Sodium-23, cesium-133 and thallium-205 NMR study of sodium,cesium and thallium complexes with large crown ethers in nonaqueous solutions

Formation constants for complexes of dibenzo-21-crown-7, dibenzo-24-crown-8 and dibenzo-27-crown-9 with the Na⁺, Cs⁺ and Tl⁺ ions have been determined by multinuclear NMR measurements in several nonaqueous solvents. The measurements of the cesium complexes were carried out at different temperatures and the enthalpy and entropy of complexation were determined from the temperature dependence of the formation constants.The stabilities of these complexes in solvents of low to medium donicity, —nitromethane, acetonitrile, acetone, methanol, and propylene carbonate, vary in the order Tl⁺>Cs⁺>Na⁺. Depending on the relative sizes of the cation and of the ligand cavity, either a three-dimensional wrap-around complex or a two-dimensional crown complex are formed. For the cesium complexes, the values of H _c ^o and S _c ^o are definitely solvent-dependent and in all cases the complexes are enthalpy stabilized but entropy destabilized. A compensating effect is observed in the variation of the enthalpy and entropy of complexation so that the overall influence of the above solvents on the stability of the complexes is rather limited.     

Complex Formation of Some Anilinium Ion Derivatives with 18-Crown-6, 1,10-diaza-18-crown-6 and Cryptand C222 in Acetonitrile, Dimethylformamide and their 1 : 1 Mixture

The complexation reactions between the protonated salts of aniline, o-hydroxy aniline, o-amino aniline and 2,3-benzo aniline (-naphthylamine) and macrocyclic ligands 18-crown-6,1,10-diaza-18-crown-6 and cryptand C222 have been studied conductometrically in acetonitrile, dimethylformamide and their 1 : 1 (mol–mol) mixture at 25 °C. Formation constants of the resulting 1 : 1 complexes were determined from the computer fitting of the molar conductance-mole ratio data. In all cases studied, the stability of the complexes decreases in the order C222 > 1,10-diaza-18-crown-6 > 18-crown-6. There is also an inverse relationship between the stabilities of the complexes and the Gutmann donor number of the solvents. It was found that, in the aromatic anilinium series used, increasing the bulkiness of the organic substituent in the ortho position results in a loss of complex stability.     

Selective determination of trace copper(II) by cathodic adsorptive stripping voltammetry with a naphthol-derivative Schiff's base

A selective and sensitive stripping voltammetric method for the determination of trace amounts of copper(II) with a recently synthesized naphthol-derivative Schiff's base (2,2'-[1,2-ethanediylbis(nitriloethylidyne)]bis(1-naphthalene)) is presented. The method is based on adsorptive accumulation of the resulting copper-Schiff's base complex on a hanging mercury drop electrode, followed by the stripping voltammetric measurement at the reduction current of adsorbed complex at -0.15 V (vs. Ag/AgCl). The optimal conditions for the stripping analysis of copper include pH 5.5 to 6.5, 8 microM Schiff's base and an accumulation potential of -0.05 V (vs. Ag/AgCI). The peak current is linearly proportional to the copper concentration over a range 2.3-50.8 ng ml(-1) with a limit of detection of 1.9 ng ml(-1). The accumulation time and RSD are 90 s and (3.2-3.5)%, respectively. The method was applied to the determination of copper in some analytical grade salts, tap water, human serum and sheep's liver.     

Effects of ion-carrier substituents on the potentiometric-response characteristics in anion-selective membrane electrodes based on iron porphyrins.

The potentiometric response characteristics with respect to salicylate anion of several membrane electrodes based on iron(III) tetraphenylporphyrin chloride (FeTPPCl) and derivatives with electrophilic and nucleophilic substituents, incorporated into plasticized polyvinylchloride (PVC) membranes were investigated. Complexes tetraphenyl porphyrin iron(III) chloride (FeTPPCl; A), tetrakis (4-methoxyphenyl) porphyrin iron(III) chloride (Fe(TOCH3PP)Cl; B), tetrakis (2,6-dichlorophenyl) porphyrin iron(III) chloride (Fe(TDClPP)Cl; C), tetrakis (4-nitrophenyl) porphyrin iron(III) chloride (Fe(TNO2PP)Cl; D), and tetrakis (pentafluorophenyl) porphyrin iron(III) chloride (Fe(TPFPP)Cl; E) were used as anion carriers in the membrane electrodes. The sensitivity, working range, detection limit, response mechanism, and selectivity of the membrane sensor toward interference shows a considerable dependence on the type of carrier substituent and the pH value of the sample solution. Potentiometric investigations in solutions of various pH show that the carrier complex containing fluoro substituents (E), which have very strong electron-accepting properties and a high ability to form hydrogen bonds, is capable of serving as a positively charged ionophore. Some other ionophores are capable of serving as both charged and neutral carriers under different conditions. The electrodes prepared in this work show super-Nernstian slopes with respect to salicylate concentration, which tend to a Nernstian response (slope near to -59 mV decade-1) upon an increase of the pH of the test solution. The results of UV/Vis absorption spectroscopy are used for interpretation of the formation of an oxene complex between salicylate and iron porphyrins.     

Nuclear magnetic resonance study of the ligand interchange of Ba2+-18-crown-6 complex in methanol solution

Exchange kinetics of Ba²⁺-18-crown-6 complex in deuterated methanol solution was studied by proton NMR line-shape analysis of a series of solutions containing equal population of free and complexed 18-crown-6, but varying concentration of the macrocycle, at various temperatures. From –33 to 37°C, the predominant mechanism for the exchange of the ligand between the two sites is a bimolecular pathway which is characterized by the following activation parameters:E _a=47±2 kJ-mol^–1; H =45±2 kJ-mol^–1; S =–8±4 J-mol^–1-K^–1. However, the contribution of a dissociative mechanism with activation parametersE _a=36±5 kJ-mol^–1, H =33±5 kJ-mol^–1 and S =104±18 J-mol^–1-K^–1 becomes more important at higher temperatures.     

Conductance Study of the Thermodynamics of Binding of Some Macrocyclic Polyethers with Tl+ Ion in Dimethylformamide-Acetonitrile Mixtures

The complexation reactions between Tl⁺ ion and dibenzo-30-crown-10 (DB30C10), dibenzo-24-crown-8 (DB24C8), dibenzo-21-crown-7 (DB21C7), and aza-18-crown-6 (A18C6) were studied in different dimethylformamide-acetonitrile mixtures at various temperatures. The formation constants of the resulting 1 : 1 complexes were determined from the molar conductance-mole ratio data and found to vary in the order A18C6 > DB30C10 > DB21C7 > DB24C8. The enthalpy and entropy of complexation were determined from the temperature dependence of the formation constants.     

Interaction of Iodine with Hexaaza-18-crown-6 and Tetraaza-14-crown-4 in Chloroform Solution

Interactions of hexaaza-18-crown-6 (HA18C6) and tetraaza-14-crown-4 (TA14C4) with iodine have been investigated spectrophotometrically in chloroform solution. The observed time dependence of the charge-transfer band and subsequent formation of I₃ ^- in solution were related to the slow transformation of the initially formed 1:1 macrocycle. I₂ outer complex to an inner electron donor-acceptor (EDA) complex, followed by fast reaction of the inner complex with iodine to form a triiodide ion, as follows: macrocycle + I₂→_fast ^K _f macrocycle.I₂ (outer complex) macrocycle.I₂ (outer complex) →^slow (macrocycle.I⁺)I^- (inner complex) macrocycle.I⁺)I^- (inner complex) + I²→^slow (macrocycle.I⁺)I₃ ^-. The pseudo-first-order rate constants at various temperatures for thetransformation process were evaluated from the absorbance-time data. The activation parameters (E_a, Δ H^?, and Δ S^?) for thetransformation were obtained from the temperature dependence of the rate constants. The stoichiometry and formation constants of the resulting EDA complexes have also been determined. It was found that the (TA14C4.I⁺)I₃ ^- is more stable the (HA18C6.I⁺)I₃ ^- complex in chloroform solution.     

Biodegradable Cell-Seeded Collagen Based Polymer Scaffolds for Wound Healing and Skin Reconstruction

In the present study a collagen-glycosaminoglycans scaffold, following chemical modifications, was prepared as an analog of the extracellular matrix (ECM) by a freeze drying method. Appropriate tests related to its use for wound healing (evaluation of the structure by scanning electron microscopy, swelling properties, biodegradability properties, Fourier transform infrared spectroscopy and cell culture) were done on the scaffolds. Amniotic fluid-derived stem (AFS) cell seeded scaffolds were shown to be potentially useful for wound healing.     

8-Hydroxyquinoline Functionalized Graphene Oxide: an Efficient Fluorescent Nanosensor for Zn<Superscript>2+</Superscript> in Aqueous Media

A nanosensor, based on 8-hydroxyquinoline functionalized graphene oxide, was developed for the fluorescence detection of Zn²⁺. It showed high selectivity and sensitivity for Zn²⁺ion in aqueous solution over other metal ions such as Li⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺, Cd²⁺, Co²⁺, Cu²⁺, Hg²⁺, Ni²⁺, Pb²⁺, Fe²⁺, Fe³⁺and Cr³⁺. Due to the linearity of the emission intensity toward Zn²⁺ concentration, fluorescent technique could be used for the detection of Zn²⁺ ion even at very low concentrations.     

The influence of surface effects on Frederiks transition in nematic liquid crystal doped with ferroelectric nanoparticles

Motivated by our recent work,in this work,we present the numerical study of the anchoring effect on the Frederiks threshold field in a nematic liquid crystal doped with ferroelectric colloidal nanoparticles.Assuming weak anchoring conditions,we employ the relaxation method and Maxwell construction to numerically solve the Euler–Lagrangian differential equation for the total free energy together the Rapini–Papoular surface energy to take into account anchoring of nematic liquid crystal molecules at the substrates.In this study,we focus our attention on obtaining the phase diagrams of Frederiks transition for different values of anchoring strength which have been not computed in our previous work.In this way,the effect of nanoparticle radius,nanoparticle volume fraction,nanoparticle polarization,and cell thickness on the Frederiks transition for different values of anchoring conditions are summarized in the phase diagrams.The numerical results show that by increasing the nanoparticles size and nanoparticle volume fraction in the ferronematic system,the Frederiks threshold field is strongly reduced.