Thermal decomposition of basic cobalt and copper carbonates

Basic cobalt and copper carbonates were prepared by precipitation from solutions of their nitrates using KHCO₃ at room temperature in CO₂ atmosphere. The thermal decomposition of the prepared basic carbonates was studied by means of TG and DTA techniques and the phases produced were identified by XRD measurements. The products obtained at 400C were subjected to different doses of gamma-rays (40–160 M rad) and the thermal stabilities of these solids were investigated.The results obtained revealed that basic cobalt carbonate decomposed at 335C to produce Co₃O₄ which remained stable up to 850 and then decomposed above this temperature giving CoO which transformed into Co₃O₄ on cooling to room temperature. Basic copper carbonate dissociated at 290C yielding CuO which yielded Cu₂O and metallic copper at 1060 and 1150C, respectively. However, the produced cuprous oxide and metallic copper solids were converted into CuO and CU₂O, respectively by cooling in air to room temperature.Gamma-irradiation decreased the thermal stability of Co₃O₄ to an extent proportional to the dose employed. On the other hand, this treatment increased the thermal stability of both CuO and Cu₂O.     

Phase equilibria in polymer/solvent systems. Part V: Thermodynamic theory of the swelling pressure equilibrium of a crosslinked substance with a solvent in various phases

A thermodynamic theory has been developed to define the swelling pressure equilibrium between a homogeneous gel and a pure solvent, where phase transitions of the solvent, such as evaporation and crystallization can occur. It is shown that the equilibrium curve, which describes the temperature dependence of the composition in the gel phase under the condition of a constant swelling pressure, has distinct bends at the transition temperatures. These bends are related to the enthalpies of transition of the pure solvent at the transition temperatures. As a consequence of the phase transition of the solvent the swelling pressure-temperature curve at constant composition of the gel shows a discontinuous behavior at the transition point. Numerical calculations with a modified Flory-Huggins expression, based on results of swelling and deswelling measurements of the system crosslinked PEG/water, are presented.The discussion includes natural systems, which are in the gel state, where water may crystallize in the extracellular space.     

Correlation of the volumes and heat capacities of solutions with their solid-liquid phase diagrams

Liquid systems which have strong non-idealities, as seen from their thermodynamic properties, often show evidence of these interactions in the solid-liquid phase diagrams. This suggests that some of the structures present in the solid state can persist in the solution state, on a time average, up to temperatures much higher than the melting point. Volumes and heat capacities of typical systems were either taken from the literature or measured to illustrate this correlation with the phase diagrams. With mixtures of aprotic solvents which show nearly-ideal simple eutectic phase diagrams, the properties of the solutions are also nearly ideal. Examples of systems investigated which show strong non-idealities are ionic surfactant solutions, alcohol-water mixtures, chloroform-triethylamine mixtures and lithium salts in aprotic solvents.Paper written in the honor of Loren Hepler on the occasion of his retirement.     

Complex forming and extraction properties of oligo benzo-fused crown ethers

The complex forming properties with alkali metal and ammonium ions of a series of oligo benzo-condensed 18-crown-6 ethers1–8 having a different gradation of lipophilicity and of molecular rigidity are investigated by voltammetry at the interface of two immiscible electrolyte solutions (ITIES) and by a liquid-liquid extraction technique. The experimental results obtained in the two phase system H₂O/nitrobenzene are discussed in relation to the structure of the crown and the cation type. The stability constants for the 1 : 1 complexes of Na⁺, K⁺, Rb⁺, Cs⁺ and NH ₄ ⁺ in nitrobenzene have been determined and compared with the extraction constants for the 1: 1 complexes of Na⁺ and K⁺ and for the 1 : 1 and 1 : 2 complexes of Cs⁺, showing the effect of oligo benzo condensation for the 18-crown-6 system.     

Solid-phase Synthesis of-Haloaldehydes from Polymer-supported 4-(Phenylseleno)morpholine

Polymer-supported reagents for organic synthesis are enjoying a renewed popularity with the emergence of combinatorial chemistry in recent years1. 4-(Arylseleno)morpholines formed in situ are useful -seleno-introducing reagents for saturated aldehydes2. -Sele- noaldehydes, as a frequently used intermediates, can be converted into -haloaldehydes by halogenating reaction3. However, organic selenium reagents always have a foul smell and are quite toxic, which is often problematic in organic sy…     

Chiral separation of enantiomers of amino acid derivatives by high-performance liquid chromatography on a norvancomycin-bonded chiral stationary phase

A novel norvancomycin-bonded chiral stationary phase (NVC-CSP) was synthesized by using the chiral selector of norvancomycin. The chiral separation of enantiomers of several dansyl-amino acids by high-performance liquid chromatography (HPLC) in the reversed-phase mode is described. The effects of some parameters, such as organic modifier concentration, column temperature, pH and flow rate of the mobile phase, on the retention and enantioselectivity were investigated. The study showed that ionic, as well as hydrophobic interactions were engaged between the analyte and macrocycle in this chromatographic system. Increasing pH of buffers usually improved the chiral resolution for dansyl--amino-n-butyric acid (Dns-But), dansyl-methionine (Dns-Met) and dansyl-threonine (Dns-Thr), but not for dansyl-glutamic acid (Dns-Glu) which contains two carboxylic groups in its molecular structure. The natural logarithms of selectivity factors (ln ) of all the investigated compounds depended linearly on the reciprocal of temperature (1/T), most processes of enantioseparation were controlled enthalpically. Interestingly, the process of enantioseparaton for dansyl-threonine was enthalpy-controlled at pH of 3.5, while at pH of 7.0, it was entropy-controlled according to thermodynamic parameters Δ_R,SΔH° and Δ_R,SΔS° afforded by Van’t Hoff plots. In order to get baseline separation for all the solutes researched, norvancomycin was also used as a chiral mobile phase additive. In combination with the NVC-CSP, remarkable increases in enanselectivity were observed for all the compounds, as the result of a “synergistic” effect.     

Phase-transfer agents as catalysts for a nucleophilic substitution reaction in microemulsions

The reaction between 4-tert-butylbenzyl bromide and potassium iodide was carried out in microemulsions based on different nonionic surfactants, and the reaction rates were compared with those obtained in two-phase systems with added phase-transfer agent, either a quaternary ammonium salt or a crown ether. The reactions were relatively fast in the microemulsions and extremely sluggish in the two-phase systems without additional phase-transfer agent. Addition of a phase-transfer agent did not accelerate the reaction when a hydrocarbon was used as organic solvent, neither in the two-phase system nor in the microemulsion. When a chlorinated hydrocarbon was used as solvent, phase-transfer catalysis became effective and the rate obtained in the two-phase system with an equimolar amount of phase-transfer agent added was higher than that obtained in the microemulsion. When a catalytic amount of phase-transfer agent was used, the rate in the two-phase system was about the same as the rate obtained in the microemulsion without the phase-transfer agent. The combined approach, that is, use of a microemulsion as the reaction medium and addition of a phase-transfer agent, gave the highest reaction rate. The quaternary ammonium salt (tetrabutylammonium hydrogen sulfate) was a more efficient catalyst in the microemulsion system than the crown ether ([18]crown-6).     

Cellulose acetate and polyethersulfone blend ultrafiltration membranes. Part I: Preparation and characterizations

The overall objective of this investigation is to achieve high‐performance membranes with respect to flux and rejection characteristics, with an interplay of blending polymers having desired qualities. Thus, cellulose diacetate and polyethersulfone as candidate materials, in the presence of polyethylene glycol 600 as a pore forming agent, were blended in 100/0, 95/5, 90/10, 85/15, 80,20 and 75/25% compositions using N,N′‐dimethylformamide as solvent and membranes were prepared by the phase inversion technique. Polymer blend composition, additive concentration, and casting and gelation conditions were standardized for the preparation of asymmetric membranes with various pore statistics and morphology. These blend membranes were characterized for compaction in ultrafiltration experiments at 414 kPa pressure in order to attain steady state flux and is reached within 4–5 hr. The pure water flux was measured at 345 kPa pressure and is determined largely by the composition of polyethersulfone and additive concentration. The flux was found to reach the highest values of 66.5 and 275 1/(cm² hr) at 0 and 10 wt% additive concentrations respectively, at 25% SPS content of the blend. Membrane hydraulic resistance derived by measuring water flux at various transmembrane pressure and by using an algorithm was found to be inversely proportional to pure water flux. Water content is estimated by simple drying and weighing procedures and found proportional to pure water flux for all the membranes. The molecular weight cut‐offs (MWCOs) of different membranes were determined with proteins of different molecular weights and found to vary from 20–69 kDa (globular proteins) depending on the PEG and SPS content in the casting dope. Skin surface porosity of the membranes were analyzed by scanning the frozen membrane samples using scanning electron microscopy (SEM) at different magnifications. The surface porosity is in direct correlation to the MWCO derived from solute retention experiments. Copyright © 2004 John Wiley & Sons, Ltd.     

The role of palladium catalyst and base in stereoselective transformations of (E)-2-chlorovinylsulfides

Stereoselective transformations of 2-chlorovinylsulfides in the presence of soluble (t-BuOK) or insoluble (solid KOH or Cs₂CO₃/18-crown-6) base and palladium catalyst (dppb)Pd(OAc)₂ have been studied. Depending on the substrate or catalytic system, the reaction leads to the formation of (E)-1,2-bis[aryl(or arylmethyl)thio]ethenes and/or (E)-1,4-bis[aryl(or arylmethyl)thio]-1-buten-3-ynes in yields of up to 93%.     

高效液相色谱-串联质谱法结合多糖衍生物手性固定相拆分氰戊菊酯

建立了以多糖衍生物为手性固定相的高效液相色谱-串联质谱(HPLC-MS/MS)直接拆分氰戊菊酯对映体的方法。在反相液相色谱条件下,考察了手性固定相的种类、流动相组成、柱温、流速对氰戊菊酯4个立体异构体分离的影响。同时,利用热力学方法对氰戊菊酯的立体异构体与固定相之间的色谱保留和分离的热力学机理进行了探讨。结果表明:采用Lux Cellulose-3(纤维素-三(4-甲基苯甲酸酯))手性色谱柱,在以流动相为乙腈-水(5 mmol/L甲酸铵)=(55:45,V:V)流速0.4 mL/min,柱温30℃的条件下,可在14 mins内实现氰戊菊酯4个立体异构体的基线分离。拓展了HPLC-MS/MS在菊酯类手性农药对映体分离及检测上的应用。