Search Results (34)

Relaxation behavior of water confined in reverse micelles under temperature-induced micelle clustering is undertaken using broadband dielectric spectroscopy in frequency range 1 Hz–20 GHz. All microemulsion systems with sufficiently noticeable micelle water pool (water/surfactant molar ratio W > 10) depict three relaxation processes, in low, high and microwave frequencies, anchoring with relaxation of shell (bound) water, orientation of surfactant anions at water-surfactant interface and relaxation of bulk water confined in reverse micelles. The analysis of dielectric relaxation processes in AOT-based w/o microemulsions under temperature induced clustering of reverse micelles were made according to structural information obtained in NMR and conductometry experiments. The “wait and switch” relaxation mechanism was applied for the explanation of results for water in the bound and bulk states under spatial limitation in reverse micelles. It was shown that surfactant layer predominantly influences the bound water. The properties of water close to AOT interface are determined by strong interactions between water and ionic AOT molecules, which perturb water H-bonding network. The decrease in micelle size causes a weakening of hydrogen bonds, deformation of its steric network and reduction in co-operative relaxation effects. Full article

In this contribution, facile synthesis of gold nanoparticles (AuNPs) at ambient conditions is reported based on the use of the polyphenolic compound quercetin (QT) as the reducing and stabilizing agent at room temperature (RT). Under alkali-induced pH adjustment of QT solution and stirring conditions at RT, QT could quickly reduce gold salt (Au³⁺) into its nanoparticle form (Au⁰), resulting in the formation of a sparkling red color colloidal solution (AuNPs) with an absorption maximum at 520 nm. Further, Fourier transform infrared spectroscopy (FTIR) was employed to showcase the role of QT in the nanomaterial’s synthesis process. The formed QT-AuNPs responded swiftly to Al³⁺ charging with color perturbation from red to grayish-purple, coupled with an absorption spectra red shift, owing to Al³⁺-induced aggregation of QT-AuNPs. However, when fluoride ion (F⁻) was pre-mixed with an optimized Al³⁺ concentration, reversed color changes from grayish-purple to red were observed with a blue shift in the absorption spectra. Simply put, F⁻ formed a complex with Al³⁺, thus preventing Al³⁺-induced aggregation of QT-AuNPs. The analytical response A₅₂₀/A₆₅₀ was linear with F⁻ concentration ranging from 25.0 to 250.0 µM and 250.0–600.0 µM, with a detection limit of 7.5 µM. The developed QT-AuNPs/Al³⁺ detection probe was selective to only F⁻ charging, in comparison with other possible interfering anions. Real sample potentiality of the developed sensor was demonstrated on tap water samples, toothpaste, and fluoride-rich mouthwash, with reliable accuracy. Full article

Fuel cells, as clean and efficient energy conversion devices, hold great potential for applications in the fields of hydrogen-based transportation and stand-alone power systems. Due to their sensitivity to load parameters, environmental parameters, and gas supply, the performance monitoring and fault diagnosis of fuel cell systems have become crucial research areas. Electrochemical impedance spectroscopy (EIS) is a widely applied analytical method in fuel cell systems. that can provide rich information about dynamic system responses, internal impedance, and transmission characteristics. Currently, EIS detection is primarily implemented by using simple topologies such as boost circuits. However, the injection of excitation signals often results in significant power fluctuations, leading to issues such as uneven temperature distributions within the cell, unstable gas supply, and damage to the proton exchange membrane. To address this issue, this paper proposes a real-time EIS detection technique for a proton exchange membrane fuel cell (PEMFC) system that connects a lithium-ion battery and injects the load voltage perturbation through a triple active bridge (TAB) converter. By applying the small-signal model of the TAB converter and designing a system controller using a decoupling control method, the PEMFC power remains stable after the disturbance injection across the entire frequency range under tests. Furthermore, the lithium-ion battery can instantly track load changes during fluctuations. The proposed EIS detection method can acquire EIS data in real time to monitor the state of the PEMFC. Simulation results validate the effectiveness and accuracy of the proposed method for EIS detection. Full article

The formation and analysis of amyloid fibers by two β-glucosidases, BglA and BglB, belonging to the GH1 enzyme family, are reported. Both proteins have the (β/α)₈ TIM-barrel fold, which is characteristic of this family and is also the most common protein structure. BglA is an octamer, whereas BglB is a monomer. Amyloid fibrillation using pH and temperature as perturbing agents was investigated using fluorescence spectroscopy as a preliminary approach and corroborated using wide-field optical microscopy, confocal microscopy, and field-emission scanning electron microscopy. These analyses showed that both enzymes fibrillate at a wide range of acidic and alkaline conditions and at several temperature conditions, particularly at acidic pH (3–4) and at temperatures between 45 and 65 °C. Circular dichroism spectroscopy corroborated the transition from an α-helix to a β-sheet secondary structure of both proteins in conditions where fibrillation was observed. Overall, our results suggest that fibrillation is a rather common phenomenon caused by protein misfolding, driven by a transition from an α-helix to a β-sheet secondary structure, that many proteins can undergo if subjected to conditions that disturb their native conformation. Full article

A new method for variable temperature infrared spectroscopy studies of minerals is presented. A sample heating/cooling apparatus incorporating a modified button sample holder with thermoelectric temperature control is described. By employing different programs to heat and cool samples with temperatures varying in different ways, various aspects of mineral powders are investigated. Infrared spectroscopy methodologies for identifying sample structural changes as a function of temperature are described. The results obtained for a variety of minerals are provided as examples. The high precision and accuracy of this approach permit the detection of subtle crystallographic unit cell distortions as a function of temperature. A < 0.25% reduction in O-H stretching vibration band intensity associated with water desorption from a quartz sample is observed at 150 °C. By employing step temperature heating profiles, reversible and irreversible sample changes can be distinguished. Variable temperature infrared spectroscopy analyses demonstrate the utility of the technique for profiling sample dehydration processes and for elucidating interactions between mineral functionalities and absorbates as a function of temperature. Full article

The vibrational dynamics of rutile (TiO₂) as a function of temperature has been studied by combining molecular dynamics (MD) simulations in conjunction with the generalized two-dimensional correlation spectroscopy analysis (2D COS) technique. Molecular dynamics simulations within the microcanonical ensemble were performed with the self-consistent charge density functional tight binding formalism at a series of different temperatures. To validate the DFTB MD results against the experimental data, the method of atomic pair distribution functions (PDFs) was used. IR absorption spectra were calculated implementing the time correlation function formalism. Subsequently, the generalized two-dimensional correlation approach was applied to obtain further insights into the temperature-dependent vibrational dynamics. The static DFTB calculations of the vibrational force field of the rutile reproduced excellently the experimental data and allowed for more exact assignments of the corresponding experimental IR/Raman spectral bands. Through the detailed analysis of the synchronous and asynchronous 2D spectra computed on the basis of MD-generated anharmonic spectra, we have provided new insights into the couplings between the modes in the studied system, as well as into the sequential character of the temperature-induced changes in the vibrational force field. The sensitivity of IR active modes to the temperature-induced perturbation of the system decreases in the order 685 cm⁻¹E_u mode > 370 cm⁻¹E_u mode > 982 cm⁻¹A_2u mode. The results presented in this study clearly demonstrate the usefulness of the combination of periodic SCC DFTB MD simulations coupled to the 2D COS analysis techniques in solid-state vibrational spectroscopy. Full article

Temperature step infrared spectroscopy is used to monitor vibrations associated with water molecules and zeolite framework during thermal dehydration and rehydration of HY and NaY. Temperature-dependent HY and NaY infrared spectrum intensity and wavenumber trends for O-H stretching vibrations, H-O-H bending vibrations, and zeolite framework vibrations are compared. Changes in hydroxyl stretching and water bending vibration bands confirm that HY has a stronger hydrogen bonding network. The intensity of a band at 3700 cm⁻¹ in NaY spectra varies with temperature and can be correlated with Na⁺ migration and zeolite water content. Spectral subtractions reveal complex intensity variations in the framework vibration band wavenumber region. Spectrum differences indicate the presence of at least four overlapping contributions in this wavenumber region. The intensities and wavenumbers of these constituents vary with temperature differently for HY and NaY. For both zeolites, infrared spectrum changes detected during thermal dehydration were mostly reversed when water re-adsorbed to the sample after cooling. Full article

In this paper, we investigate the solubility effects of Co in ZnO (Zn_1−xCo_xO, where x = 0, 0.03, 0.05, 0.1, 0.2, 0.25, 0.4, 0.8, and 1) by combining the results of perturbed angular correlation (PAC) spectroscopy using highly diluted ¹¹¹Cd as probe nuclei and ab initio calculations based on spin-density functional theory (SDFT). This combined approach enables us to characterize the local structure around Cd ions, where, through PAC technique, it was possible to measure the EFG as a function of temperature and Co concentration and thereby monitor the changes in the structure and the Co solubility threshold in ZnO and the ZnO/CoO/Co₃O₄ mixed phase. The full-potential linear augmented plane wave plus local orbital (APW+lo) formalism were used here to describe the electronic structure of the supercells, including the atomic relaxations. These Ab initio calculations show an interesting behavior of the Cd and Co impurity levels in the band structure of ZnO, which explains the experimental results in terms of the origin of EFG and the evidence of ferromagnetic response. Full article

During the dissolution of amorphous solid dispersion (ASD) formulations, the gel layer that forms at the ASD/water interface strongly dictates the release of the active pharmaceutical ingredient (API) and, hence, the dissolution performance. Several studies have demonstrated that the switch of the gel layer from eroding to non-eroding behavior is API-specific and drug-load (DL)-dependent. This study systematically classifies the ASD release mechanisms and relates them to the phenomenon of the loss of release (LoR). The latter is thermodynamically explained and predicted via a modeled ternary phase diagram of API, polymer, and water, and is then used to describe the ASD/water interfacial layers (below and above the glass transition). To this end, the ternary phase behavior of the APIs, naproxen, and venetoclax with the polymer poly(vinylpyrrolidone-co-vinyl acetate) (PVPVA64) and water was modeled using the perturbed-chain statistical associating fluid theory (PC-SAFT). The glass transition was modeled using the Gordon–Taylor equation. The DL-dependent LoR was found to be caused by API crystallization or liquid-liquid phase separation (LLPS) at the ASD/water interface. If crystallization occurs, it was found that API and polymer release was impeded above a threshold DL at which the APIs crystallized directly at the ASD interface. If LLPS occurs, an API-rich phase and a polymer-rich phase are formed. Above a threshold DL, the less mobile and hydrophobic API-rich phase accumulates at the interface which prevents API release. LLPS is further influenced by the composition and glass transition temperature of the evolving phases and was investigated at 37 °C and 50 °C regarding impact of temperature of. The modeling results and LoR predictions were experimentally validated by means of dissolution experiments, microscopy, Raman spectroscopy, and size exclusion chromatography. The experimental results were found to be in very good agreement with the predicted release mechanisms deduced from the phase diagrams. Thus, this thermodynamic modeling approach represents a powerful mechanistic tool that can be applied to classify and quantitatively predict the DL-dependent LoR release mechanism of PVPVA64-based ASDs in water. Full article

Raman spectroscopy is a promising method for the analysis of natural gas. It is necessary to account for the broadening effects on spectral lines to improve measurement accuracy. In this study, the broadening coefficients for methane lines in the region of the ν₂ band perturbed by propane, n-butane, and isobutane at room temperature were measured. We estimated the measurement errors of the concentration of oxygen and carbon dioxide in the case of neglecting the broadening effects on the methane spectrum by the pressure of C₂–C₆ alkanes. The obtained data are suited for the correct simulation of the methane spectrum in the hydrocarbon-bearing gases and can be used to improve the accuracy of the analysis of natural gas by Raman spectroscopy. Full article

The Al₄Ba crystal structure is the most common structure among binary intermetallic compounds. It is well suited for accommodating large atoms of group II elements and is often the intermediate phase closest to the terminal phase. It is, therefore, of interest to characterize diffusion properties of compounds with this tetragonal crystal structure. In the present study, ¹¹¹In perturbed angular correlation spectroscopy was used to study solute site occupation and atom movement in In₄Ba, Al₄Ba, Al₄Eu, Al₄Sr, and Ga₄Sr. The indium tracer and its daughter cadmium were found to occupy only the two Al-type sublattices in these compounds through detection of nuclear quadrupole interactions with axially symmetric EFGs. Measurements with increasing temperature revealed merging of signals due to dynamical averaging of these interactions as Cd atoms jumped at increasing rates between alternating sublattices. The jump rates were estimated to be between 8 kHz and 2 MHz at about 350 °C for Al₄Eu and at about 450 °C for In₄Ba and Al₄Ba. Fits of spectra using Blume’s stochastic model allowed determination of activation enthalpies for average Cd jump rates between alternating Al sublattices in Al₄Sr and Ga₄Sr to be 1.16(3) eV and 1.47(3) eV, respectively. This result was used to estimate transverse diffusivities of Cd. Full article

Ni-H-Beta catalysts for ethylene oligomerization (EO) were prepared by ion exchange of NH₄-Beta and H-Beta zeolites with aqueous Ni(NO₃)₂ and characterized by H₂-temperature-programmed reduction (TPR), NH₃-temperature-programmed desorption (TPD), and diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS). Quadruple exchange of NH₄-Beta at 70 °C resulted in 2.5 wt.% Ni loading corresponding to a Ni²⁺/framework aluminum (FAl) molar ratio of 0.52. [NiOH]⁺ and H⁺ are the primary charge-compensating cations in the uncalcined catalyst, as evidenced by TPR and DRIFTS. Subsequent calcination at 550 °C in air yielded a Ni-H-Beta catalyst containing primarily bare Ni²⁺ ions bonded to framework oxygens. Quadruple exchange of H-Beta at 70 °C gave 2.0 wt.% Ni loading (Ni²⁺/FAl = 0.41). After calcination at 550 °C, the resulting Ni-H-Beta catalyst comprises a mixture of bare Ni²⁺ ions: [NiOH]⁺ and NiO species. The relative abundance of [NiOH]⁺ increases with the number of exchanges. In situ pretreatment at 500 °C in flowing He converted the [NiOH]⁺ species to bare Ni²⁺ ions via dehydration. The bare Ni²⁺ ions interact strongly with the Beta framework as evidenced by a perturbed antisymmetric T-O-T vibration at 945 cm⁻¹. DRIFT spectra of CO adsorbed at 20 °C indicate that the Ni²⁺ ions occupy two distinct exchange positions. The results of EO testing at 225 °C and 11 bar (ethylene) suggested that the specific Ni²⁺ species initially presented (e.g., bare Ni²⁺, [NiOH]⁺) did not significantly affect the catalytic performance. Full article

In this work, we report on the hyperfine parameters of the foreign ¹⁸¹Ta probe in the rutile structure of the single crystal TiO₂ using the e−γ and γ−γ time differential perturbed angular correlation (TDPAC) technique. We implanted ¹⁸¹Hf ions into a sample of single crystal rutile TiO₂ in the Bonn Isotope Separator. The implanted sample was then thermally annealed at a temperature of 873 K for 315 min in a vacuum. The ¹⁸¹Hf radioisotopes decayed by β⁻ emission, followed by a cascade to the ground of γ rays or conversion electrons into a stable state ¹⁸¹Ta. The ¹⁸¹Ta probe substitutes the Ti lattice site with a unique nuclear quadrupole interaction, allowing for the precise measurement of the largest electric field gradient (V_zz) and asymmetry parameter (η). The hyperfine parameters obtained from the e−γ TDPAC spectroscopy agree with those of the γ−γ TDPAC spectroscopy at room temperature, apart from a calibration factor, both from our experiments and the literature. This suggests that the electronic recombination following the internal conversion of the L shell electron takes less time (ps) than the intermediate lifetime of the metastable ¹⁸¹Ta state (ns). Full article

Profound understanding of the local electronic and defect structure in semiconductors always plays a vital role in the further developing of applications of such materials. In the present work an investigation of the electronic structure in hydrogenated TiO${}_2$ (rutile) thin films is conducted by virtue of Time-Differential $\gamma$-$\gamma$ Perturbed Angular Correlation spectroscopy (TDPAC or PAC) with ${}^{111m}$Cd/Cd isotope, produced and implanted at ISOLDE/CERN. The measurements were performed at 581 K as a function of the temperature of the samples during hydrogenation. Despite the fact, that rutile single crystals usually show the presence of two local environments, when are studies with Cd/In isotopes, the current pristine thin films sample had a single electric field gradient. Upon various degrees of hydrogenation, Cd probe atoms showed underwent alterations, resulting in up to 3 different local surroundings, generally with high electric field gradients. Broad EFG distributions are likely due to randomly distributed point defects in the neighbourhood of Cd acceptors. Observed results suggest that hydrogenations performed at RT and 423 K are not able to promote unique defect configurations, while in the range of 473-573 K the formation of such configurations is observed. Therefore, one may assume that the formation of Cd-defect complexes (Cd-H pairs) is temperature enhanced. At higher levels of hydrogenation (663 K), the samples become partly amorphous that further hinders any atomistic studies with strong damped PAC spectra. Cd-H complexes seem to be stable up to annealing up to 581 K annealing. The obtained results give a deep insight into complex hydrogen defects, their interactions and bond formations with Cd acceptor. Full article

In this study we aimed to investigate the effect of heat treatment on the spectral pattern of honey using near infrared spectroscopy (NIRS). For the research, sunflower, bastard indigo, and acacia honeys were collected from entrusted beekeepers. The honeys were not subject to any treatment before. Samples were treated at 40 °C, 60 °C, 80 °C, and 100 °C for 60, 120, 180, and 240 min. This resulted in 17 levels, including the untreated control samples. The 5-hydroxymethylfurfural (HMF) content of the honeys was determined using the Winkler method. NIRS spectra were recorded using a handheld instrument. Data analysis was performed using ANOVA for the HMF content and multivariate analysis for the NIRS data. For the latter, PCA, PCA-LDA, and PLSR models were built (using the 1300–1600 nm spectral range) and the wavelengths presenting the greatest change induced by the perturbations of temperature and time intervals were collected systematically, based on the difference spectra and the weights of the models. The most contributing wavelengths were used to visualize the spectral pattern changes on the aquagrams in the specific water matrix coordinates. Our results showed that the heat treatment highly contributed to the formation of free or less bonded water, however, the changes in the spectral pattern highly depended on the crystallization phase and the honey type. Full article