Dynamic scaling for gelation of a thermosensitive chitosan-β-glycerophosphate hydrogel

This paper investigates the thermal gelation of a chitosan-β-glycerophosphate system using rheometry. The gelation, performed for two polymer concentrations, was achieved in isothermal conditions to examine the effect of various temperatures (between 35 and 65 °C) on the sol-gel transition. The oscillatory shear data was used to verify the scaling behavior (G′ ~ G″ ~ ω ⁿ ) at the gel point, and the power-law index n was found to be dependent to some extent on chitosan concentration and temperature. The solutions zero shear viscosity (η ₀) and the gels low frequency modulus (G _e) were analyzed in terms of the relative distance to gelation time (ɛ). The two properties showed power–law relationships with ɛ, i.e., η ₀ ~ ɛ ^−s and G _e ~ ɛ ^z, and the indexes slightly increased with temperature. The dynamic scaling theory predicts a relationship between the indexes, i.e., n  =  z/(z  +  s), and this was in reasonably good agreement with the experimental findings.     

Reentrant behavior of poly(vinyl alcohol)–borax semidilute aqueous solutions

 The reentrant behavior of Poly(vinyl alcohol) (PVA)–borax aqueous semidilute solutions with a PVA concentration of 20 g/l and borax concentrations varies from 0.0 to 0.20 M was investigated using dynamic light scattering (DLS) and dynamic viscoelastic measurements. Two (fast and slow modes) and three (fast, middle, and slow) relaxation modes of PVA semidilute aqueous solutions without and with the presence of borax, respectively, were observed from DLS measurements. The fast and middle relaxation modes were q ²-dependent (q is the scattering vector) characteristic of diffusive behavior; however, the slow modes were q ³-dependent, characteristic of intraparticle dynamics. The experimental results showed that the slow relaxation mode dominates the DLS relaxation. The DLS slow mode relaxation time, τ_s, and the viscoelastic modulus G′(ω) and G′′(ω) data had a similar trend and demonstrated reentrant behavior as the borax concentration was increased from 0.0 to 0.20 M, i.e. τ_s, G′(ω), and G′′(ω) fluctuated with increasing borax concentration. The excluded-volume effect of polymers, charge repulsion among borate ions bound on PVA molecules, and intermolecular cross-linking didiol–borate complexation caused an expansion of the polymer chain; however, the screening effect of free Na⁺ ions on the negative charge of the borate ions bound on PVA and intramolecular cross-linking didiol–borate complexation led to a shrinkage of the polymer chain. The reentrant behavior was the consequence of the balance between expansion and shrinkage of the PVA–borate complex. Received: 26 March 1999/Accepted in revised form: 3 September 1999     

An EXAFS study of the molecular structure of heterometallic chalcogenide clusters

Applicability of EXAFS spectroscopy for determination of the molecular structure of heterometallic chalcogenide clusters in the crystalline state and in solution was examined. The spatial structure of the metal core of the FeMoW(μ₃-Se)(CO)₇(η⁵-C₅H₅) cluster was determined using EXAFS data obtained at the K absorption edge for Fe, Mo, and Se and at the L_III-edge for W. The geometric parameters (the bond lengths and bond angles) obtained from EXAFS data are close to those determined by X-ray analysis. The Mo K-edge EXAFS study of the structurally similar FeMo₂Te(CO)₇(η⁵-C₅H₅) cluster both in the crystalline state and ino-xylene solution confirmed that the geometry of the metal core of the cluster is retained in solution. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1395–1398, August, 2000.     

Reaction of a naphthalene complex C10H8Yb(THF)2 with cyclopentadienyl-substituted alcohols and amines as a convenient synthetic route to the half-sandwich complexes of divalent ytterbium

The reactions of ytterbium naphthalene complex C₁₀H₈Yb(THF)₂ with 2-cyclopentadienylethanol, 1-cyclopentadienylpropan-2-ol, 3-cyclopentadienyl-1-butoxypropan-2-ol, and cyclopentadienyldimethylsilyl-tert-butylamine were studied. The bivalent ytterbium complexes with chelate bifunctional cyclopentadienyl ligands [(η⁵−C₅H₅)CH₂CH₂(η¹−O)]Yb(THF), [(η⁵−C₅H₅)CH₂CH₂(η¹−O)]Yb(DME). [(η⁵−C₅H₅)CH₂CH(Me)(η¹−O)]Yb(THF), [(η⁵−C₅H₅)CH₂CH(CH₂OC₄H₉)(η¹−O)]Yb(THF), and [(η⁵−C₅H₅)SiMe₂(η¹−N(Bu¹))]Yb(THF) were obtained and characterized. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 742–745, April, 2000.     

Aqueous polysaccharide blends based on hydroxypropyl guar gum and carboxymethyl cellulose: synergistic viscosity and thixotropic properties

Aqueous polysaccharide blends, formed from 2.5% (w/v) solution of hydroxypropyl guar gum (HPG) and 2.5% (w/v) solution of carboxymethyl cellulose (CMC) according to different blending ratios, were investigated at 20 °C in terms of their shear-dependent viscosity and thixotropic properties. The Cross viscosity equation was found to fit the shear-dependent viscosity data with reasonable accuracy. When the HPG solution with the mass fraction (f _HPG) of 0.87 was mixed, the zero shear viscosity (η _o) of the corresponding blend was found to be 168.5753 Pa s, while the η _o values of component HPG and CMC solutions were found to be 3.3859 and 98.6525 Pa s, respectively. For the aqueous HPG/CMC blends investigated, the resulting zero shear viscosity was observed to be much greater than the combined zero shear viscosity of the component polysaccharide solutions, showing a synergistic viscosity property. The quantitative determination of the hysteresis loop area, developed during viscometer tests on shear rate–shear stress reverse paths, was used to describe the thixotropic behavior. When compared with aqueous solutions of the component polysaccharides, these polysaccharide blends could afford enhanced thixotropic property. Maximum thixotropy synergism was observed for the HPG/CMC blend with the f _HPG of 0.67.     

Synthesis and Characterizations of Ferrocenyl Carbonyl Chromium and Cobalt Clusters

Two novel bimetallic complexes, [Cr(CO)₃(η ⁶-C₆H₅)–C≡C–C₆H₄–Fc] (Fc = C₅H₅FeC₅H₄] (1) and [Cr(CO)₃(η ⁶-C₆H₅)–C ≡ C–Fc–C(CH₃)₂–Fc] (3), were synthesized by the Sonogashira coupling reaction. By using of (1) and (3) as ligands to react with Co₂(CO)₈, two others novel polymetallic complexes, [Cr(CO)₃(η ⁶-C₆H₅){Co₂(CO)₆-η ²-μ ²-C≡C–}–C₆H₄–Fc] (2) and [Cr(CO)₃(η ⁶-C₆H₅){Co₂(CO)₆-η ²-μ ²-C≡C–}Fc–C(CH₃)₂–Fc] (4) were obtained. Four carbonyl complexes were characterized by elemental analysis, FT-IR, NMR and MS. The molecular structures of complexes (1), (2) and (4) were determined by single crystal X-ray diffraction. The interactions among the ferrocenyl, Cr(CO)₃ and Co₂(CO)₆-η ²-μ ²-C≡C– units were investigated by cyclic voltammetry.     

Synthesis and structures of nickel(II) trimethylacetate dimers with coordinated diamines

Thermal decomposition of the tetranuclear nickel(II) complex Ni₄lη²-o-(NH₂)(NHPh)C₆H₄|₂(MeCN)₂(μ-OOCCMe₃)₄(η²-OOCCMe₃)₂ (I) under an inert atmosphere (o-xylene, 140 °C) was investigated. Under these conditions, the asymmetric binuclear complex Ni|η²-o-(NH₂)(NHPh)C₆H₄‖(η¹-o-(NH₂))(NHPh)C₆H₄|(η²,η-O,O-OOCCMe₃)(η²-OOCCMe₃) (2) was formed at the first stage. Complex2 was converted into the symmetric dimer Ni|η¹-o-(NH₂)(NHPh)C₆H₄|(μ-OOCCMe₃)₄ (3) upon recrystallization from benzene. The structures of complexes2 and3 were established by X-ray diffraction analysis. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1915–1918, November, 2000.     

Synthesis and structures of fulvene-bridged diruthenium complexes

Three diruthenium carbonyl complexes, namely (η ³:η ⁵-C₅H₄C(CH₂)₂)Ru₂(CO)₅ (1), (η ³:η ⁵-C₅H₄C(CHCH₂)(C₂H₅))Ru₂(CO)₅ (2), and (η ¹:η ⁵-C₅H₄C₅H₈)Ru₂(CO)₆ (3), were obtained from the reactions of C₅H₄C(Me)₂, C₅H₄C(Et)₂, and C₅H₄C(CH₂)₄, respectively, with Ru₃(CO)₁₂ in refluxing xylene. The complexes were characterized by elemental analysis, IR and ¹H NMR spectra. Single-crystal X-ray diffraction analysis for complexes 1 and 2 revealed that the fulvene ligands bridge two ruthenium atoms in η ³:η ⁵ fashion.     

Synthesis, Characterization, and Fluxional Behaviour of Binuclear Palladium Complexes with a Half-A-Frame Structure

 Partial removal of chloride anions from the dimer [Pd(η³-2-CH₃*C₃H₄)(μ-Cl)]₂ with AgTf(Tf = CF₃SO₃) followed by addition of dppm affords [Pd₂(η³-Me*C₃H₄)₂(μ-Cl)(μ-dppm)]Tf (1). The substitution of Cl⁻ by X⁻ (X = pz, SC₆F₅, S _py ) using the appropriate salts yields the new derivatives [Pd₂(η³-Me*C₃H₄)₂(μ-X)(μ-dppm)]Tf (2–4). All complexes exhibit a dinuclear half-A-frame structure with two isomers present in solution. The isomers differ in the relative orientation of the two allyl groups (cis or trans). The isomer interconversion was studied by variable temperature ¹H NMR spectroscopy. The molecular structures of 2 and 4 were solved by X-ray diffraction studies. A distorted boat conformation of the seven- or six-membered metallacycle was found in both cases.     

Preparation and physico-chemical characterisation of iron(III) complexes containing substituted benzimidazole and Schiff base moieties

New complexes of iron(III) of the type [(μ-Cl)₂Fe₂{η ²-(opbz)₄] (1) and [(Cl)₂Fe{η ²-(opbz)] (2) [Hopbz = {2-(o-hydroxyphenyl)}-benzimidazole, η ² = Number of connectivity sites involved in bonding the metal] have been synthesized by the interactions of iron(III) chloride with the corresponding ligand (Hopbz) in 1:2 and 1:1 molar ratio(s) in hot EtOH. The complex (1) was further treated with various sodium salts of Schiff bases (sb) [Na(o-smab), Na(p-smab) and Na(sap)], alkoxo-Na(OPrⁱ), aryloxo-Na(OAr) and tetraisopropoxyaluminate [Na{Al(OPrⁱ)₄] in a 1:1 molar ratio (in THF–benzene medium) to produce derivatives of the type: {(η ²-(sb) Fe{η ²-opbz)₂] (3), (4) and (5), [(μ-OPrⁱ)₂Fe₂{η-(opbz)₄}] (6) [(μ-OAr)₂Fe₂{η-(opbz)₄}] (7) and [(μ-OPrⁱ)₂Al(OPrⁱ)₂-Fe{η-(opbz)₂}] (8). All these derivatives were characterised by elemental analysis, spectral (IR, UV–visible and FAB-mass) and magnetic susceptibility measurements. On the basis of these studies, a distorted octahedral (dimeric) geometry around iron(III) for the complexes (1), (6), (7) and (8) have been suggested.     

A note on overpotential dependence of AC impedance data

Charge-transfer resistance [R _ct = (dη/di)_{η = 0}] and Tafel plots of current density (i) versus overpotential (η) data are generally known to yield values of the energy-transfer coefficient (α) and exchange current density (i _o) of an electrochemical reaction. In the present investigation, the resistance (dη/di)_η≠0 that could be calculated by differentiating a wide range of i−η curves was also shown to provide the values of α and i _o, by plotting ln(dη/di)_η≠0 against η. Since α and i _o could also be evaluated directly from the experimental DC polarization data, the procedure was not of significant importance. Nevertheless, it was considered important in evaluating α and i _o from AC impedance data, because the procedure was based on data analysis, which was much simpler than that reported in the literature. A cobalt electrode prepared from fine metal powder was used in 1 M KOH electrolyte and the hydrogen evolution reaction was studied by AC impedance at several potentials. The resistance values measured from the complex plane impedance diagram were plotted against the potential, and the values of α and i _o were evaluated. Received: 8 October 1998 / Accepted: 11 January 1999     

Synthesis, Characterization, and Fluxional Behaviour of Binuclear Palladium Complexes with a Half-A-Frame Structure

Summary.  Partial removal of chloride anions from the dimer [Pd(η³-2-CH₃*C₃H₄)(μ-Cl)]₂ with AgTf(Tf = CF₃SO₃) followed by addition of dppm affords [Pd₂(η³-Me*C₃H₄)₂(μ-Cl)(μ-dppm)]Tf (1). The substitution of Cl⁻ by X⁻ (X = pz, SC₆F₅, S _py ) using the appropriate salts yields the new derivatives [Pd₂(η³-Me*C₃H₄)₂(μ-X)(μ-dppm)]Tf (2–4). All complexes exhibit a dinuclear half-A-frame structure with two isomers present in solution. The isomers differ in the relative orientation of the two allyl groups (cis or trans). The isomer interconversion was studied by variable temperature ¹H NMR spectroscopy. The molecular structures of 2 and 4 were solved by X-ray diffraction studies. A distorted boat conformation of the seven- or six-membered metallacycle was found in both cases. Received June 7, 2000. Accepted June 20, 2000     

Reaction of cyclopentadienyllutetium anthracenide with azobenzene. Synthesis and molecular and crystal structure of the binuclear complex [C5H5(THF)Lu(μ−η2:η2−PhN—NPh)]2(THF)2 and its dynamic behavior in solution

The reaction of the cyclopentadienyllutetium anthracenide, C₅H₅Lu(C₁₄H₁₀)²⁻(THF)₂ (1), with azobenzene yielded the [C₅H₅(THF)Lu(μ−η²:η²−PhN—NPh)]₂(THF)₂ (2) binuclear complex. The structure of the reaction product was established by X-ray structural analysis. The dynamic behavior of complex2 in a THF-d₈ solution was studied by¹H NMR spectroscopy in the temperature range of 265–330 K. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1667–1671, September, 1997.     

Reduction of bis[(2-phosphinoethyl)tetramethyl-cyclopentadienyl]zirconium dichlorides: intramolecular C-H activation <Emphasis Type="Italic">vs.</Emphasis> stabilization of the low-valence metal center

Reduction of the R₂P-functionalized zirconocene dichlorides [C₅Me₄(CH₂)₂PR₂] (C₅Me₅)ZrCl₂ (R = Me (1) and Ph (2)) and [C₅Me₄(CH₂)₂PMe₂][C₅Me₄(CH₂)₂PR₂]ZrCl₂ (R = Me (3) and Ph (4)) with amalgamated magnesium was studied. In the reduction of compounds 1 and 2, intramolecular C-H activation highly selectively afforded the fulvene hydride complexes Zr(H)(η⁵−C₅Me₅)[η⁵:η²(C,P)−(CH₂)C₅Me₃CH₂CH₂PR₂] (R = Me (7), Ph (8)); in the case of compound 2, the aryl hydride Zr(H)(η₅:C₅Me₅)[η⁵:η¹(C)−C₅Me₄CH₂CH₂PPh(o−C₆H₄)] (9) was also formed. The reduction of complexes 3 and 4 gave the Zr^II derivatives Zr[η⁵:η¹(P)− C₅Me₄CH₂CH₂PMe₂]₂ (12) and Zr[η⁵:η₁(P)−C₅Me₄CH₂CH₂PMe₂][η⁵:η¹(P)−C₅Me₄CH₂ CH₂PPh₂] (14) stabilized by two phosphine groups. The second product in the reduction of compound 4 was the fulvene hydride complex Zr(H)(η⁵−C₅Me₄CH₂CH₂PPh₂)[η⁵:η²(C,P)−(CH₂)C₅Me₃CH₂CH₂PMe₂] (15). The reaction of compound 7 with an excess of MeI resulted selectively in replacement of the hydride ligand by iodide to give the complex ZrI(η⁵−C₅Me₅)[η⁵:η²(C,P)−(CH₂)C₅Me₃CH₂CH₂PMe₂] (10). In contrast, in the reaction of compound 7 with Me₂Si(H)Cl, the Zr-CH₂ bond underwent cleavage to give the chloride hydride complex Zr(H)Cl(η₅−C₅Me₅)[η⁵:η¹(P)−C₅Me₃(CH₂SiMe₂H)CH₂CH₂PMe₂] (11). In the reaction of complex 12 with CO, a phosphine group was replaced by CO to form the complex Zr(CO)(η5−C₅Me₄CH₂CH₂PMe₂)[η⁵:η¹(P)−C₅Me₄CH₂CH₂PMe₂] (13). The results obtained were compared with analogous reduction reactions of MeO-, MeS-, and Me₂N-functionalized zirconocene dichlorides. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 65–74, January, 2008.     

Effects of temperature and molecular weight on dissolution of cellulose in NaOH/urea aqueous solution

Dissolution of cellulose having different viscosity-average molecular weight (M _η ) in 7 wt%NaOH/12 wt%urea aqueous solution at temperature from 60 to −12.6°C was investigated with optical microscope, viscosity measurements and wide X-ray diffraction (WXRD). The solubility (S_a) of cellulose in NaOH/urea aqueous solution strongly depended on the temperature, and molecular weight. Their S_a values increased with a decrease in temperature, and cellulose having M _η below 10.0 × 10⁴ could be dissolved completely in NaOH/urea aqueous solution pre-cooled to −12.6°C. The activation energy of dissolution (E_a,s) of the cellulose dissolution was a negative value, suggesting that the cellulose solution state had lower enthalpy than the solid cellulose. The cellulose concentration in this system increased with a decrease of M _η to achieve about 8 wt% for M _η of 3.1 × 10⁴. Moreover, cellulose having 12.7 × 10⁴ could be dissolved completely in the solvent pre-cooled to −12.6°C as its crystallinity (χ _c) decreased from 0.62 to 0.53. We could improve the solubility of cellulose in NaOH/urea aqueous system by changing M _η , χ _c and temperature. In addition, the zero-shear viscosity (η ₀ ) at 0°C for the 4 wt% cellulose solution increased rapidly with an increase of M _η , as a result of the enhancement of the aggregation and entanglement for the relatively long chains.     

Competition of Viscosity Correlation Equations in Isobutyric Acid + Water Binary Mixtures Near and Far Away from the Critical Temperature

Shear viscosity deviations Δη have been investigated by using density (ρ) and kinematic viscosity (ν) measurements for isobutyric acid + water (IBA + W) mixtures over the entire range of mole fractions at atmospheric pressure and at two temperatures (301.15 and 315.15 K). This study extends the temperature range from the five other temperatures investigated in a previous work, 1.055 K≤(T−T _c )≤14.055 K, both far from and close to the critical temperature. This system exhibits very large positive values of Δη due to increased hydrogen bonding interactions and the correlation length between unlike molecules in the critical region, and to very large differences between the molar volumes of the pure components at low temperatures. The results were also fitted with the Redlich–Kister polynomial equations and the recently proposed Herráez correlation equation. Comparisons between the two models at different temperatures and number of parameters are discussed. We note that, in this system where the shear viscosity η as a function of mole fraction (x ₁) of IBA presents a maximum, experimental data are in agreement with the two correlation models when more than three parameters are employed, especially for temperatures far from the critical temperature.     

New Cyclosiloxanolate Cluster Complexes of Transition Metals

New cyclosiloxanolate transition metal cluster complex derivatives were prepared. PhSiO₂K reacted with NiX₂ (X₂ = Cl₂ or acac) to give K₂{[η⁶−(PhSiO₂)₆]₂[μ₃−(OH)]₂Ni₄K₄}, a mixed group 1–group 10 metal complex. PhSiO₂Na reacted with Ni(NH₃)₆I₂ to give Na{[η⁶−(PhSiO₂)₆]₂Ni₆(μ₆−I)} as the first example of “encapsulated” I⁻ ion in siloxanolate complexes. The macrocyclic Na₄{[η¹²−(PhSiO₂)₁₂]Cu₄} complex reacted with η⁶−(1,3,5−C₇H₈)Cr(CO)₃ to give the heterobimetallic adduct Na₄{[η¹²−(PhSiO₂)₁₂]Cu₄}· [Cr(CO)₃]₃ as one of the rare examples of heterobimetallic complexes with different oxidation numbers of the metals. The copper derivative {[η⁶−(PhSiO₂)₆]₂Cu₆(n−BuOH)₅} reacted in MeOH/CHCl₃ (1:6) with Et₄NCN to give the hexanuclear complex {[η⁶−(PhSiO₂)₆]₂Cu₆(η²−C₃H₅N₂O₂)₂}, containing 2-amino-2-oxoetanimidic acid methyl ester monoanion ligands, product of an unexpected C–C coupling reaction. This latter complex was characterized also by X-ray diffraction crystal and molecular structure determination. This paper is dedicated to the 70th birthday of Professor Dr. Gunter Schmid (Essen), pioneer of large cluster chemistry, known to friends as GOLD-Schmid, because of his famous discovery of the Au₅₅ cluster. The Authors are proud to be within his many friends.     

The effect of the electrolyte concentration in the solution on the voltammetric response of insertion electrodes

A solid-state redox reaction involving an insertion of ions is analyzed with respect to the influence of the concentration of inserting ions in the solution phase. The voltammetric response is independent of the mass transfer in the solution provided that z = (D _ss/D _aq)^1/2 ρ/[C⁺]* is smaller than 0.1 (D _ss: diffusion coefficient of the cation C⁺ in the crystal; D _aq: diffusion coefficient of the cation C⁺ in the solution; ρ: density of the solid compound; [C⁺]*: concentration of cations in the bulk of the solution). In real cases this condition will be satisfied at solution concentrations above 1 mol/l. Received: 15 December 1997 / Accepted: 5 March 1998     

Study of the electronic structure of polynuclear cobalt trimethylacetate complexes by Co3<Emphasis Type="Italic">s</Emphasis> and Co3<Emphasis Type="Italic">p</Emphasis> X-ray photoelectron spectroscopy

The electronic structure of mono-, hexa-, and nonanuclear cobalt trimethylacetate complexes was studied by XPS. The Co3s- and Co3p X-ray photoelectron spectra of the complexes were recorded. The Co3p spectrum of bivalent cobalt was calculated in the isolated-ion intermediate-coupling approximation. Spectrum analysis showed that the [Co(N-Phobsqdi)2(η′-N-Ph-opda)(OOCCMe₃)] complex is a strong-field complex with Co(III) in the diamagnetic state; the [Co(dipy)₂(OOCCMe₃)₂], [Co(dipyam)(OOCCMe₃)₂], and [Co₉(μ₃-OH)₆(μ-OOCCMe₃)₁₂(OCMe₂)₄] are high-spin weak-field Co(II) complexes; and the [Co₆(μ₄-O)₂(OOCCMe₃)₁₀(THF)₄] complex contains both the Co(II) and Co(III) atoms. The energy position of major Co3s- and Co3p spectral maxima were found to be sensitive to the nature of the nearest environment of cobalt atoms. The data correlate well with X-ray crystallographic data.     

Synthesis and characteristics of novel pentanuclear complexes [(OC)3Mn(η1,η5-C5H4)Fe(CO)2(η1,η5-C5H4CO2)]2M(η5-C5H5)2 (M=Ti, Zr)

The reaction of Cp₂MCl₂ complexes (M=Ti and Zr) with 2 equiv. of (OC)₃Mn(η¹,η⁵-C₅H₄)Fe(CO)₂(η⁵-C₅H₄COONa) results in the formation of the pentanuclear complexes (OC)₃Mn(η¹,η⁵-C₅H₄)Fe(CO)₂(η⁵-C₅H₄CO₂)]₂M(η⁵-C₅H₅)₂, which are characterized by IR and¹H NMR spectroscopy and cyclic voltammetry. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1055–1058, May, 1997.