data enabled us to estimate the isentropic compressibility of the surfactant in both forms. The results are compared with relevant literature data for alkyltrimethylammonium bromides.     

The exo-selectivity of the new non-natural chiral auxiliary (+)-(1R,endo)-2-benzonorbornenol in an asymmetric aza-Diels-Alder reaction

The recently reported compound (+)-(1R,endo)-2-benzonorbornenol (2) proved to be an efficient chiral auxiliary in the asymmetric aza-Diels-Alder reaction between cyclopentadiene and the N-benzyl imine of its glyoxylate, which afforded a virtually all-exo mixture of cycloaducts with a 1S:1R diastereomeric ratio of 63:37.     

Resolution of benzo[a]pyrene in complex mixtures of other polycyclic aromatic hydrocarbons. Comparison of two spectrofluorimetric methods applied to water samples

Two spectrofluorimetric methods, second-derivative constant-energy synchronous luminescence (SDCESL) and constant-wavelength synchronous luminescence (CWSL) in combination with multiple linear regression (MLR), for the quantification of benzo[a]pyrene (BaP) at sub-ng mL-1 levels, in the presence of benzo[b]fluoranthene (BbFt), benzo[k]fluoranthene (BkFt), benzo[ghi]perylene (BghiP) and indeno[1,2,3-cd]pyrene (IP), were developed and compared in detail. SDCESL presents lower limits of detection and quantification than CWSL/MLR and also gives more exact and precise results for levels close to the quantification limit. For BaP, SDCESL achieved quantification limits of 0.019 ng mL-1 in river waters and 0.007 ng mL-1 in drinking waters. This work offers a sensitive, precise, accurate, rapid, simple and economic methodology for monitoring BaP in waters for public consumption, meeting all the requirements of the EC Directive 98/83/CE that fixes the maximum admissible limit for this polycyclic aromatic hydrocarbon in drinking waters at of 0.010 ng mL-1.