Jean-François Verchère

Molecular recognition of carbohydrates by a resorcinarene. Selective transport of alditols through a supported liquid membrane.

A supported liquid membrane (SLM) containing a resorcinarene carrier has been used for the selective transport of erythritol, threitol, ribitol and xylitol from concentrated (1.0-0.01 M) aqueous solutions. The membrane is made of a microporous polytetrafluoroethylene film impregnated with a 0.01 M solution of the carrier in CCl4. The permeabilities of the SLM for all alditols were calculated. On the basis of the flux dependence on the initial concentrations of carrier and alditol, the rate-determining step in the transport mechanism is shown to be the migration of the 1:1 carrier-carbohydrate complex in the immobilized organic phase. The flux of sugar is related to the initial concentration of alditol in the feed phase by a saturation law, which allowed the determination of the apparent diffusion coefficients and the stability constants of the resorcinarene complexes of alditols formed in the liquid membrane.

Novel Metal-Complexing Membrane Containing Poly(4-vinylpyridine) for Removal of Hg(II) from Aqueous Solution

A novel poly(vinyl alcohol)/poly(4-vinylpyridine) (PVA/P(4)VP) complexing membrane for removal of Hg(II) ions from aqueous solutions represents a significant improvement over a previously reported PVA/poly(ethylenimine) (PEI) membrane. This membrane was prepared by the semi-interpenetrating polymer network technique, and its cross-linking by three different agents was studied. The best results were obtained with gaseous 1,2-dibromoethane at 140 degrees C for 1 h that gave a membrane with a swelling ratio of 0.66. The sorption reaction of Hg(II) followed a first-order rate law, and the rate-limiting step was shown to be the association of Hg(II) ions with the complexing sites of P(4)VP. Sorption experiments at pH 2.5 showed that the retention ratio could reach 100% under optimized conditions for the initial concentration of Hg(II) and mass of membrane, c(0) = 100 mg L(-1) and m(D) = 100 mg, respectively. The retention ratio was remarkably insensitive to water hardness or the presence of NaCl, suggesting possible use for the purification of real wastewaters. The retention capacity of the membrane was 450 mg g(-1) compared to 311 mg g(-1) reported for the PVA/PEI membrane. Sorption isotherms were determined at various temperatures, according to the Langmuir model, for the determination of the thermodynamical parameters. When T increased, mercury uptake at equilibrium did not change, whereas the sorption coefficient b decreased and the change in free energy DeltaG degrees decreased. This result is probably due to a large favorable entropic effect, ascribed to the displacement of protons from the protonated sites of P(4)VP while they bind with Hg(II) ions. The membrane could be regenerated by 0.5 M nitric acid with less than 3% loss of efficiency. The membrane was used for filtration experiments. The elimination ratio was 99.9% or more for filtration of Hg(II) solutions in the c(0) = 16.6-89.1 mg L(-1) range.

Facilitated transport of boric acid by 1,3-diols through supported liquid membranes

Boric acid is selectively transported by 1,3-diols from a source aqueous phase into a receiving alkaline aqueous phase through a phase of organic solvent (o-dichlorobenzene). Measurements of the transport rates were made using a U-tube apparatus. The process was adapted to liquid membranes supported on polypropylene films. The factors which influence the stability of the supported liquid membrane are discussed. Assuming that the transport of the neutral 1:1 diol-boric acid complex within the organic phase is diffusion-limited, a model is proposed that realistically represents the fluxes of boric acid as a function of the initial concentrations of boric acid and diol. The experimental diffusion coefficients were determined and agreed satisfactorily with the calculated values.

Relationship between structures and stabilities of molybdate complexes of alditols: A potentiometric, 13C and 95Mo nmr study

Abstract Dinuclear molybdate complexes of alditols have been studied in aqueous acidic solution by 95 Mo and 13 C NMR. Their formation constants were determined by potentiometry. In all complexes, the dimolybdate group was chelated by four vicinal hydroxyls. Two types of complexes were characterized by NMR, depending on the erythro or threo configuration of the central diol group. Their structures were precised using literature crystal data. A single asymmetrical complex of the first type was formed by symmetrical erythro ligands (erythritol, galactitol), while the asymmetrical d -arabinitol and D -mannitol formed pairs of isomeric species of this type. Ligands with complexing sites involving a central threo group ( DL -threitol, xylitol) were complexed in a symmetrical manner. D -Glucitol formed four complexes as pairs of each type. Rules relating the stabilities of the complexes to the structures of the ligands are presented.

Stability constants and structures of homologous dinuclear molybdate and tungstate complexes of aldoses by potentiometry and 13C and 95Mo NMR

Abstract A potentiometric study showed that aldoses of the lyxo -series: d -lyxose, d -mannose and l -rhamnose, form dinuclear anionic complexes with tungstate or molybdate ions in acidic solutions. 95 Mo NMR confirmed this result by revealing the presence of two non-equivalent metal atoms in the molybdate complexes. The corresponding formation constants were determined. The d -lyxose complexes had the higher stabilities, and the W VI complexes were more stable than their Mo VI homologues. The 13 C NMR spectra were reassigned by means of a 2-D experiment and showed that all three carbohydrates were complexed in pyranose form. The chelating hydroxyl groups were strongly deshielded and displayed increased direct coupling constants 1 J CH . It can be concluded that the prevailing species in aqueous acidic solution cannot be furanose complexes similar to that characterized in solid state from molybdate— d -lyxose mixtures.

Formation of neutral complexes of boric acid with 1,3-diols in organic solvents and in aqueous solution

Abstract From solubility measurements in organic media and extraction experiments, the values of the stability constants of neutral 1 : 1 complexes (HBL) formed between boric acid (HB) and 1,3-diols (L) have been obtained in aqueous solution (Kc(w)) and in organic solvents (Kc(s)). The values of the partition coefficients of the diols (PL) and the complexes (PHBL) between water and the organic solvents have also been calculated. Generally, the addition of a diol to an aqueous solution of borax is accompanied by a decrease of pH due to the prevailing formation of borate species but, in the case of 2-methyl-2,4-pentanediol, the pH increased, in agreement with the formation of a single neutral complex. It is shown that a modification of a previous potentiometric procedure yields simultaneously the values of the stability constants of the neutral and anionic complexes in aqueous solution.

Structures of the borate complexes of D-allose, D-talose, and D-psicose in aqueous solution: an 11B- and 13C-N.M.R. study

Abstract The formation of 1:1 or 1:2 borate-sugar complexes by sugars having ribo configurations has been studied by 11B- and 13C-n.m.r. spectroscopy. Two 1:2 complexes can be formed, depending on whether the sugar is α or β. The main species involved cis-HO-1,2. d -Psicose formed a single complex at HO-2,3. A second species was formed by d -talose (10%), d -ribose (30%), and d -allose (30%), which involved cis-HO-2,3 with HO-1,2 trans. The order of stabilities of the complexes was d -psicose > d -ribose > d -talose > d -allose. The high affinity of ribo sugars towards borate is discussed. There was no correlation between the stability constants and the relative proportions of 1:2 complexes.

Acyclic molybdate complexes of aldoses in the arabino and xylo series and their application to the determination of the proportion of acyclic forms in aqueous solution

Abstract Aldoses of the arabino and xylo series with molybdate ions in aqueous acidic medium, form tetradentate acyclic complexes which are much weaker than those of the related alditols. The formation constants ( K f ) of these complexes were obtained by two independent methods, potentiometry and UV spectrophotometry, which gave values in good agreement. The structures of the complexes with aldoses were shown by 13 C NMR spectroscopy to involve the ligand in its acyclic form. A study of the complexes of carbohydrate derivatives proved that the magnitude of K f was mainly dependent on the configuration of the site of chelation. The lower stabilities of the aldose complexes were due to the endergonic opening of the pyranose heterocycle in the first step of the complex formation. An application to the determination of the equilibrium constant for the ring-opening reaction of aldopyranoses is described. It ultimately allowed the calculation of the proportion of acyclic forms (aldehyde and hydrate) in aqueous solutions of the aldoses in the arabino and xylo series.

A 13C and 183W NMR study of the structures of tungstate and molybdate complexes of volemitol

Abstract 1 H, 13 C, and 183 W NMR spectroscopic methods were used for the structural characterization of the tungstate and molybdate complexes of volemitol ( d - glycero - d - manno -hepitol) in aqueous solution. The major species (type E) are a pair of isomeric complexes formed at the arabino site in reversed orientation, i.e., HO-1,2,3,4 and HO-4,3,2,1. A single, minor complex is formed at the HO-3,4,5,6 altro site and is shown to be isostructural with the known single complexes of ribitol and d -altritol (type E′). The present NMR results support the hypothesis that complexes of type E′ are weaker than those of type E because they are destabilized by a steric strain due to the interaction of the side chain of the ligand with the site of chelation.

A 13C-n.m.r. study of the tungstate and molybdate complexes of perseitol, galactitol, and d-mannitol

Abstract Perseitol ( d - glycero - d - galacto -heptitol) forms dinuclear complexes with tungstate that have a stability higher than those of galactitol and d -mannitol, and all of the formation constants are three orders of magnitude higher than those for molybdate. The 13 C-n.m.r. data showed that the tungstate and molybdate complexes had similar structures that involved four vicinal hydroxyl groups. The sites of chelation involved the galacto group in galactitol and perseitol, and the arabino group HO-3,4,5 and HO-6 in d -mannitol. d -Mannitol and perseitol formed pairs of isomeric complexes that involved the same site of chelation but in reversed orientations.