Argon–water system at low temperatures

The molecular dynamics method is used to simulate argon solutions in water and a thin water film–argon system at low temperatures. The correlation in motions of two closely spaced argon atoms is of another nature than the correlation of two neon atoms in a neon solid solution in ice II. The structure of hydrate shells of argon atoms contains five-membered rings composed of water molecules. The solubility of argon in a water film at low temperatures is noticeably higher than at room temperature. If a water film is first cooled to the glassy state and then argon atoms are added to it, then approximately as many argon atoms are absorbed on the film surface as they are present in a cooled film in equilibrium with the argon atmosphere. Argon atoms migrate from one pit to another on the rough surface of a solid water film.     

Stabilization of CCl4− radical anions in glassy matrices at low temperatures

Optical spectrophotometry has been used to investigate the product of electron addition to CCl₄ in solvents of different nature. It is shown that in “rigid” matrices (η > 10¹⁹ P) CCl₄⁻ radical anions are formed, whereas in a “soft” environment (η < 10¹⁹ P) CCl₄⁻ undergoes dissociation into CCl₃ radicals and Cl⁻ ions. The possible reasons for stabilization of the CCl₄⁻ radical anions in solid media are discussed.     

Separation and quantification of quantum dots and dissolved metal cations by size exclusion chromatography–ICP-MS

The prevalence of engineered metallic nanoparticles within electronic products has evoked a need to assess their occurrence and fate within environmental systems upon potential release of these nanoparticles. Quantum dots (QDs) are mixed-metal nanocrystals with the smallest of particle sizes (2–10 nm) that readily leach heavy metal cations in water, potentially creating a co-occurrence of nanoparticulate and dissolved metal pollutants. In this report, we develop a size exclusion chromatography–inductively coupled plasma–mass spectrometry method (SEC-ICP-MS) for the rapid separation and quantification of ~5-nm-sized CdSe/ZnS QDs and dissolved Cd²⁺ and Zn²⁺ cations in water. The SEC-ICP-MS method provided a wide chromatographic separation of CdSe/ZnS QDs and dissolved Cd²⁺ and Zn²⁺ cations only when using the smallest SEC column pore size available and an eluent composition that prevented loss of metals to column polymer surfaces by using a surfactant to ensure elution of QDs (ammonium lauryl sulfate) and a complexing ligand to ensure elution of metal cations (ethylenediaminetetraacetate). Detection limits were between 0.2 and 2 µg L^–1 for Cd²⁺ and Zn²⁺ among dissolved cation and QD phases, and ranges of linearity covered two to three orders of magnitude. Gold nanoparticles of sizes 5, 10, 20 and 50 nm were also effectively separated from dissolved Au³⁺ cations, illustrating the method applicability to a wide range of nanoparticle sizes and compositions. QD and dissolved metal concentrations measured by SEC-ICP-MS were comparable to those measured using the more conventional method of centrifuge ultrafiltration on split samples for dissolved and total metals. The applicability of the SEC-ICP-MS method to environmental systems was verified by measuring QDs and dissolved metals added to samples of natural waters. The method was also applied to monitoring CdSe/ZnS dissolution kinetics in an urban river water. The SEC-ICP-MS developed here may offer improved automation for characterising heterogeneous suspensions containing >1 µg L^–1 heavy metals.     

Debye–Einstein model and anomalies of heat capacity temperature dependences of solid solutions at low temperatures

An approach is proposed for analysing the deviations of the heat capacity C_p(T) of solid solutions from the Kopp–Neumann rule (KNR) ΔC(T)?=?C_p(T)???C_KNR(T). Temperature dependences of the heat capacity C_p(T) of selected compositions of systems (InP)_x (InAs)_1?x and (GaAs)_x (InAs)_1?x at temperatures of 5–300 K are analysed in the Debye–Einstein approximation. It was established that in the case of substitution of atoms in the cation subsystem (Ga³⁺???In³⁺) with the same subsystem of anions (As^3?), the positive values of ΔC(T) at T?<?100 K are due to the appearance of the low-frequency Einstein mode, whereas the negative values of ΔC(T) at T?>?100 K are the result of a decrease in the fraction of the Debye contribution without changing the upper limit of the oscillation frequency. In the case of substitution in the cation subsystem (P^3????As^3?) with the invariant cation subsystem (In³⁺) to the low-temperature positive contribution of the additional low-frequency Einstein mode, a positive part is added from the modified Debye mode having the characteristic temperature θ_D below the additive value θ_DKNR. The adequacy of this model is confirmed by Raman scattering data.

     

Resonance Raman and UV–vis characterization of charge transfer complexes of TCNQ and aromatic amines

Although the molecular charge transfer complexes formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) and several different donors have been widely investigated in the past, in this paper it is shown for the first time the vibrational spectroscopic characterization of the complexes formed by TCNQ and the simplest series of substituted anilines in solution. The UV–vis spectra indicate the formation of TCNQ complexes stabilized by two aromatic amines in a sandwich π type complex. It was observed that the charge transfer transition energies of the complexes follow a linear correlation with the ionization potential of the amines. The resonance Raman spectra revealed, by the analysis of the ν(CC) and ν(CN) stretching modes of TCNQ, that the complexes keep their neutral character in the ground electronic state, and not as a TCNQ²⁻ dianion species as reported before. The observed enhancement of the TCNQ bands as the amine donor capacity increases, confirmed the charge transfer nature of the electronic transitions. The DFT and TDDFT results were obtained and the theoretical Raman and electronic spectra data supported the experimental findings. The results of the model systems presented herein can contribute to a deep understanding of the photoinduced charge transfer process in complexes of TCNQ, which is a key step in the designing of organic molecular devices.     

Orientational order of guest molecules in aligned liquid crystal as measured by EPR and UV–vis techniques

Orientational order of guest molecules in aligned liquid crystal 4-cyano-4′-pentylbiphenyl (5CB) is studied via optical dichroism and electron paramagnetic resonance (EPR) spectra measurements. The guest molecules used are bifunctional molecules bearing paramagnetic nitroxide group and photochromic azobenzene moiety. The bifunctional probe with rigidly bonded nitroxide and azobenzene moieties was found to align as a whole, while flexible long spacer between the moieties provides independent alignment for the nitroxide and azobenzene parts. Intermolecular interactions responsible for the alignment of azobenzene and nitroxide moieties of the probe molecules are discussed. The molecules with cis-configuration of azobenzene moiety are able to align in the liquid-crystalline medium, but to a lesser extent than the molecules with trans-configuration. Directions of orientational axes and characteristics of rotational mobility of spin probes are determined. Second, fourth and, in some cases, sixth rank order parameter values are found.     

Kinetics of direct and water-mediated tautomerization reactions of nucleobases at low temperatures ⩽200 K

Detailed chemical kinetic mechanisms for the synthesis of complex organic molecules in the interstellar medium are at an early stage of developement. That such synthesis must take place is well-known from chemical analysis of sampled asteroids. As molecular complexity increases the number of possible structural isomers also increases with the consequence that the nascent species may adopt a different spatial arrangement, to the lowest energy one. As part of a program of investigations of the hydrogen atom transfer reaction or tautomerization of imidic acid–amide species H-O=C-N- $\rightleftharpoons$ O=C-N-H we have studied the kinetics for a number of nucleobases, namely cytosine, thymine and uracil where a cyclic form of tautomerism (lactim–lactam) is encountered. Together with a fourth, 5-aza-uracil (1,3,5-triazine-2,4(1H,3H)-dione), we report on the rates of reaction at low temperatures 50–200 K for both the direct unimolecular process and the similar transformation mediated by an additional water molecule. We show that these tautomerization reactions can be categorized into three classes, and highlight the importance of quantum mechanical tunneling on the rate constants at these low temperatures. We further present some thermochemistry data, such as formation enthalpies, entropies, isobaric heat capacities and enthalpy functions.     

Formation of radical cations and dose response of α-terthiophene-cellulose triacetate films irradiated by electrons and gamma rays

The radiation-induced UV-vis spectrum of α-terthiophene radical cation in solid is reported. The radical cation initiates an oligomerization in the CTA matrix producing permanently coloured conjugated polarons. The specific net absorbance at 465 nm is linearly related with dose up to 2×10⁶ Gy, for electrons and gamma irradiation. The decrease of the UV typical absorption (355 nm) and of four IR bands of α-terthiophene is linear with dose, as well. Although sensitivity is influenced by dose rate, it turned out that a linear relationship holds between sensitivity and log dose rate, in the range from 2 to 10⁵ Gy/min. These findings suggest a potential application of the system for dosimetric purposes over a wide range of dose and dose rate.     

Molar conductivities of concentrated lithium chloride–glycerol solutions at low and high temperatures: application of a quasi-lattice model

Conductivities of concentrated solutions of lithium chloride in glycerol were measured for concentrations ranging from 0.005 to 1.5 mol.dm^?3. The conductivity dependencies were analysed successively using the Debye–Huckel–Onsager limiting law (DHO) at very low concentrations, the Fuoss equation of 1978 up to 0.1 mol.dm^?3, the Casteel–Amis empirical equation and the quasi-lattice model (QLM) at moderate and higher concentrations. The molar conductivities at infinite dilution, obtained using DHO and QLM were quite different from each other, because the salt forms contact pairs which were underestimated in the Λ = f(C^1/3) in QLM, as it may well be proved by Raman spectroscopy. Besides, the value of Madelung constant suggests that LiCl crystallises face centred cubic (FCC) at higher concentrations. On the basis of Raman spectroscopy analysis of previous lithium salts, we assume that the dissociation coefficient varies slightly with concentration and fraction of paired ion constant, the QLM equation is applied successfully in the concentration range used in this study. The temperature dependency of conductivity was also described using the Vogel–Tamman–Fulcher (VTF) empirical equation where the Arrhenius type was found. The results also suggest that as NaCl, LiCl can be considered as a structure maker electrolyte.     

UV–vis,IR and 1H NMR spectroscopic studies and characterization of ionic-pair crystal violet–oxytetracycline

The present study shows the formation and characterization of the ionic-pair between the antibiotic oxytetracycline and the dye crystal violet in ammonia solution pH 9.0 ± 0.2 extracted into chloroform. The characterization was demonstrated using UV–vis spectrophotometry, ¹H NMR, measurement of relaxation times T₁ and IR spectroscopy, using a comparison between the signals of individual pure compounds with the signals with the mixture CV–OTC in different alkaline media. The formation of ionic-pair was also corroborated by new signals and chemical shifts. (2D) NMR spectroscopy experiments show that the interaction is electrostatic.     

Sodium chlorite–iodine–methyl acetoacetate oscillatory reaction investigated by UV–vis spectrophotometric method

A new sodium chlorite–iodine–methyl acetoacetate chemical oscillatory reaction was studied using UV–vis spectrophotometric method. The initial concentrations of methyl acetoacetate, sodium chlorite, iodine, sulfuric acid, and the pH value have great influence on the oscillation observed at wavelength of 585 nm. There is a pre-oscillatory or induction period; the amplitude and the number of oscillations are associated with the initial concentration of reactants. The equations for the I₃ ^?–starch complex reaction rate changing with reaction time and the initial concentrations in the oscillation stage were obtained. The time of induction period decreases with the initial concentration of methyl acetoacetate or sulfuric acid, and increases with the initial concentration of sodium chlorite. A good linear relationship exists. Oscillatory reaction can be accelerated by increasing temperature. The apparent activation energies in terms of the induction period and the oscillation period were 114.28 and 64.92 kJ/mol, respectively. It may indicate that the two stages have different reaction mechanisms. The reaction of producing enol isomer by keto-enol tautomerism is an important step to constrain the time of induction period.     

An optical and electrochemical anion sensor of F− investigated by UV–vis, 1H NMR and cyclic voltammetry

A new anion receptor: 1,1′-di-(2″,4″-di-nitrophenylhydrazino-β-carbonyl)-ferrocene (1) based on ferrocene has been designed and synthesized as a highly colorimetric and electrochemical sensitive sensor for F^?. The binding mode with F^? was further investigated by UV–vis titration and ¹H NMR titration experiments. In addition, the cyclic voltammetry (CV) was performed to discuss the electrochemical sensing for F^?.     

Liquid chromatographic analysis of Hg(II) binding by thiol-rich peptides using both UV–vis and electrochemical detection

An existing method for HPLC determination of thiol-containing peptides has been successfully adapted to the analysis of mixtures of glutathione (GSH) and some related peptides with their Hg(II) complexes as a first approach to the study of phytochelatin extracts. The separation was achieved in a C18 column with a mobile phase of 0.1% trifluoroacetic acid (TFA) and 0.1% TFA/acetonitrile. Non-derivative UV–vis detection at 202 nm used in the original method has been complemented with amperometric detection at 1.2 V on glassy carbon electrode. Two different Hg(II)–GSH complexes were observed by both detection modes and confirmed by mass spectrometry.     

Dissolution and gelation of α-chitin nanofibers using a simple NaOH treatment at low temperatures

In this study, both α-chitin powders and nanofibers were successfully dissolved in 20 wt% NaOH solutions at low temperatures. Elemental analysis confirmed that this NaOH-freezing treatment did not cause deacetylation of chitin. After heating and neutralizing with water, chitin hydrogels could be prepared. X-ray diffraction and field emission scanning electron microscope data demonstrated that these two hydrogels formed typical regenerated microporous structures with low crystallinity, which was caused by the dissolution process. Based on this result, cold ethanol was used for α-chitin nanofibers during the initial stages of neutralization, which effectively prevented the dissolution and decrease in crystallinity and was even able to preserve the continuous network structure of original nanofiber. This gelation behavior seems to be attributed to interdigitation and aggregation between neighboring nanofibers in cold alkali solution leading to the shrinkage of the hydrogel. In general, by avoiding the dissolution process, highly crystalline hydrogel based on α-chitin nanofibers was prepared by a simple NaOH treatment without use of any other chemical solvents or cross-linking agents. We expect this new type of hydrogel could be promoted to wide applications and research studies as novel green nanomaterials.     

Volume expansion behavior of water–hydrocarbon mixtures at high temperatures and pressures as studied by infrared spectroscopy

Infrared spectra of binary mixtures of water with toluene and ethylbenzene have been measured at temperatures and pressures in the 473–623 K and 100–350 bar ranges, respectively. Concentrations of water and hydrocarbons in the hydrocarbon-rich phase have been estimated from the integrated band intensities, and using these results, densities of the hydrocarbon-rich phase have been obtained as a function of temperature and pressure. In order to characterize the density of the hydrocarbon-rich phase, the experimental densities have been compared with the average densities before mixing, which were calculated from the literature densities of pure water and pure hydrocarbon with the experimental concentrations. All the experimental densities of the mixtures are lower than the average densities before mixing at the same condition. Relative volume change for mixing has been estimated and an anomalously large increase in volume has been found in the vicinity of the critical region of the water–hydrocarbon mixtures. This volumetric behavior is very similar to that previously found for water–benzene mixtures, and may be characteristic of the critical behavior of fluid mixtures of water and hydrocarbons.     

Characterization of iron promoter in tungstated zirconia catalysts by Mössbauer spectroscopy at very low temperatures

A tungstated zirconia (WZ) catalyst with iron promoter used for the conversion of n-pentane into isopentane has been characterized by M?ssbauer spectroscopy. The M?ssbauer spectra have been recorded in zero magnetic field in the temperature range 0.05-295 K and with a magnetic field up to 7 T between 4.2 and 50 K. Both the recording of M?ssbauer spectra with an applied magnetic field and at extremely low temperature allowed for the demonstration that iron is present in the catalysts as (i) hematite (alpha-Fe2O3) particles a few 10 nm in size, (ii) very small oligomeric Fe(III) species, probably in solid solution in zirconia, and (iii) Fe(III) oxide clusters showing magnetic ordering, probably embedded in the first surface layer and thus forming "rafts". These latter clusters form two ensembles with quite different sizes: one with diameters of about 3 nm, the other with diameters larger than 30 nm. These results are in agreement with those recently obtained by X-ray absorption spectroscopy and electron paramagnetic resonance.     

Study of the Complexation Behavior of Calixarene with Transition Metal Cations by UV—vis and Fluorescent Spectra

A new fluorescent compound based on calix[4] arene skeleton was synthesized.Its complexation ability with transition metal ions,such as Fe^3 ,Co^2 ,Ni^2 ,Cu^2 ,Zn^2 and Ag^ ,Was investigated by UV-vis and fluorescent spectra.     

Preparation of titanium carbide powders by sol–gel and microwave carbothermal reduction methods at low temperature

Titanium carbide ultrafine powders were prepared from tetrabutyl titanate and sucrose by sol–gel and microwave carbothermal reduction. The influences of reaction temperature and molar ratio of Ti to C on the synthesis of titanium carbide were studied. The results show that excess amount of carbon plays a positive effect on the carbothermal reduction of TiO₂ at low temperature. The inceptive carbothermal reduction temperature of TiO₂ and formation of titanium oxycarbide was below 900 °C, and pure TiC can be prepared at 1,200 °C, which was considerably lower compared to that by conventional carbothermal reduction using a mixture of TiO₂ and carbon powders as raw materials. The morphology and particle size of synthesized TiC powder were examined by field emission-scanning electron microscopy (FE-SEM) and the quantities of the phases of the powders were analyzed by Rietveld refinement method, the particle sizes of the TiC powders synthesized at 1,300 °C distribute over 0.1–0.5 μm.     

Probing the chemical reactivity of interfaces: Investigation on the interaction of dehydroindigo with Laponite by UV–vis,Raman and infrared spectroscopy

In this work, the interaction between dehydroindigo (an intermediary oxidized form of indigo) and Laponite clay was investigated. Dehydroindigo (DHI) has been detected when indigo is adsorbed by clay minerals, but it is relatively unstable and in the presence of water it turns back into indigo. It is, therefore, important to understand the factors that extend its stability and Laponite was chosen because the small aspect ratio implies in a large amount of silanol groups (SiOH) which would thus favor the DHI interaction through hydrogen bonding.A significant bathochromic shift (65 nm) of the DHI π®π* transition band in the visible region and changes in the relative intensities and position of the Raman bands at 1530, 1378 and 1167 cm⁻¹ assigned to ν(NCCN), δ(CN) and ν(CN) respectively, indicate that the interaction is stronger than expected for the van der Waals and polarization forces involved in the external surfaces interactions with the siloxane groups. It was also observed that DHI presents an enhanced photochemical stability when interacting with Laponite. These results indicate that hydrogen bonding between a DHI nitrogen atom and the −SiOH or MOH groups is mainly responsible for the behavior present in the DHI + Lap system.