Contrasting Two Transformation-based Methods for Obtaining Absolute Extrema

In this note, we contrast two transformation-based methods to deduce absolute extrema and the corresponding extremizers. Unlike variation-based methods, the transformation-based methods of Carlson and Leitmann and the recent one of Silva and Torres are direct in that they permit obtaining solutions by inspection.     

Solution Chemistry of Arsenic Anions in the Presence of Metal Cations

The interaction of arsenic(V) and arsenic(III) oxyanions with metal cations was investigated by potentiometry under temperature and ionic strength conditions approaching those prevailing in natural waters. The selection includes the major metal cations and some other ions of high environmental relevance. Ionic pairs [M(As^VO₄)]^?, [M(HAs^VO₄)] and [M(H₂As^IIIO₃)]⁺ formation is suggested for all +2 metal cations, based on the potentiometric results. These ion-pairs between arsenic anions and other metal cations are hardly ever mentioned or taken into account when arsenic speciation in natural waters is considered. These results provide the basis for studying arsenic speciation in natural aquatic systems, on which environmental fate, bioavailability and toxicity of the element depend. Some extrapolations to the conditions of the natural waters are presented as well as some insights into the adsorption process onto hydrous oxides.     

Tandem Michael addition–Claisen-type condensation of anions of O-ethyl carbonates of cyanohydrins to cyclohex-2-en-1-one

A one pot method for the synthesis of ethyl 2-acetoxy-6-aroylcyclohex-1-ene-1-carboxylates and ethyl 2-acetoxy-6-heterocarbonylcyclohex-1-ene-1-carboxylates through Michael addition of the anions of ethyl carbonates of cyanohydrins to cyclohex-2-en-1-one and subsequent reaction with acetic anhydride is described. These compounds are potential intermediates for the synthesis of 9,10-anthraquinone and heterofused 1,4-naphthoquinone derivatives.     

Aggregation behavior of Brij-35/perfluorononanoic acid mixtures

The mixed system of a nonionic hydrocarbon surfactant, polyoxyethylene (23) lauryl ether (Brij-35), and a perfluorinated surfactant, perfluorononanoic acid, was investigated by a combination of methods. The critical micelle concentrations (cmc's) have been determined over a wide range of sample compositions by fluorescence and UV-visible spectrophotometry using pyrene and N-(4-nitrophenyl) perfluorononanamide, respectively, as molecular probes. The values of the cmc's obtained were considerably different with the two techniques employed. Measurements of the (19)F nuclear magnetic resonance chemical shift of the same mixtures showed two breaks in the plots of Δδ(f) versus molar fraction of the perfluorinated surfactant. Conductivity and surface tension measurements also showed two breaks. The behavior is attributed to the formation of mixed micelles that change their composition when the fraction of the fluorinated compound increases and some segregation of the fluorinated compound takes place at a high total surfactant concentration.     

The mobility in disordered molecular systems with energies given by a charge-induced dipoles interaction

We investigate the field dependence of the mobility in a model for a disordered molecular system containing spatial and energetic disorders. In this model we assign an isotropic polarizability to each site and take the site energies to be the site polarization energies, the interaction energy of a charge in the given site with the induced dipoles in the neighboring sites. This model was shown, in a previous publication, to contain short-ranged energetic correlations and we show in this work that this correlation produces a charge mobility proportional to the exponential of the square root of the applied field, the Poole-Frenkel dependence observed in various disordered organic materials, over a significant range of fields. We present an expression for the field dependence of the mobility in terms of the average intersite separation and of the isotropic polarizability of the electronic states, the two model parameters.     

Selenite and Selenate Speciation in Natural Waters: Interaction with Divalent Metal Ions

Selenium is a trace element of environmental relevance. Studies on its solution chemistry are scarce and were mostly carried out under experimental conditions of little relevance to environmental research. Thus, we have performed new studies of selenium speciation in solutions of low ionic strength, in contrast to those prevailing in the literature data. In this work, potentiometric titrations (at 20.0 °C, and I=0.15 mol⋅L⁻¹ NaClO₄) were carried out for systems containing Se(VI) or Se(IV) oxyanions and divalent metal ions (Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg, and Pb). Ion pairs such as [M(SeO₄)] and [M(HSeO₄)₂], or [M(HSeO₃)]⁺ and [M(SeO₃)], exist in solutions. The data reported here provide the basis for determining selenium speciation in natural aquatic systems, on which the bioavailability and toxicity of this element depends.