The study of the existence of unknown and unstable compounds through mixed crystal formation using radioactive tracers

A heterogeneous distribution study with internally formed lead chromate as host and¹¹⁰Ag as guest indicates the existence of a new variety of lead chromate, which forms mixed crystals with silver chromate, and differs in morphology from the aged variety. It is inferred that the formation of mixed crystals is probably due to breeding of a morphologically analogous and unstable species by the guest component. By means of study of the homogeneous distribution coefficient, D, of the system consisting of KFe(SO₄)₂·12H₂O as host and¹¹⁰Ag as guest, the existence of an unknown alum, AgFe(SO₄)₂·12H₂O, in trace amounts has been demonstrated. A distribution study of the system shows that KFe(SO₄)₂·12H₂O forms mixed crystals with¹¹⁰Ag up to 10°C, after which the uptake follows an adsorption pattern. The transition temperature of the guest alum, indicated by a sharp change in the D values, is found to be 10°C. Application of radioactive tracers in the study of new and unstable compounds have been stressed.     

Enhancement of hydrolysis through the formation of mixed hetero-metal species

In order to analyze the formation of hetero-metal polynuclear hydrolytic species, in this paper, we reported some results of an investigation (at I = 0.16 mol L⁻¹ in NaNO₃, at t = 25 °C by potentiometry, ISE-H⁺, glass electrode) on the hydrolysis of several mixtures (in different ratios) of two couples of cations: dioxouranium(VI)/copper(II) and dioxouranium(VI)/diethyltin(IV). The elevated total concentrations of cations 0.005 ≤ ΣC_M mol L⁻¹ ≤ 0.05) adopted in these measurements induced us to study again the hydrolysis of uranyl, for which no suitable literature data are available in these particular experimental conditions. All measurements were performed by two different operators, using completely independent instruments and reagents. Many different speciation models were considered in the calculations, including the simultaneous refinement of homo- and hetero-metal species, and a statistical analysis of obtained results was proposed too. Main results can be summarized as follows: UO₂²⁺ and Cu²⁺ form three hetero-metal polynuclear hydrolytic species [(UO₂)Cu(OH)³⁺, (UO₂)Cu₂(OH)²⁺ and (UO₂)₂Cu₄(OH)²⁺, with log β_pqr = −2.93 ± 0.01, −7.34 ± 0.03 and −13.78 ± 0.03, respectively], all those common to their simple speciation without the other cation; UO₂²⁺ and (C₂H₅)₂Sn²⁺ form seven mixed hydrolytic species [(UO₂)(DET)(OH)³⁺, (UO₂)(DET)₂(OH)²⁺, (UO₂)₂(DET)₄(OH)²⁺, (UO₂)(DET)₂₄(OH)²⁺, (UO₂)₂(DET)⁺₅(OH), (UO₂)(DET)₂⁺₅(OH) and (UO₂)₂(DET)⁻₇(OH), with log β_pqr = −2.5 ± 0.2, −4.74 ± 0.02, −10.70 ± 0.06, −10.34 ± 0.03, −15.70 ± 0.06, −15.58 ± 0.06 and −27.9 ± 0.1, respectively] that are of the same kind of those formed by uranyl; formation of mixed hydrolytic species causes a significant enhancement of the percentage of hydrolyzed metal cations, modifying the solubility and, therefore, the bioavailability of these cations. We also determined, for dioxouranium(VI)/copper(II) system, the corresponding complex formation enthalpies and entropies by direct calorimetric measurements. We obtained ΔH₁₁₂ = 47.9 ± 0.6 and ΔH₂₁₄ = 92.9 ± 0.5 kJ mol⁻¹, TΔS₁₁₂ = 6 ± 1 and TΔS₂₁₄ = 14 ± 1 kJ mol⁻¹ (±S.D.), respectively, for the formation of (UO₂)(Cu)₂(OH)²⁺ and (UO₂)₂(Cu)₄(OH)²⁺ species (according to reaction 2). We also calculated the single enthalpic and entropic contributes to the extra-stability that these species show with respect to the corresponding homo polynuclear ones.     

Single crystal particles of a mesoporous mixed transition metal oxide with a wormhole structure

A new type of mesoporous mixed transition metal oxide of Nb and Ta (NbTa-TIT-1) has been prepared through a two-step calcination, which consists of single crystal particles with wormhole mesoporous structure.     

The relevance of glass transition temperature for the process of tablet formation

The aim was to determine the relevance of the glass transition temperature (T_g) on the compressibility and compactibility of different excipients as celluloses, cellulose derivatives, lactoses, starch, maltodextrin and carrageenan. Their T_g was determined, they were tableted on an instrumented eccentric tableting machine and crushing force was analyzed. Using force, time and displacement tableting behavior was analyzed by 3D modeling. The parameters obtained, d (time plasticity), e (pressure plasticity) and w (fast elastic decompression), show different deformation mechanisms for the materials in relation to their T_g. Further, if the T_g can be reversibly exceeded during tableting, crushing force is high, otherwise crushing force is lower. This revised version was published online in July 2006 with corrections to the Cover Date.     

Deuteration triggered downward shift of dielectric phase transition temperature in a hydrogen-bonded molecular crystal

Deuteration of hydrogen-bonded phase transition crystals can increase the transition temperatures due to the isotope effect. But rare examples show the opposite trend that originates from the structural changes of the hydrogen bond, known as the geometric H/D isotope effect. Herein, we report an organic crystal, diethylammonium hydrogen 1,4-terephthalate, exhibits a reversible structural phase transition and dielectric switching. Structural study shows the cations reside in channels formed by on...     

Thermotropic homopolyesters. IV. Study of fiber formation

We report a melt spinning and viscosity study of two semiflexible homopolyesters containing both rigid and flexible segments in the repeating unit. Single filaments of the polyesters formed from 4,4′-diacetoxybiphenyl and azelaic acid (PB7) and sebacic acid (PB8), and from 4′-hydroxyphenyl-4-hydroxycinnamate and azelaic acid (C7), were spun at temperatures between 205 and 255°C. The temperature dependence of the Newtonian melt viscosity of PB7 and C7 was investigated, and a range of molecular weights was studied for PB7. The spinning parameters, fiber characteristics, and viscosity-temperature behavior are related to the type of mesophase formed. The mechanical properties of fibers spun from both the nematic and smectic phases of these semiflexible chain polymers were poor. Increasing the polymer molecular weight or extrusion rate only afforded a modest improvement in fiber properties. Most polymers could not be spun at temperatures corresponding to the existence of the single mesophase. Hence the low viscosity typical of the nematic mesophase is not necessarily an advantage in fiber formation from the melt. It appears from these results that this type of polyester does not possess adequate chain extension to develop ultrahigh-modulus properties. The director, which describes the local orientation of molecules within the mesophase, may undergo more frequent variations than is the case for rigid chain polyesters. Mechanisms relevant to flexible polymers may contribute to the development of orientation for this class of nematogenic melts.     

Infrared polarized spectra of single crystal of chloropentamethylbenzene above and below the temperature of transition

The i.r. spectra of polycrystalline sample and of single crystals of CPMB having ac and ab as predominant faces have been investigated above and below the temperature of transition to get information concerning the symmetrical modifications on the molecule or on the crystalline cell. Bands assigned to the stretching and bending vibrations CCH₃ groups become broader and less intense. Bands assigned to the rocking vibrations remain relatively unaffected. The general splitting of bands observed at low temperature is due to crystal field effect. Our results indicate a decrease of molecular symmetry through the phase transition I and III.     

Monte Carlo simulation for the formation of a mixed crystal from two solids in contact

The study focuses on nucleation and growth of a binary mixed crystal phase from two pure crystals in contact. Monte Carlo simulations of this process are conducted, with the dynamics proceeding via activated atom-vacancy exchanges. Intermolecular interactions, ranging up to next-nearest neighbors, are of size typical of hydrogen bonded systems. The process is driven by the formation of strong AB bonds at the expense of weaker AA and BB bonds. In the resulting model, the material is channeled and transported through the mixed phase crust along antiphase boundaries. The flow of molecules through the channels is directed, due to molecular energy lowering via gradual acquisition of an increasing number of nearest neighbors of the second species. On the other hand, defect motion is quasirandom. The model accounts partially for the t(1/alpha) (alpha>3) time dependence observed for conversion of nanoparticles of HBr dihydrate to monohydrate, by exposure to acid adsorbate.     

Synthesis,X-ray crystal structures,and luminescent properties of two coordination polymers constructed from transition metal complexes of mixed flexible–rigid ligands

Two new coordination polymers [Ni(C₆H₁₀O₄)(C₁₀H₈N₂)] _n (1) and [Zn₃(C₆H₁₀O₄)₃(C₁₂N₂H₈)₂] _n (2) have been synthesized under hydrothermal conditions. Complex (1) exhibits an interesting three-dimensional interpenetrating network, whereas complex (2) has a two-dimensional network. The IR and TG properties of these two complexes are studied. Furthermore, complex (2) exhibits photoluminescence. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Study of pH-triggered heteroaggregation and gel formation within mixed dispersions

This work involves an investigation of pH-triggered heteroaggregation and gelation within mixed dispersions of polystyrene (PS) and pigment particles. The PS particles were stabilised by a carboxylated alkyl ethoxylate surfactant which is pH-responsive. The pigment used was beta-copper phthalocyanine. The pigment particles contained a co-surfactant system consisting of the carboxylated alkyl ethoxylate and a non-ionic surfactant. The latter was a beta-naphthol ethoxylate. The PS and pigment particles were characterised using SEM, TEM, photon correlation spectroscopy and electrophoretic mobility measurements. The PS dispersions exhibited pH-triggered aggregation when the pH was decreased to below a critical value (pH(crit)), which was 1.9. Concentrated PS dispersions formed stable particle gels at pH values less than or equal to pH(crit). Dilute pigment dispersions were found to have a pH(crit) of 3.45. However, concentrated pigment dispersions did not form gels when the pH was decreased to below pH(crit). A phase diagram for the mixed dispersions was constructed which showed a gel phase existed at pH values between 2.0 and 3.0, which corresponds to a pH region higher than pH(crit) for the PS particles. This implicates PS-pigment inter-particle bonds in the gel structure. The heteroaggregate gels were investigated using dynamic rheological measurements and it was apparent that the highest elastic modulus values were obtained in the pH range of approximately 2 to 3. SEM images provided evidence of heteroaggregates with diameters of a few micrometers. These primary heteroaggregates are suggested to be the network forming unit for the gels formed in mixed dispersions. The data from the study are used to propose a conceptual model for the structure of the heteroaggregate gels.     

Spectroscopic flame temperature measurements and their physical significance—IV Importance of the transition probability factor

A comparison of flame temperatures measured by the well-known spectroscopic “slope” method for Fe thermometric species residing in an isothermal, optically-thin environment shows that a range of 150°K is obtained when several different sets of recommended transition probabilities are used for the determinations. A similar lack of self-consistency is also shown by the temperatures obtained from two groups of Fe lines when the same set of transition probabilities is employed. These uncertainties have not been generally recognized.     

Molecular motions and λ phase transition: Raman and far-ir studies of neat and isotopic mixed hexamethylbenzene crystal

Raman and far-IR studies of hexamethylbenzene (HMB) low temperature crystalline phases (neat and isotopic mixed) are presented. The Raman phonon spectrum changes drastically during the λ-phase transition. Site splittings on two intramolecular modes of HMB-h₁₈ (but on only one corresponding HMB-d₁₈ mode) are observed in the room temperature phase (phase II) spectrum. These splittings disappear in the lower temperature phase (phase III). The methyl torsional bands are identified and a significant shift is observed for them during the phase transition. Also, while the λ-phase transition takes place at 113°K (± 2°K) for HMB-h₁₈ crystal, the transition temperature is 133°K (± 2°K) for HMB-d₁₈. Our results suggest that (for phonon interactions) the symmetry in phase Ill is close to D_3d and reduces to C_i in phase II. Furthermore, the results support the mechanism of phase transition which involves a tilting of the methyl groups out of the benzene ring.     

Effect of temperature on triazole and bistriazole formation through copper-catalyzed alkyne-azide cycloaddition

Temperature is an important factor in the copper catalyzed alkyne azide cycloaddition under oxidative conditions. 1,2,3-Triazoles were obtained in high yields when several alkynes and azides were reacted at methanol reflux using catalytic amounts of both copper iodide and sodium hydroxide. On the other hand, bistriazoles were major products when reactions were performed at −35 °C using excess sodium hydroxide.     

Calorimetric study on the temperature dependence of the formation of mixed ionic/nonionic micelles

The differential excess enthalpy of mixed micelle formation was measured at different temperatures by mixing nonionic hexa(ethylene glycol) mono n-dodecyl ether with anionic sodium dodecyl sulfate or cationic dodecylpyridinium chloride. The experimental data were obtained calorimetrically by titrating a concentrated surfactant solution into a micellar solution of nonionic surfactant. The composition and the size of the mixed nonionic/ionic micelles at different surfactant concentrations were also determined. Pronounced differences in both composition and excess enthalpy were found between the anionic and the cationic mixed system. For both systems, the excess enthalpies become more exothermic with increasing temperature, but for the anionic mixed system an additional exothermic contribution was found which was much less temperature dependent. Temperature dependence of the excess enthalpy was attributed to the effect of the ionic headgroup on the hydration of the ethylene oxide (EO) groups in the mixed corona. Ionic headgroups located in the ethylene oxide layer cause the dehydration of the EO chains resulting in an additional hydrophobic contribution to the enthalpy of mixing. A high affinity of sodium dodecyl sulfate for nonionic micelles and an extra exothermic and less temperature dependent contribution to the excess enthalpy found for the SDS-C(12)E(6) system might be attributed to specific interactions (hydrogen bonds) between the sulfate headgroup and the partly dehydrated EO chain.     

Nonmonotonic temperature dependence of heat capacity through the glass transition within a kinetic model

The heat capacity of a supercooled liquid subjected to a temperature cycle through its glass transition is studied within a kinetic model. In this model, the beta process is assumed to be thermally activated and described by a two-level system. The alpha process is described as a beta relaxation mediated cooperative transition in a double well. The overshoot of the heat capacity during the heating scan is well reproduced and is shown to be directly related to delayed energy relaxation in the double well. In addition, the calculated scan rate dependencies of the glass transition temperature T(g) and the limiting fictive temperature T(f) (L) show qualitative agreement with the known results. Heterogeneity is found to significantly reduce the overshoot of heat capacity. Furthermore, the frequency dependent heat capacity has been calculated within the present framework and found to be rather similar to the experimentally observed behavior of supercooled liquids.     

Ambient temperature rapid ATRP of methyl acrylate, methyl methacrylate and styrene in polar solvents with mixed transition metal catalyst system

Ambient temperature atom transfer radical polymerization (ATRP) of methyl acrylate (MA), methyl methacrylate (MMA) and styrene (Sty) in the presence of polar solvents (dimethyl sulfoxide: DMSO, dimethylformamide: DMF and acetonitrile: MeCN) with a mixed transition metal catalyst system (Fe(0) as initial activator and CuBr₂/Me₆TREN complex as deactivator) provides a rapid synthesis of polymers with very low polydispersity (PDI) values and predetermined molecular weights. The polymethylacrylate (PMA) prepared using this novel approach contains the Br-terminated chain ends (functionality ∼100%) and can be successfully used for block copolymer synthesis (as demonstrated on the chain extension experiment performed using the PMA–Br macroinitiator). The key elementary reactions involved in this novel ATRP system and some preliminary mechanistic aspects of the process are also discussed.     

Syntheses, crystal structures, and properties of six new lanthanide(III) transition metal tellurium(IV) oxyhalides with three types of structures

Solid-state reactions of lanthanide(III) oxide (and lanthanide(III) oxyhalide), transition metal halide (and transition metal oxide), and TeO(2) at high temperature lead to six new lanthanide transition metal tellurium(IV) oxyhalides with three different types of structures, namely, DyCuTe(2)O(6)Cl, ErCuTe(2)O(6)Cl, ErCuTe(2)O(6)Br, Sm(2)Mn(Te(5)O(13))Cl(2), Dy(2)Cu(Te(5)O(13))Br(2), and Nd(4)Cu(TeO(3))(5)Cl(3). Compounds DyCuTe(2)O(6)Cl, ErCuTe(2)O(6)Cl, and ErCuTe(2)O(6)Br are isostructural. The lanthanide(III) ion is eight-coordinated by eight oxygen atoms, and the copper(II) ion is five-coordinated by four oxygens and a halide anion in a distorted square pyramidal geometry. The interconnection of Ln(III) and Cu(II) ions by bridging tellurite anions results in a three-dimensional (3D) network with tunnels along the a-axis; the halide anion and the lone-pair electrons of the tellurium(IV) ions are oriented toward the cavities of the tunnels. Compounds Sm(2)Mn(Te(5)O(13))Cl(2) and Dy(2)Cu(Te(5)O(13))Br(2) are isostructural. The lanthanide(III) ions are eight-coordinated by eight oxygens, and the divalent transition metal ion is octahedrally coordinated by six oxygens. Two types of polymeric tellurium(IV) oxide anions are formed: Te(3)O(8)(4)(-) and Te(4)O(10)(4)(-). The interconnection of the lanthanide(III) and divalent transition metal ions by the above two types of polymeric tellurium(IV) oxide anions leads to a 3D network with long, narrow-shaped tunnels along the b-axis. The halide anions remain isolated and are located at the above tunnels. Nd(4)Cu(TeO(3))(5)Cl(3) features a different structure. All five of the Nd(III) ions are eight-coordinated (NdO(8) for Nd(1), Nd(2), Nd(4), and Nd(5) and NdO(7)Cl for Nd(3)), and the copper(I) ion is tetrahedrally coordinated by four chloride anions. The interconnection of Nd(III) ions by bridging tellurite anions resulted in a 3D network with large tunnels along the b-axis. The CuCl(4) tetrahedra are interconnected into a 1D two-unit repeating (zweier) chain via corner-sharing. These 1D copper(I) chloride chains are inserted into the tunnels of the neodymium(III) tellurite via Nd-Cl-Cu bridges. Luminescent studies show that ErCuTe(2)O(6)Cl and Nd(4)Cu(TeO(3))(5)Cl(3) exhibit strong luminescence in the near-IR region. Magnetic measurements indicate the antiferromagnetic interactions between magnetic centers in these compounds.     

Dynamics of a low temperature phase transition in liquid crystal TB10A probed by Raman spectroscopy

Raman spectra of TB10A were recorded in the regions, 925-1000 and 1140-1220cm-1 from room temperature down to 20 K, during cooling and heating cycles. The subtle changes in the spectral features of the bands at 975 and 1195cm-1 at 47 K were attributed to a hitherto unreported stable-metastable phase transition in TB10A. The dynamics of the new phase transition have been explained in terms of splitting of the non-planar mode at 975cm-1 owing to strong intermolecular interaction due to close molecular packing in the low temperature phase. The spectral anomaly of the 975cm-1 band, in terms of variation of relative intensity with respect to temperature, also shows the hysterisis linked with the process of stable-metastable-stable modification in TB10A.