The effect of layer charge and exchangeable cations on sorption of biphenyl on montmorillonites

Montmorillonies separated from the bentonites SAz-1 (Cheto, AZ, USA), and Cressfield (New South Wales, Australia) were used as starting materials. Reduced charge montmorillonites (RCMs) were prepared from these chemically different and Li-saturated montmorillonites via heating at temperatures in the range of 120–300°C. The residual exchangeable Li⁺ cations were then replaced with tetramethylammonium (TMA⁺) or hexadecyltrimethylammonium (HDTMA⁺) cations and the ability of the modified montmorillonites to adsorb biphenyl was investigated. Lower adsorption was observed for Li-montmorillonites than for the organoclays. The extent of adsorption was dependent on both the layer charge of montmorillonite and the size of alkylammonium cations. HDTMA-forms prepared from unheated Li-montmorillonites adsorbed biphenyl better than the organoclays prepared from RCMs. In contrast, the TMA-samples prepared from the Li-montmorillonites that were not heated showed low uptake of biphenyl probably due to high content of TMA⁺ cations. Reduction of the layer charge, resulting in lower content of TMA⁺ cations, increased sorption efficiency of both TMA-montmorillonites. The best adsorbents of biphenyl were HDTMA-SAz-1 prepared from the unheated Li-SAz-1 and TMA-Cressfield prepared from the Li-form heated at 180°C. These samples removed about 80% of biphenyl from its aqueous solutions      

Additivity model calculations of UHF spin densities and charge densities in methyl-substituted radical cations

By use of a heteroatom model for the methyl group and an additivity model for spin densities, the unrestricted Hartree-Fock after annihilation (UHFAA) results for the radical cations of naphthalene, 1-methylnaphthalene and 2-methylnaphthalene are used to predict the spin densities in the -electron approximation in the corresponding cations of di-, tri- and tetramethylnaphthalenes. The additivity model approach is shown to be equally successful for charge densities.     

Nucleogenic silylium cations and their analogs

Radiochemical studies of nucleogenic trivalent cations of the 14th group elements (silicon and germanium) generated by the β-decay of tritium atoms incorporated into organometallic molecules are reviewed. The radiochromatographic analysis of the neutral labeled products of reactions of these cations with various nucleophiles combined with quantum chemical data allowed us to obtain new information about the reactivity and rearrangements of silylium and germylium cations.     

Molecular complexes of crown ethers. Part 3. Effect of cations on charge transfer complexes with TCNE

The intermolecular charge transfer complexes (CT) of two crown ethers (CE), viz, B15C5 and DB18C6 (as donors), and tetracyanoethylene (TCNE), as acceptor, were studied in the UV-visible region in dichloroethane (DCE), at 298.2 K. The sequence of addition of the cation was varied in the case of B15C5 such that in one system the sequence was (CE+Cation)+TCNE and in the other (CE+TCNE)+cation. These two systems were found to be non-interchangeable, even under reflux conditions, giving differentK _c values which were explained as being due to the different geometries of the CE. For the first sequence, the values most affected depended on the fit of the metal cation with the ether cavity, thus in B15C5, Na⁺ showed the greatest effect, while for DB18C6 it was K⁺.     

Anion dependent molecular recognition of cations

In DMF-d₇ tetrabenzimidazole cavitands 2 exist as monomeric species and vase-like conformers. Several possible arrangements of the four benzimidazole NHs are indicated by ¹H NMR spectroscopy. The cavitands form 1:1 inclusion complexes with tetraethyl ammonium and phosphonium cations only when strong hydrogen bonding anions like chloride or acetate are present. These complexes are stable on the NMR time scale at 295 K feature a C_2V-symmetrical arrangement of benzimidazole functions. The stability of the C_2V-symmetrical tetramethylammonium acetate complex is independent of the temperature. In contrast, tetramethylammonium and phosphonium chloride complexes exist at 233 K as several isomers. This complicated behavior is, in part, attributed to the hydrogen bonding interactions between the anions and the NH groups of benzimidazole functions.     

Molecular electric field mapping of some anions and cations of 2- aminopurine and 6- thioguanine

Electric fields of the anions, cations and neutral forms of 2-aminopurine and 6-thioguanine have been mapped. Certain important features of the maps are similar to those found earlier in the neutral and ionic forms of adenine and guanine. The computed electric field patterns satisfactorily explain reactive sites and biological activity of the molecules.     

Performance of ion chromatography in the determination of anions and cations in various natural waters with elevated mineralization

The performance of ion chromatography in the determination of anions and cations in natural mineral waters of different composition and different total mineralization was evaluated. Up to 12 ions of the 20 usually included in extended chemical analysis of natural waters were successfully determined by ion chromatography alone. At least 98.60% and up to 99.96% of total cation composition of mineral waters was determined by ion chromatography. Hydrogen carbonate predominated in anion composition of mineral waters and was determined titrimetrically. The percentage of anions determined by ion chromatography in the remaining anion composition of mineral waters was between 98.90% and 99.96%. The agreement between total concentrations of anions and cations in individual mineral waters determined predominantly by ion chromatography is very good and the performance of ion chromatography for the basic and for the extended chemical analysis of highly mineralized water samples is very high. Method development was assisted by previously developed algorithms and appropriate experimental conditions are also discussed.     

Ion chromatographic determination of some anions and alkaline cations in liquid crystal materials after ultraviolet irradiation

A method was developed for the quantitative determination of inorganic cations and anions present in liquid crystal materials pretreated with ultraviolet irradiation by ion chromatography. The samples were irradiated at less than 20±2 °C and then pretreated with OnGuard RP and P cartridges to remove the interferential organic compounds prior to ion chromatographic analysis for both cations and anions. With a high capacity and very hydrophilic anion exchange column, four anions (fluoride, chloride, bromide, iodide) were determined in 20 min with gradient of sodium hydroxide. By employing a cation exchange column, five cations (sodium, ammonium, potassium, magnesium, calcium) could be determined in 12 min. The spiked recoveries for the cations and anions were 84.0–108.1% and 86.1–108.2%, respectively.     

Cyclic distiba- and dibismadiazenium cations

Think positive, keep the anions away: Hitherto unknown mono- and dications of the type [XE(μ-NTerE)](+) and [E(2) (μ-NTer)(2) ](2+) (E=Sb, Bi; X=Cl, I) were studied and fully characterized for the first time. Salts bearing these highly reactive cations can be obtained from terphenyl-substituted cyclo-dipnictadiazanes [XE(μ-NR)](2) by halide abstraction or triflate substitution.     

Simultaneous separation of inorganic anions and cations by capillary zone electrophoresis

A new capillary electrophoretic approach for simultaneous separation of fast anions and cations is demonstrated. Indirect UV detection at 214 nm in conjunction with electromigration sampling from both ends of the capillary was developed. Two electrolyte systems based on imidazole-nitrate and copper(II)-ethylenediamine-nitrate were investigated for the simultaneous separation of chloride, sulphate, hydrocarbonate, potassium, ammonium, calcium, sodium and magnesium ions. Experimental parameters that were evaluated included a nature of UV chromophore, pH of electrolyte, a nature of complexing agent. The method permits the excellent separation of three anions and five cations in only 4 min using electrolyte system containing 2.5 mmol l⁻¹ Cu(NO₃)₂, 5 mmol l⁻¹ ethylenediamine and 1 mmol l⁻¹ fumaric acid at pH 8.5 adjusted with tetraethylammonium hydroxide.     

Microsolvation of small cations and anions

Recent advances in the theoretical treatment of microsolvation of small ions, a problem with practical implications in chemistry, physics, and biology, are exposed. In particular, we discuss sound stochastic approaches to sample complex energy landscapes and delve into the nature of bonding interactions that dictate both structural and energetical preferences. An in-depth analysis of the effect of formal charges in the surrounding network of solvent to solvent hydrogen bonds is also presented. The problem, as expected, is more complicated than simple definitions may forecast.     

Synthesis and selectivity of sym-hydroxydibenzo-14crown-4 ionophores for protons,alkali metal cations,and alkaline earth cations in polymeric membranes

Six derivatives of sym-hydroxydibenzo-14-crown-4 have been prepared and incorporated into solvent-polymeric membranes. Responses of the membranes to protons, alkali metal cations, and alkaline earth cations have been determined. The preferred uptake of protons is attributed to proton complexation by stable crown ether alcohol and diol monohydrate species.     

Capillary electrochromatography with contactless conductivity detection for the determination of some inorganic and organic cations using monolithic octadecylsilica columns

A fast separation of alkali and alkaline earth metal cations and ammonium was carried out by capillary electrochromatography on monolithic octadecylsilica columns of 15 cm length and 100 μm inner diameter using water/methanol mixtures containing acetic acid as mobile phase. On-column contactless conductivity detection was used for quantification of these non-UV-absorbing species. The method was also extended successfully to the determination of small amines as well as of amino acids, and the separation selectivity was optimized by varying the composition of the mobile phase. Detection limits of about 1 μM were possible for the inorganic cations as well as for the small amines, while the amino acids could be quantified down to about 10 μM. The separation of 12 amino acids was achieved in the relatively short time of 10 min.     

Electronegativity force of cations and thermal decomposition of complex fluorides

The thermal decompositions of anhydrous fluorosilicates M₂SiF₆ (M=Li, Na, K, Rb, Cs) and MSiF₆ (M=Ca, Sr, Ba) were investigated. The decompositions proceeded according to a simple acid-base mechanism with evolution of SiF₄. The influence of the cationic counter-ion acidity (expressed by means of the electronegativity force) on the course of thermal decomposition was estimated quantitatively.     

Removal and regeneration of aqueous divalent cations by boehmite

Boehmite (Al(OH)O) was employed for the removal of aqueous Mg(2+), Cu(2+), Cd(2+), Pb(2+), and Co(2+) at 298 K. Although boehmite was able to remove these divalent cations, the greater removal rate with boehmite of Pb(2+) (28.7%) than with Mg(2+), Cu(2+), Cd(2+), and Co(2+) (5.6, 25.3, 10.9, and 13.3%, respectively) was observed under acidic conditions. Under stronger alkaline conditions, in which the lead species was completely dissolved, a greater removal rate of Pb(2+) (more than 80%) was observed under the corresponding conditions employed for the acidic conditions. The removed lead species could not be dissolved from boehmite in an acidic solution while an evident dissolution of lead species was detected using an aqueous NaOH solution. The results shown in the present study reveal that boehmite can be employed as a reagent for the removal and regeneration of aqueous metal cations.     

Quantitative determination of inorganic minor cations in sodium-, calcium-, magnesium-matrix simulated samples by capillary electrophoresis

The analysis of ammonium, alkali and alkaline-earth trace cations (0.5 ppm) in samples with a calcium, sodium or magnesium matrix (500 ppm) has been achieved using 10 mM imidazole (pH 4.5) electrolyte to which a complexing agent (15-crown-5, oxalic acid or dipicolinic acid) has been specifically added in order to decrease the electrophoretic mobility of the matrix cation and thus to allow the separation of higher mobility cations at sub-ppm concentrations. The influence of several experimental parameters (complexing agent concentration, buffer pH and temperature) have been studied in order to optimize the separation. The complexing agent concentration appears to be the main parameter governing the selectivity of the cations during the analysis of matrix samples. In optimized conditions, we have checked that the separation between minor inorganic cations is not significantly altered by an increase in the matrix cation concentration. As the concentration of the matrix cation increases, the migration times of minor cations remain unchanged even for a 1000 ppm concentration of the matrix cation. Finally, these optimized buffers allow the quantitation of minor cations down to 0.05% (w/w) for calcium- or magnesium- matrix simulated samples and 0.2% (w/w) for sodium-matrix simulated samples.