Search

[EN] The use of EDTA as a complexing agent in electrodialysis has gained increasing attention in several industrial applications. Variations in pH at the membrane surface are known to alter the chemical equilibrium of EDTA species during electrodialysis. In this work, we investigate the simultaneous transport of EDTA species and hydroxyl ions through an anion-exchange membrane under alkaline conditions, and the influence of their relative concentrations on relevant membrane transport properties. Chronopotentiometric and current-voltage curves were obtained using solutions at different pH and electrolyte concentrations. Intense oscillations in membrane potential drop and a reduction in membrane resistance over time were verified for the solution at pH 11, where EDTA(4-) and OH- ions presented similar molar concentrations. While the intense oscillations can be associated with the onset of electroconvection, the reduction in membrane potential drop can be explained based on the alteration of the chemical equilibrium in the system. At an advanced stage of concentration polarization, the increased exclusion of protons in the membrane phase induces an equilibrium shift and the dissociation of HEDTA(3-) into EDTA(4-) and H+ ions. The formed EDTA(4-) ions migrate to the enriched solution, while protons are released back to the diluting diffusion boundary layer, increasing the conductivity in this region. The equilibrium shift in the membrane/electrolyte system and intense electroconvection at the membrane surface were also noted with the solution at pH 12. When the concentrations of OH- and EDTA species are similar, the equilibrium shift and the occurrence of overlimiting phenomena become more prominent.

[EN] The use of EDTA as a complexing agent in electrodialysis has gained increasing attention in several industrial applications. Variations in pH at the membrane surface are known to alter the chemical equilibrium of EDTA species during electrodialysis. In this work, we investigate the simultaneous transport of EDTA species and hydroxyl ions through an anion-exchange membrane under alkaline conditions, and the influence of their relative concentrations on relevant membrane transport properties. Chronopotentiometric and current-voltage curves were obtained using solutions at different pH and electrolyte concentrations. Intense oscillations in membrane potential drop and a reduction in membrane resistance over time were verified for the solution at pH 11, where EDTA(4-) and OH- ions presented similar molar concentrations. While the intense oscillations can be associated with the onset of electroconvection, the reduction in membrane potential drop can be explained based on the alteration of the chemical equilibrium in the system. At an advanced stage of concentration polarization, the increased exclusion of protons in the membrane phase induces an equilibrium shift and the dissociation of HEDTA(3-) into EDTA(4-) and H+ ions. The formed EDTA(4-) ions migrate to the enriched solution, while protons are released back to the diluting diffusion boundary layer, increasing the conductivity in this region. The equilibrium shift in the membrane/electrolyte system and intense electroconvection at the membrane surface were also noted with the solution at pH 12. When the concentrations of OH- and EDTA species are similar, the equilibrium shift and the occurrence of overlimiting phenomena become more prominent.

Cyclic secondary amines and 2-hydroxybenzaldehydes or related ketones react to furnish benzo[e][1,3]oxazine structures in generally good yields. This overall redox-neutral amine α-C-H functionalization features a combined reductive N-alkylation/oxidative α-functionalization and is catalyzed by acetic acid. In contrast to previous reports, no external oxidants or metal catalysts are required. Reactions performed under modified conditions lead to an apparent reductive amination and the formation of o-hydroxybenzylamines in a process that involves the oxidation of a second equivalent of amine. A detailed computational study employing density functional theory compares different mechanistic pathways and is used to explain the observed experimental findings. Furthermore, these results also reveal the origin of the catalytic efficiency of acetic acid in these transformations. © 2014 American Chemical Society.

This article concerns the adsorption of the diblock polyampholyte poly(methacrylic acid)-block-poly((dimethylamino)ethyl metharylate) (PMAA-b-PDMAEMA) from aqueous solution on silicon substrates. The investigated polyampholyte is characterized by a small molecular weight around 15,000 g/mol and a big positively charged PDMAEMA block. The adsorbed amount determined by ellipsometry was strongly influenced by the pH of the adsorption solution. Using dynamic light scattering polyampholytic structures with diameters around 50 nm were found in aqueous solution. The hydrodynamic diameter was hardly affected by changing the pH of the polymer solution. Analogous regular structures were also found by scanning force microscopy (SFM) and grazing incidence, small angle X-ray scattering (GISAXS) at the silicon surface after the adsorption process. While SFM provides a topographical image of a small part of the adsorbed polyampholytic layer, GISAXS was used to get a statistical description of the lateral surface structures. The adsorbed structures were highly regular and their sizes were nearly pH independent over a large pH region. Only directly at the isoelectric point of the polyampholyte larger adsorbed structures were observed. Compared with earlier investigated PMAA-b-PDMAEMA systems we are now able to prepare highly regular polyampholytic structures at silicon surfaces. There are two kinds of interactions for the adsorbed micelles. First, the charged block of the chains is directly attracted to the substrate via electrostatic interactions, while the uncharged part of the chains is only hydrophobically attracted via the hydrophobic core of the adsorbed micelle. © 2001 Academic Press.

This article concerns the adsorption of the diblock polyampholyte poly(methacrylic acid)-block-poly((dimethylamino)ethyl metharylate) (PMAA-b-PDMAEMA) from aqueous solution on silicon substrates. The investigated polyampholyte is characterized by a small molecular weight around 15,000 g/mol and a big positively charged PDMAEMA block. The adsorbed amount determined by ellipsometry was strongly influenced by the pH of the adsorption solution. Using dynamic light scattering polyampholytic structures with diameters around 50 nm were found in aqueous solution. The hydrodynamic diameter was hardly affected by changing the pH of the polymer solution. Analogous regular structures were also found by scanning force microscopy (SFM) and grazing incidence, small angle X-ray scattering (GISAXS) at the silicon surface after the adsorption process. While SFM provides a topographical image of a small part of the adsorbed polyampholytic layer, GISAXS was used to get a statistical description of the lateral surface structures. The adsorbed structures were highly regular and their sizes were nearly pH independent over a large pH region. Only directly at the isoelectric point of the polyampholyte larger adsorbed structures were observed. Compared with earlier investigated PMAA-b-PDMAEMA systems we are now able to prepare highly regular polyampholytic structures at silicon surfaces. There are two kinds of interactions for the adsorbed micelles. First, the charged block of the chains is directly attracted to the substrate via electrostatic interactions, while the uncharged part of the chains is only hydrophobically attracted via the hydrophobic core of the adsorbed micelle. © 2001 Academic Press.

The adsorption of the diblock polyampholyte poly (methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate) from aqueous solution on silicon substrates was investigated as a function of polymer concentration and pH. Dynamic light scattering and electrokinetic measurements were used to characterize the polyampholyte in solution. The amount of polymer adsorbed was determined by ellipsometry and lateral structures of the polymer layer were investigated by scanning force microscopy. The amount of polymer adsorbed was found to be strongly influenced by the pH of the polymer solution, while the size of the polyampholyte micelles adsorbed on the surface was hardly affected by pH during adsorption. From investigations by scanning force microscopy well-seperated micelles were seen in the dried monolayers adsorbed directly from solution. The structures at the surface are correlated to structures in solution, and the adsorbed amount depends on the relative charge of the micelles and the surface.

2-Methyl-3-chloro-9-hydroxypyrido[1,2-a]pyrimidin-4-one (I) and bis(2-methyl-3-chloro-9-hydroxypyrido[1,2-a]pyrimidin-4-onium) perchlorate (Ia) are synthesized and studied by X-ray diffraction and IR spectroscopy. It is shown that compound I exists in the molecular form in crystals and in the zwitterion form in solutions. The structural units of compound Ia are partially protonated centrosymmetric dimeric cations and perchlorate anions. The dimeric cation involves two molecules I in the zwitterion form. These molecules are linked together by a strong symmetric hydrogen bond formed with the participation of a proton located at the inversion center. The structures of a series of pyrido[1,2-a]pyrimidines are investigated and compared. It is demonstrated that the tautomeric equilibrium revealed in solutions of compounds of this class depends on the solvent type and pH of the medium. © 2004 MAIK "Nauka/Interperiodica".