H-bond in chitosan-based hydrogels

N-alkyl chitosan derivatives, including N-ethyl, butyl, heptyl and lauryl forms were prepared via Schiff's base intermediates. The study of H-bonds in chitosan-based hydrogel was performed by DSC, NMR, XKD and PALS. The content of unfrozen bound water increases with increasing length of the alkyl side chain and the degree of substitution. The X-ray diffraction peak centered at 2θ=26.5° is sensitive to water. The free volume enhances in the order, chitosan (CS) < N-ethyl CS < N-butyl CS < N-heptyl CS << N-lauryl CS. Moreover the free volume of the derivatives decreases as water content rises but the situation of chitosan is opposite. The dynamic properties of water molecules within the networks were studied through the variation of the proton line width in the ¹H-NMR spectrum. The proton line width of the water peak of N-alkyl chitosan increases in the sequence, N-ethyl CS < N-butyl CS < N-heptyl CS < N-lauryl CS. The same results arise from the spin-lattice relaxation time (T₁) for these N-alkyl chitosan derivatives.     

Palladium and platinum guanine complexes

Summary Reaction of [MCl₂(H₄Y)] or (H₆Y)[MCl₄] compounds (M = Pd or Pt; Y = EDTA, PDTA or CDTA) with guanine (Gu-H) in 0.5 M HCl yields new complexes MCl₂(Gu-H)(H₂O which have been characterized by elemental analyses in conjunction with electronic, i.r. and n.m.r. spectra. A dimeric structure with two bridging chloride ligands, including coordination of guanine through atom N(9), is proposed for both compounds.Abbreviations used in the paper EDTA 1,2-diaminoethane-N,N,N,N-tetraacetate - PDTA 1,2-diaminopropane-N,N,N,N-tetraacetate - CDTA trans-1,2-diaminocyclohexane-N,N,N,N-tetraacetate - Gu-H guanine - 9-EtGu-H 9-ethylguanine     

Palladium and platinum dihalide complexes with dl-selenomethionine

Palladium and platinum dihalides react with dl-selenomethionine (sem), yielding the complexes [M(sem)X2](M=Pd,X=Cl or Br;M=Pt,X=Cl) and, in the presence of N,N-dimethylformamide (dmf), the species [M(sem)X2]·dmf (M=Pd, X=I; M=Pt, X=Cl, Br or I). The complexes were characterized by i.r. and proton n.m.r. spectroscopy and by thermogravimetric analysis, and their properties were compared with those of the dl-methionine analogues [M(Met)Cl2] and [Pt(Met)Cl2]·dmf. On the basis of n.m.r. data in deuteriated dimethyl sulfoxide, the platinum complexes undergo ligand rearrangement to form [Pt(sem)2]2+ moieties whereas the solvent does not seem to interact with the palladium coordination sphere, which contains the chelated N, Se ligand.     

Thermodynamic properties of chitosan-based hydrogels in the range 0–350 K

The heat capacity, the temperature, and the enthalpy of physical transformations of hydrogels based on the copolymer of acrylamide and chitosan with N,N-methylene-bis(acrylamide) as a crosslinking agent, hydrogels based on the mixture of poly(vinylpyrrolidone) and chitosan with glutaric aldehyde as the crosslinking agent, and dehydrated hydrogels has been studied in the range 85–350 K with the use of adiabatic vacuum calorimetry. The temperatures and enthalpies of melting of free water in hydrogels have been determined. From the experimental data obtained, the thermodynamic functions C° _p (T), H° (T) ? H°(0), S° (T) ? S°(0), and G° (T) ? H°(0) have been calculated for the temperature range 0–350 K. The ratio of free and bound water in the hydrogels under study has been determined by calorimetry.     

Palladium and platinum organochalcogenolates and their transformation into metal chalcogenides

Platinum group metal chalcogenides find extensive applications in catalysis and in the electronic industry. To develop an efficient low temperature clean preparation of these materials, molecular routes have been explored. Thus the chemistry of mononuclear organochalcogenolates of the type [M(ER’)₂(PR₃)₂], binuclear benzylselenolates, [M₂Cl₂(μ-SeBz)₂(PR₃)₂], allylpalladium complexes [Pd₂(μ-ER)₂(η³-C₄H₇)₂] and palladium/platinum sulphido/selenido-bridged complexes, [M₂(μ-E)₂L₄] (M = Pd or Pt; E = S, Se or Te; L = tertiary phosphine ligand) has been investigated. All the complexes have been characterized by elemental analysis, NMR (¹H,³¹P,⁷⁷Se,¹⁹⁵Pt) spectroscopy and in some cases by X-ray diffraction. The thermal behaviour of these complexes has been studied by TGA. The pyrolysis of allylpalladium complexes in refluxing xylene yields Pd₄E as established by analysis and XRD patterns.     

New polyfunctional anion exchangers for platinum metal sorption

The platinum(IV) and palladium(II) sorption properties under static conditions were examined for new polyfunctional anion exchangers based on resorcinol diglycidyl ether, monoethanolamine vinyl ether, allyl bromide, and various amines.     

Ultrasonic characterization of the kinetics of water sorption in hydrogels

A complex mechanism characterizes the water uptake kinetics in hydrogels, as a consequence of the strong structural changes occurring in the material during the sorption process. Water acts as a plasticizer, reducing the glass transition temperature of the polymer below the sorption temperature and determining a glass transition in the polymer. In this study the changes in the ultrasonic attenuation and velocity in semicrystalline Poly-vinyl-alcohol (PVA) hydrogel films during water sorption are measured by a pulse-echo system. The ultrasonic wave propagation is applied to monitor the position of the swollen/unswollen fronts and to the measurement of velocity and attenuation. The structural changes in PVA hydrogels, monitored by Wide Angle X-ray Diffractometry (WAXD), performed during the sorption process, are correlated with the ultrasonic data.     

Effect of carbon support on the hydrogen sorption of platinum

The hydrogen sorption of carbon-supported platinum catalysts, studied by the potentiodynamic method, differs significantly from that of unsupported platinum. The difference depends on the preparation method and on the nature of the support. Possible explanations are suggested as (i) some kind of interaction between metal and support atoms during impregnation, (ii) high dispersity (amorphous structure) and (iii) hydrogen spillover onto the carbon support.

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Pt/C, . . (I) , (II) ( ), (III) .

     

Palladium electrodeposits: Dependence of structure and sorption properties on the deposition potential

A series of Pd electrodeposits (edPd) on Pt substrates is prepared at deposition potentials of -0.05 to 0.55 V with respect to a reversible hydrogen electrode in 0.5 M H₂SO₄. Their nanostructure is characterized by scanning tunneling microscopy. The size distribution of particles is estimated, and dependences of its maximum and half width on the deposition potential are determined. A comparative coulometric study of adsorption of copper and oxygen on edPd shows that real surface areas determined from these data substantially differ. The average size of particles for edPd, estimated within the model of equal-size spheres, is shown to be incorrect. The assumption that particles in the deposits essentially coalesce is substantiated. It is shown that the equilibrium hydrogen content in thea and β hydrides is anomalously high for the deposits whose growth was accompanied by deep hydrogenation of Pd. At a given effective pressure, for the α-phase, this value is always substantially higher as compared with less defective materials. In the β-phase, the hydrogen concentration can be either lower or higher     

Immobilization of carboxyl-modified multiwalled carbon nanotubes in chitosan-based composite membranes for U(VI) sorption

Journal of Radioanalytical and Nuclear Chemistry - The carboxyl-modified multiwalled carbon nanotubes were immobilized in chitosan-based composite membranes (CS-CNTs) which were used as efficient...     

Palladium(II) and platinum(II) complexes with N-substituted methionines

Summary As an approach to systems containing methionine residues, 3-acetyl-4-hydroxy-6-methyl-2H-pyran-2-one (HDh, dehydroacetic acid) was treated with L-methionine (MetH) or L-methionine methylester (MetM). By condensation at the acyl group and transfer of the phenolic hydrogen on the nitrogen atom, the related ligands DhMetH and DhMetM, were isolated, and form complexes of formula [MX₂(L)₂](M = Pd or Pt, L = DhMetM, X = Cl, Br or I; L = DhMetH, X = Cl or Br) and [MI₂(DhMetH)] with palladium and platinum dihalides. The reaction of the DhMetK carboxylate with MCl₂ in various media is discussed. Ligands and complexes were characterized by i.r. and n.m.r. (¹H and¹³C) spectroscopy and, in some cases, by thermogravimetric measurements. The ligands behave as monodentate sulphur donors, the 12 complexes showing atrans geometry except for [PtCl₂(DhMetH)₂], which is probably a mixture ofcis andtrans isomers.     

Water sorption properties of poly(ethyl acrylate-co-hydroxyethyl methacrylate) macroporous hydrogels

Synthetic porous hydrogels are becoming more and more important in the field of biomaterials. Different studies demonstrate that the porous structure promotes the colonisation of living cells and improves the biocompatibility of the implants. The macroporous structure allows not only the control of cellular ingrowth morphology but also the mechanical integration and the regulation of nutrient and hydraulic flow in the hydrogel. In this work poly(ethyl acrylate-co-hydroxyethyl methacrylate) (PEA/PHEMA) copolymers were polymerized using 2% of ethylene glycol dimethacrylate as cross-linking agent and azoiso-botyronitrile as initiator. Five samples were prepared with the EA/HEMA weight ratios of 75/25, 50/50, 25/75 and pure PEA and PHEMA polymers, obtaining different degrees of hydrophilicity. The macroporous structure was obtained by adding poly(acrylonitrile) fibres to the monomers. After polymerization the fibres were eliminated by dissolution in dimethyl formamide. The holes are cylinders of approximately 40μm diameter and are all, more or less, in the same direction, although they are not uniformly distributed. Water sorption isotherms and diffusion properties of the macroporous samples are compared with the samples without holes.     

Palladium(II) and platinum(II) complexes of dithiocarbamates and L-methioninol

The [M(dithiocarbamato)(Mol)]Cl complexes [M = Pd or Pt; dithiocarbamato = DMDT (Me₂NCS^– ₂) or ESDT (EtO₂CCH₂MeNCS^– ₂); Mol = L-methioninol (L-2-amino-4-methylthio-1-butanol)] have been prepared by reacting methioninol with the appropriate [M(dithiocarbamato)Cl] _n complex in a 1:1 molar ratio in chlorinated hydrocarbons. By operating at a 1:2 molar ratio, the binuclear species [M₂(dithiocarbamato)₂(Mol)Cl₂] were obtained. The complexes were characterized by i.r., n.m.r. and electrospray ionisation (ESI) mass spectra and by t.g.a. The [M(dithiocarbamato)(Mol)]Cl species are ionic and contain S,N-chelated methioninol. The ligand forms an S,N bridge between two metal atoms in the binuclear species, whose formation is confirmed by the presence of the deprotonated molecular ion in the ESI negative ion mode.     

Palladium and platinum complexes of a benzannulated N-heterocyclic plumbylene with an unusual bonding mode

Reaction of the N,N'-diisobutyl-substituted benzannulated N-heterocyclic plumbylene (NHPb) 1 with [Pd(PPh(3))(4)] and [Pt(PPh(3))(4)] gave the complexes [M(NHPb)(PPh(3))(3)] (M = Pd [2], Pt [3]). X-ray diffraction studies of both complexes showed an angle of ~125° between the plumbylene plane and the transition-metal-Pb axis, indicating coordination of the transition metal to the empty π orbital of the plumbylene Pb atom. The experimentally determined metric parameters of complexes [2] and [3] are discussed on the basis of DFT calculations.     

Palladium(II) and platinum(II) dichloro complexes containing diamine-estrone derivatives

The preparation ofcis-dichloro Pt(II) andcis-dichloro Pd(II) complexes ofN-[3-hydroxyestra 1:3:5 (10)trien-17β]ethylendiamine,N-[3-hydroxyestra 1:3:5 (10)trien-17β]1,3-propylendiamine, andN-[3-hydroxyestra 1:3:5 (10)trien-17β]2-aminomethylpyridine is reported. The complexes have been characterized by chemical analysis, infrared spectroscopy and molar conductivity.     

Palladium(II) and platinum(II) complexes of rhodanine and 3-methylrhodanine

Summary The following palladium(II) and platinum(ll) complexes of rhodanine (HRd) and 3-methylrhodanine (MRd) have been prepared: Pd(HRd)_1.5Cl₂, Pd(HRd)₂Br₂, Pd(HRd)₂Br₂ · 0.25 EtOH, M(MRd)₂X₂ [M = Pd, X = Cl (0.25 EtOH) or Br; M = Pt, X = Cl or Br], Pd(MRd)₃Br₂, and M(MRd)₄(ClO₄)₂ (M = Pd or Pt). The ligands are coordinated to the metal through the thiocarbonylic sulphur atom. Pd(HRd)_1.5Cl₂ has presumably a structure such as (X = Cl or Br) complexes have a trans-planar coordination. Pd(MRd)₂X₂ (X = Cl or Br) complexes arecis-planar coordinated. Pd(MRd)₃Br₂ has presumably a square coordination with two MRd molecules and two CI ionscis-coordinated in the equatorial plane, and a MRd molecule and a Cl ion weakly bonded in apical position. The M(MRd)₄(ClO₄)₂ complexes have square planar coordination.Author to whom all correspondence should be addressed.     

Palladium and platinum complexes of chiral and achiral calix[4]arene bisphosphite ligands

Chiral and achiral p-tert-butyl-calix[4]arene bisphosphites (L¹–L³) have been synthesized by the reaction of p-tert-butyl-calix[4]arene and the phosphorodichloridites, ROPCl₂ [R = (1S,2R,5R)-(+)-iso-menthyl (L¹), (1R,2S,5R)-(−)-menthyl (L²) or C₆H₄Bu^t-4 (L³)]. These bisphosphites function as chelating ligands in palladium(II) and platinum(II) complexes which are formed in good yields by the reaction of PdCl₂(PhCN)₂, MCl₂(COD) (M = Pd or Pt) or PdMeCl(COD) with the respective calix[4]arene bisphosphite. Single crystal X-ray diffraction studies performed on the complexes [PdCl₂(L¹)], [PdCl₂(L²)], [PdCl₂(L³)] and [PtCl₂(L³)] reveal a near square planar geometry around the metal with the two chloride ligands in a cis disposition. The crystal packing in the complexes [PdCl₂(L¹)] and [PdCl₂(L²)], which crystallize in the chiral (P6₁22) space group, shows different hydrophobic channels with intermolecular C–H?Cl hydrogen bonding. The complexes [PdCl₂(L³)] and [PtCl₂(L³)] are isostructural and the molecules in the crystal lattice are linked by intermolecular C–H?Cl and C–H?O hydrogen bonds.     

Water sorption and polymer dynamics in hybrid poly(2-hydroxyethyl-co-ethyl acrylate)/silica hydrogels

This work probes the hydration properties and molecular dynamics of hybrid poly(hydroxyethyl-co-ethyl acrylate)/silica hydrogels. Two series of hybrid copolymers were prepared by simultaneous polymerization and silica preparation by sol-gel method, the first with hydroxyethyl acrylate/ethyl acrylate (HEA/EA) composition at 100/0, 90/10, 70/30, 50/50, 30/70, 10/90 and fixed silica content at 20 wt.%, and the second with fixed HEA/EA organic composition at 70/30 and 0, 5, 10 and 20 wt.% of silica. The hydration properties of these systems were studied at 25 °C by exposure to several controlled water vapor atmospheres (water activities 0-0.98) in sealed jars and by immersion in distilled water. Finally, the molecular dynamics of the hydrated hybrids at several levels of hydration was probed with Thermally Stimulated Depolarization Currents (TSDC) in the temperature interval between −150 and 20 °C. The results indicate that a critical region of silica content between 10 and 20 wt.% exists, above which silica is able to form an inorganic network. This silica network prevents the expansion of water clusters inside the hydrogels and subsequently the total stretching of the polymer network without obstructing the water sorption at the first stages of hydration from the dry state. As concerns the copolymer composition, the presence of EA reduces water sorption and formation of water clusters affecting directly to the hydrophilic regions. The TSDC thermograms reveal the presence of a single primary main broad peak denoted as α_cop relaxation process, which is closely related to the copolymer glass transition, and of a secondary relaxation process denoted as β_sw relaxation, which originates from the rotational motions of the lateral hydroxyl groups with attached water molecules. The single α_cop implies structural homogeneity at the nanoscale in HEA-rich samples (x_HEA > 0.5), while for high EA content (x_EA ? 0.5) phase separation is detected. Both relaxation processes show strong dependence on water content and organic phase composition.     

Sequestration of agrochemicals from aqueous media using cross-linked chitosan-based sorbents

The sorption properties are reported for chitosan and its cross-linked forms (chitosan-glutaraldehyde; CG) with some model agrochemical sorbates [pentachlorophenol (PCP), 2,4-dichlorophenol (2,4-DCP) and 2,4-dichlorophenoxy acetic acid (2,4-D), dicamba and carbofuran]. The CG cross-linked materials were prepared at variable C:G monomer mole ratios: 1:0.5 (CG1), 1:1 (CG2), (CG3). The sorbents were characterized using diffuse reflectance infrared Fourier transform spectroscopy, thermogravimetric analysis and a dye sorption method using phenolphthalein. The sorption studies were carried out in aqueous solution at pH 9 except for dicamba and carbofuran (pH 7). The isotherm results were evaluated by the Sips, Freundlich, and Langmuir models. The Sips model provided the “best-fit” results where the sorption capacity increased as the cross-linker content of the CG materials increased. The relative uptake for chitosan and its cross-linked forms adopted the following order: PCP > 2,4-DCP > 2,4-D. In the case of dicamba and carbofuran, the former had a higher sorptive uptake. The variable uptake of the sorbates were attributed to their relative lipophilicity where the main driving force of these solid-solution systems relates to hydrophobic effects, in accordance with the tunable physicochemical properties of the chitosan sorbent materials.